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DOI： 10.1088/2515-7639/acd27a

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DOI： 10.1038/s41535-023-00562-x

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DOI： 10.1103/PhysRevB.107.184435

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DOI： 10.1103/PHYSREVB.107.075410

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DOI： 10.1038/s41535-022-00530-x

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DOI： 10.1103/PhysRevMaterials.6.114408

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevb.106.085114

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.106.085114/fulltext

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DOI： 10.1038/s41524-021-00689-0

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevb.104.224508

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.104.224508/fulltext

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DOI： 10.1103/PhysRevB.104.184519

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevmaterials.5.104405

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevMaterials.5.104405/fulltext

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DOI： 10.1103/PhysRevLett.126.207201

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Publishing type：Research paper (scientific journal)   Publisher：Royal Society of Chemistry ({RSC})  

<p>Superconducting transition temperature <italic>T</italic>_c shows a dependence on an angle formed between Te–Pd–Te in a PdTe₆ octahedron (inset).</p>

DOI： 10.1039/D1CP01071A

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Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media {LLC}  

DOI： 10.1038/s41467-020-15594-1

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/PhysRevB.102.155118

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/PhysRevB.101.220408

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/PhysRevB.101.180511

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society ({APS})  

DOI： 10.1103/PhysRevB.101.075134

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DOI： 10.1103/PHYSREVLETT.125.037204

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DOI： 10.1103/PhysRevB.101.035150

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DOI： 10.1016/J.POLY.2020.114705

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevlett.123.267001

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Other Link： https://link.aps.org/article/10.1103/PhysRevLett.123.267001

Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media {LLC}  

DOI： 10.1038/s41535-019-0202-z

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/PhysRevB.100.094525

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

© 2019 American Physical Society. We investigate the connection between highly frustrated kagome based Hamiltonians and a recently synthesized family of materials containing Ti3+S=12 ions. Employing a combination of all electron density functional theory and numerical diagonalization techniques, we establish the Heisenberg Hamiltonians for the distorted kagome antiferromagnets Rb2NaTi3F12,Cs2NaTi3F12, and Cs2KTi3F12. We determine magnetization curves in excellent agreement with experimental observations. Our calculations successfully clarify the relationship between the experimental observations and the magnetization-plateau behavior at 13 height of the saturation and predict characteristic behaviors under fields that are higher than the experimentally measured region. We demonstrate that the studied Ti(III) family of materials interpolates between the kagome strip and kagome lattice.

DOI： 10.1103/PhysRevB.99.140410

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DOI： 10.1103/PhysRevB.100.155113

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DOI： 10.1103/PhysRevX.9.011005

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DOI： 10.1103/PhysRevB.100.064417

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DOI： 10.1038/s41467-019-12805-2

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We synthesized a kagome fluoride Cs2LiTi3F12 with S = 1/2 spins, and studied magnetic properties of the compound. The temperature dependence of the magnetic susceptibility indicates that it has dominant antiferromagnetic interactions and that it has no magnetic order down to 2 K. We found two magnetization plateaus in its magnetization process approximately at 1/3 and 0.8 mu(B) per Ti. The monoclinic crystal structure gives four inequivalent nearest-neighbor exchange interactions. Our density functional theory calculations suggest that three of them are antiferromagnetic and one of them is weakly ferromagnetic, resulting in a magnetic system composed of antiferromagnetically coupled linear chains and Delta chains. This explains the observed suppression of magnetic order. Numerical diagonalization gives a magnetization curve in good agreement with the experimental results.

DOI： 10.1103/PhysRevB.100.174401

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DOI： 10.1103/PhysRevX.9.011035

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier B.V.  

We report an efficient algorithm for calculating momentum-space integrals in solid state systems on modern graphics processing units (GPUs). Our algorithm is based on the tetrahedron method, which we demonstrate to be ideally suited for execution in a GPU framework. In order to achieve maximum performance, all floating point operations are executed in single precision. For benchmarking our implementation within the CUDA programming framework we calculate the orbital-resolved density of states in an iron-based superconductor. However, our algorithm is general enough for the achieved improvements to carry over to the calculation of other momentum integrals such as, e.g. susceptibilities. If our program code is integrated into an existing program for the central processing unit (CPU), i.e. when data transfer overheads exist, speedups of up to a factor ∼130 compared to a pure CPU implementation can be achieved, largely depending on the problem size. In case our program code is integrated into an existing GPU program, speedups over a CPU implementation of up to a factor ∼165 are possible, even for moderately sized workloads.

DOI： 10.1016/j.cpc.2018.04.022

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

By applying measurements of the dielectric constants and relative length changes to the dimerized molecular conductor κ-(BEDT-TTF)2Hg(SCN)2Cl, we provide evidence for order-disorder type electronic ferroelectricity that is driven by the charge order within the (BEDT-TTF)2 dimers and stabilized by a coupling to the anions. According to our density functional theory calculations, this material is characterized by a moderate strength of dimerization. This system thus bridges the gap between strongly dimerized materials, often approximated as dimer-Mott systems at 1/2 filling, and nondimerized or weakly dimerized systems at 1/4 filling, exhibiting a charge order. Our results indicate that intradimer charge degrees of freedom are of particular importance in correlated κ-(BEDT-TTF)2X salts and can create novel states, such as electronically driven multiferroicity or charge-order-induced quasi-one-dimensional spin liquids.

DOI： 10.1103/PhysRevLett.120.247601

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Wiley-VCH Verlag  

Single-component conductors based on neutral organic radicals have received a lot of attention due to the possibility that the unpaired electron can serve as a charge carrier without the need of a previous doping process. Although most of these systems are based on delocalized planar radicals, we present here a nonplanar and spin localized radical based on a tetrathiafulvalene (TTF) moiety, linked to a perchlorotriphenylmethyl (PTM) radical by a conjugated bridge, which exhibits a semiconducting behavior upon application of high pressure. The synthesis, electronic properties, and crystal structure of this neutral radical TTF-Ph-PTM derivative (1) are reported and implications of its crystalline structure on its electrical properties are discussed. On the other hand, the non-radical derivative (2), which is isostructural with the radical 1, shows an insulating behavior at all measured pressures. The different electronic structures of these two isostructural systems have a direct influence on the conducting properties, as demonstrated by band structure DFT calculations.

DOI： 10.1002/chem.201800881

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DOI： 10.1103/PhysRevMaterials.2.034802

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

We investigate superconductivity in In1-xSnxTe (x≤0.5) synthesized at high pressures of up to 2 GPa and observe an enhancement of the superconducting transition temperature Tc for increasing tin concentration x. These compounds have not been accessible in rocksalt structure via conventional ambient pressure synthesis. While the lattice constant smoothly increases with x, Tc saturates around x=0.4. Electronic structure calculations indicate that the Tc modulation is brought on by the change of the density of states in the vicinity of the Fermi energy [N(EF)]. However, differences between the calculated N(EF) and the observed electronic specific-heat coefficient indicate that the phonon dispersion plays an important role in the system and that the mechanism of superconductivity may not be the same in the entire doping range.

DOI： 10.1103/PhysRevB.97.104511

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Beilstein-Institut Zur Forderung der Chemischen Wissenschaften  

The interaction of trimethyl(methylcyclopentadienyl)platinum(IV) ((C5H4CH3)Pt(CH3)3) molecules on fully and partially hydroxylated SiO2 surfaces, as well as the dynamics of this interaction were investigated using density functional theory (DFT) and finite temperature DFT-based molecular dynamics simulations. Fully and partially hydroxylated surfaces represent substrates before and after electron beam treatment and this study examines the role of electron beam pretreatment on the substrates in the initial stages of precursor dissociation and formation of Pt deposits. Our simulations show that on fully hydroxylated surfaces or untreated surfaces, the precursor molecules remain inactivated while we observe fragmentation of (C5H4CH3)Pt(CH3)3 on partially hydroxylated surfaces. The behavior of precursor molecules on the partially hydroxylated surfaces has been found to depend on the initial orientation of the molecule and the distribution of surface active sites. Based on the observations from the simulations and available experiments, we discuss possible dissociation channels of the precursor.

DOI： 10.3762/bjnano.9.66

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Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/PhysRevLett.121.137001

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DOI： 10.1103/PhysRevB.98.064427

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We theoretically investigate the low-temperature phase of the recently synthesized Lu2Mo2O5N2 material, an extraordinarily rare realization of a S = 1/2 three-dimensional pyrochlore Heisenberg antiferromagnet in which Mo5+ are the S = 1/2 magnetic species. Despite a Curie-Weiss temperature (Theta(CW)) of -121( 1) K, experiments have found no signature of magnetic ordering or spin freezing down to T* approximate to 0.5 K. Using density functional theory, we find that the compound is well described by a Heisenberg model with exchange parameters up to third nearest neighbors. The analysis of this model via the pseudofermion functional renormalization group method reveals paramagnetic behavior down to a temperature of at least T = |Theta(CW)|/100, in agreement with the experimental findings hinting at a possible three-dimensional quantum spin liquid. The spin susceptibility profile in reciprocal space shows momentum-dependent features forming a "gearwheel" pattern, characterizing what may be viewed as a molten version of a chiral noncoplanar incommensurate spiral order under the action of quantum fluctuations. Our calculated reciprocal space susceptibility maps provide benchmarks for future neutron scattering experiments on single crystals of Lu2Mo2O5N2.

DOI： 10.1103/PhysRevMaterials.1.071201

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The spinel/perovskite heterointerface gamma-Al2O3/SrTiO3 hosts a two-dimensional electron system (2DES) with electron mobilities exceeding those in its all-perovskite counterpart LaAlO3/SrTiO3 by more than an order of magnitude, despite the abundance of oxygen vacancies which act as electron donors as well as scattering sites. By means of resonant soft x-ray photoemission spectroscopy and ab initio calculations, we reveal the presence of a sharply localized type of oxygen vacancies at the very interface due to the local breaking of the perovskite symmetry. We explain the extraordinarily high mobilities by reduced scattering resulting from the preferential formation of interfacial oxygen vacancies and spatial separation of the resulting 2DES in deeper SrTiO3 layers. Our findings comply with transport studies and pave the way towards defect engineering at interfaces of oxides with different crystal structures.

DOI： 10.1103/PhysRevB.96.161409

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Since the discovery of high-temperature superconductivity in the thin-film FeSe/SrTiO3 system, iron selenide and its derivates have been intensively scrutinized. Using ab initio density functional theory calculations we review the electronic structures that could be realized in iron selenide if the structural parameters could be tuned at liberty. We calculate the momentum dependence of the susceptibility and investigate the symmetry of electron pairing within the random phase approximation. Both the susceptibility and the symmetry of electron pairing depend on the structural parameters in a nontrivial way. These results are consistent with the known experimental behavior of binary iron chalcogenides and, at the same time, reveal two promising ways of tuning superconducting transition temperatures in these materials: on one hand by expanding the iron lattice of FeSe at constant iron-selenium distance and, on the other hand, by increasing the iron- selenium distance with unchanged iron lattice.

DOI： 10.1103/PhysRevB.96.125107

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The series of compounds AAg(2)Cr[VO4](2), with A = Ag, K, or Rb, is layered S = 3/2 triangular-lattice (TL) systems in which the magnetic exchange interactions between Cr3+ (3d(3)) ions are mediated by nonmagnetic [VO4](3-) entities. Here, the relative orientation of the vanadate is altered with respect to the TL as a function of the A site, which corresponds to an induced symmetry change of the [CrO6] complex. All members of this series of compounds belong to the class of frustrated TL antiferromagnets. We find that the distorted TL (A = Ag) exhibits collinear antiferromagnetic long-range order (LRO) at T-N approximate to 10 K, whereas the high-symmetry cases (A = K, Rb) evade LRO in zero field down to 0.03 K, the lowest temperature of our experiments. The latter members of the series belong to the undistorted TL and are candidates for spin-liquid ground states presumably not related to Ising anisotropy or dimerization.

DOI： 10.1103/PhysRevB.96.064404

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER HEIDELBERG  

The insulator SrTiO3 can host high-mobility two-dimensional electron systems on its surfaces and at interfaces with other oxides. While for the bare surface a two-dimensional electron system can only be induced by oxygen vacancies, it is believed that the metallicity of heterostructure interfaces as in LaAlO3/SrTiO3 is caused by other mechanisms related to the polar discontinuity at the interface. Based on calculations using density functional and dynamical mean-field theory as well as on experimental results using photoemission spectroscopy we elucidate the role of oxygen vacancies, thereby highlighting their importance for the electronic and magnetic properties of the systems under study.

DOI： 10.1140/epjst/e2017-70059-7

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We investigate the phase diagram of the spin-orbit-coupled three orbital Hubbard model at arbitrary filling by means of dynamical mean-field theory combined with the continuous-time quantum Monte Carlo method. We find that the spin-freezing crossover occurring in the metallic phase of the nonrelativistic multiorbital Hubbard model can be generalized to a J-freezing crossover, with J = L + S, in the spin-orbit-coupled case. In the J-frozen regime the correlated electrons exhibit a nontrivial flavor selectivity and energy dependence. Furthermore, in the regions near n = 2 and n = 4 the metallic states are qualitatively different from each other, which reflects the atomic Hund's third rule. Finally, we explore the appearance of magnetic order from exciton condensation at n = 4 and discuss the relevance of our results for real materials.

DOI： 10.1103/PhysRevLett.118.086401

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

The 5K superconductor La2Sb shares some structural similarities with the iron-pnictides and -chalgogenides, especially the so-called 122-compounds, despite lacking a 3d-metal component. In order to look for similarities and dissimilarities of this layered metal pnictide with the higher-T-c Fe-based superconductors, we have performed a combined experimental and theoretical investigation of the effects of hydrostatic pressure. Magnetic measurements reveal a linear increase of T-c with pressure at a rate dTc/dP=(0.71 +/- 0.01)K GPa(-1). Band structure calculations show that (i) the states at the Fermi level have predominantly La 5d character and (ii) that the density of states increases linearly with increasing pressure. By using these theoretical results and a BCS formalism, a quantitative description of the experimentally revealed pressure dependence of T-c is possible within a reasonable range of parameters. Our results indicate that La2Sb, despite its structural similarities to the 122 Fe-based superconductors, exhibits a BCS-type of superconductivity. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssb.201600168

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Unconventional superconductivity in iron pnictides and chalcogenides has been suggested to be controlled by the interplay of low-energy antiferromagnetic spin fluctuations and the particular topology of the Fermi surface in these materials. Based on this premise, one would also expect the large class of isostructural and isoelectronic iron germanide compounds to be good superconductors. As a matter of fact, they, however, superconduct at very low temperatures or not at all. In this work we establish that superconductivity in iron germanides is suppressed by strong ferromagnetic tendencies, which surprisingly do not originate from changes in bond angles or bond distances with respect to iron pnictides and chalcogenides, but are due to changes in the electronic structure in a wide range of energies happening upon substitution of atom species (As by Ge and the corresponding spacer cations). Our results indicate that superconductivity in iron-based materials may not always be fully understood based on d or d-p model Hamiltonians only.

DOI： 10.1103/PhysRevLett.118.017204

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

The discovery of iron pnictides and iron chalcogenides as a new class of unconventional superconductors in 2008 has generated an enormous amount of experimental and theoretical work that identifies these materials as correlated metals with multi-orbital physics, where magnetism, nematicity, and superconductivity are competing phases that appear as a function of pressure and doping. A microscopic understanding of the appearance of these phases is crucial in order to determine the nature of superconductivity in these systems. Here, we review our recent theoretical efforts to describe and understand from first principles the properties of iron pnictides and chalcogenides with special focus on (i) pressure dependence, (ii) effects of electronic correlation, and (iii) origin of magnetism and superconductivity. (C) 2016 WILEY- VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssb.201600164

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We investigate the electronic properties of the three-dimensional stripyhoneycomb gamma-Li2IrO3 via relativistic density functional theory calculations as well as exact diagonalization of finite clusters and explore the details of the optical conductivity. Our analysis of this quantity reveals the microscopic origin of the experimentally observed (i) optical transitions and (ii) anisotropic behavior along the various polarization directions. In particular, we find that the optical excitations are overall dominated by transitions between j(eff) = 1/2 and 3/2 states and the weight of transitions between jeff = 1/2 states at low frequencies can be correlated to deviations from a pure Kitaev description. We furthermore reanalyze within this approach the electronic excitations in the known two-dimensional honeycomb systems alpha-Li2IrO3 and Na2IrO3 and discuss the results in comparison to gamma-Li2IrO3.

DOI： 10.1103/PhysRevB.95.045129

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Language：English   Publisher：AMER PHYSICAL SOC  

DOI： 10.1103/PhysRevLett.117.259902

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We study the effect of oxygen vacancies on the electronic structure of the model strongly correlated metal SrVO3. By means of angle-resolved photoemission spectroscopy (ARPES) synchrotron experiments, we investigate the systematic effect of the UV dose on the measured spectra. We observe the onset of a spurious dose-dependent prominent peak at an energy range where the lower Hubbard band has been previously reported in this compound, raising questions on its previous interpretation. By a careful analysis of the dose-dependent effects we succeed in disentangling the contributions coming from the oxygen vacancy states and from the lower Hubbard band. We obtain the ARPES spectrum in the limit of a negligible concentration of vacancies, where a clear signal of a lower Hubbard band remains. We support our study by means of state-of-the-art ab initio calculations that include correlation effects and the presence of oxygen vacancies. Our results underscore the relevance of potential spurious states affecting ARPES experiments in correlated metals, which are associated with the ubiquitous oxygen vacancies as extensively reported in the context of a two-dimensional electron gas at the surface of insulating d(0) transition metal oxides.

DOI： 10.1103/PhysRevB.94.241110

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Materials with a perfect kagome lattice structure of magnetic ions are intensively sought after, because they may exhibit exotic ground states like a quantum spin liquid phase. Barlowite is a natural mineral that features perfect kagome layers of copper ions. However, in barlowite there are also copper ions between the kagome layers, which mediate strong interkagome couplings and lead to an ordered ground state. Using ab initio density functional theory calculations we investigate whether selective isoelectronic substitution of the interlayer copper ions is feasible. After identifying several promising candidates for substitution we calculate the magnetic exchange couplings based on crystal structures predicted from first-principles calculations. We find that isoelectronic substitution with nonmagnetic ions significantly reduces the interkagome exchange coupling. As a consequence, interlayer-substituted barlowite can be described by a simple two-parameter Heisenberg Hamiltonian, for which a quantum spin liquid ground state has been predicted.

DOI： 10.1103/PhysRevB.94.125136

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

There is a growing interest in the development of single-component molecular conductors based on neutral organic radicals that are mainly formed by delocalized planar radicals, such as phenalenyl or thiazolyl radicals. However, there are no examples of systems based on nonplanar and spin localized C-centered radicals exhibiting electrical conductivity due to their large Coulomb energy (U) repulsion and narrow electronic bandwidth (W) that give rise to a Mott insulator behavior. Here we present a new type of nonplanar neutral radical conductor attained by linking a tetrathiafulvalene (TTF) donor unit to a neutral polychlorotriphenylmethyl radical (PTM) with the important feature that the TTF unit enhances the overlap between the radical molecules as a consequence of short intermolecular S center dot center dot center dot S interactions. This system becomes semiconducting upon the application of high pressure thanks to increased electronic bandwidth and charge reorganization opening the way to develop a new family of neutral radical conductors.

DOI： 10.1021/jacs.6b02888

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We have investigated the charge transfer mechanism in single crystals of DTBDT-TCNQ and DTBDT-F(4)TCNQ (where DTBDT is dithieno[2,3-d;2',3'-d'] benzo[1,2-b;4,5-b'] dithiophene) using a combination of near-edge X-ray absorption spectroscopy (NEXAFS) and density functional theory calculations (DFT) including final state effects beyond the sudden state approximation. In particular, we find that a description that considers the partial screening of the electron-hole Coulomb correlation on a static level as well as the rearrangement of electronic density shows excellent agreement with experiment and allows to uncover the details of the charge transfer mechanism in DTBDT-TCNQ and DTBDT-F-4 TCNQ, as well as a reinterpretation of previous NEXAFS data on pure TCNQ. Finally, we further show that almost the same quality of agreement between theoretical results and experiment is obtained by the much faster Z+1/2 approximation, where the core hole effects are simulated by replacing N or F with atomic number Z with the neighboring atom with atomic number Z+1/2. Published by AIP Publishing.

DOI： 10.1063/1.4958659

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The symmetry of the superconducting order parameter in quasi-two-dimensional bis-ethylenedithio-tetrathiafulvalene (BEDT-TTF) organic superconductors is a subject of ongoing debate. We report ab initio density-functional-theory calculations for a number of organic superconductors containing.-type layers. Using projectiveWannier functions, we derive the parameters of a common low-energy Hamiltonian based on individual BEDT-TTF molecular orbitals. In a random-phase approximation spin-fluctuation approach, we investigate the evolution of the superconducting pairing symmetry within this model, and we point out a phase transition between extended s + d(x)(-y)(2)(2) and dxy symmetry. We discuss the origin of the mixed order parameter and the relation between the realistic molecule description and the widely used dimer approximation. Based on our ab initio calculations, we position the investigated materials in the obtained molecule model phase diagram, and we simulate scanning tunneling spectroscopy experiments for selected cases. Our calculations show that many kappa-type materials lie close to the phase-transition line between the two pairing symmetry types found in our calculation, possibly explaining the multitude of contradictory experiments in this field.

DOI： 10.1103/PhysRevB.94.024515

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We present a general framework for deriving effective spin Hamiltonians of correlated magnetic systems based on a combination of relativistic ab initio density functional theory calculations, exact diagonalization of a generalized Hubbard Hamiltonian on finite clusters, and spin projections onto the low-energy subspace. A key motivation is to determine anisotropic bilinear exchange couplings in materials of interest. As an example, we apply this method to the pyrochlore Lu2V2O7 where the vanadium ions form a lattice of corner-sharing spin-1/2 tetrahedra. In this compound, anisotropic Dzyaloshinskii-Moriya interactions (DMIs) play an essential role in inducing a magnon Hall effect. We obtain quantitative estimates of the nearest-neighbor Heisenberg exchange, the DMI, and the symmetric part of the anisotropic exchange tensor. Finally, we compare our results with experimental ones on the Lu2V2O7 compound.

DOI： 10.1103/PhysRevB.94.014410

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Two-dimensional electron systems with fascinating properties exist in multilayers of standard semiconductors, on helium surfaces, and in oxides. Compared to the two-dimensional (2D) electron gases of semiconductors, the 2D electron systems in oxides are typically more strongly correlated and more sensitive to the microscopic structure of the hosting lattice. This sensitivity suggests that the oxide 2D systems are highly tunable by hydrostatic pressure. Here we explore the effects of hydrostatic pressure on the well-characterized 2D electron system formed at LaAlO3-SrTiO3 interfaces [A. Ohtomo and H. Y. Hwang, Nature (London) 427, 423 (2004)] and measure a pronounced, unexpected response. Pressure of similar to 2 GPa reversibly doubles the 2D carrier density n(s) at 4 K. Along with the increase of ns, the conductivity and mobility are reduced under pressure. First-principles pressure simulations reveal the same behavior of the carrier density and suggest a possible mechanism of the mobility reduction, based on the dielectric properties of both materials and their variation under external pressure.

DOI： 10.1103/PhysRevB.93.235117

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We have studied a disordered N-c x N-c plaquette Hubbard model on a two-dimensional square lattice at half-filling using a coherent potential approximation (CPA) in combination with a single-site dynamical mean field theory (DMFT) approach with a paramagnetic bath. Such a model conveniently interpolates between the ionic Hubbard model at N-c = root 2 and the Anderson model at N-c = 8 and enables the analysis of the various limiting properties. We confirmed that within the CPA approach a band insulator behavior appears for noninteracting strongly disordered systems with a small plaquette size N-c = 4, while the paramagnetic Anderson insulator with nearly gapless density of states is present for large plaquette sizes N-c = 48. When the interaction U is turned on in the strongly fluctuating random potential regions, the electrons on the low energy states push each other into high energy states in DMFT in a paramagnetic bath and correlated metallic states with a quasiparticle peak and Hubbard bands emerge, though a larger critical interaction U is needed to obtain this state from the paramagnetic Anderson insulator (N-c = 48) than from the band insulator (N-c = 4). Finally, we observe a Mott insulator behavior in the strong interaction U regions for both N-c = 4 and N-c = 48 independent of the disorder strength. We discuss the application of this model to real materials.

DOI： 10.1103/PhysRevB.93.224203

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

In this study, we reanalyze the magnetic interactions in the Kitaev spin-liquid candidate materials Na2IrO3, alpha-RuCl3, and alpha-Li2IrO3 using nonperturbative exact diagonalization methods. These methods are more appropriate given the relatively itinerant nature of the systems suggested in previous works. We treat all interactions up to third neighbors on equal footing. The computed terms reveal significant long-range coupling, bond anisotropy, and/or off-diagonal couplings which we argue naturally explain the observed ordered phases in these systems. Given these observations, the potential for realizing the spin-liquid state in real materials is analyzed, and synthetic challenges are defined and explained.

DOI： 10.1103/PhysRevB.93.214431

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We report on a combined theoretical and experimental investigation of the superconducting state in the quasi-two-dimensional organic superconductor kappa-(ET)(2)Cu[N(CN)(2)]Br. Applying spin-fluctuation theory to a low-energy, material-specific Hamiltonian derived from ab initio density functional theory we calculate the quasiparticle density of states in the superconducting state. We find a distinct three-peak structure that results from a strongly anisotropic mixed-symmetry superconducting gap with eight nodes and twofold rotational symmetry. This theoretical prediction is supported by low-temperature scanning tunneling spectroscopy on in situ cleaved single crystals of kappa-(ET)(2)Cu[N(CN)(2)]Br with the tunneling direction parallel to the layered structure.

DOI： 10.1103/PhysRevLett.116.237001

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We present a structural analysis of the substituted system (Ba1-xSrx)CuSi2O6, which reveals a stable tetragonal crystal structure down to 1.5 K. We explore the structural details with low-temperature neutron and synchrotron powder diffraction, room-temperature, and cryogenic high-resolution NMR, as well as magnetic-and specific-heat measurements and verify that a structural phase transition into the orthorhombic structure which occurs in the parent compound BaCuSi2O6, is absent for the x = 0.1 sample. Furthermore, synchrotron powder-diffraction patterns show a reduction of the unit cell for x = 0.1 and magnetic measurements prove that the Cu dimers are preserved, yet with a slightly reduced intradimer coupling J(intra). Pulse-field magnetization measurements reveal the emergence of a field-induced ordered state, tantamount to Bose-Einstein-condensation (BEC) of triplons, within the tetragonal crystal structure of I4(1)/acd. This material offers the opportunity to study the critical properties of triplon condensation in a simple crystal structure.

DOI： 10.1103/PhysRevB.93.174121

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Electronic states with non-trivial topology host a number of novel phenomena with potential for revolutionizing information technology. The quantum anomalous Hall effect provides spin-polarized dissipation-free transport of electrons, while the quantum spin Hall effect in combination with superconductivity has been proposed as the basis for realizing decoherence-free quantum computing. We introduce a new strategy for realizing these effects, namely by hole and electron doping kagome lattice Mott insulators through, for instance, chemical substitution. As an example, we apply this new approach to the natural mineral herbertsmithite. We prove the feasibility of the proposed modifications by performing ab-initio density functional theory calculations and demonstrate the occurrence of the predicted effects using realistic models. Our results herald a new family of quantum anomalous Hall and quantum spin Hall insulators at affordable energy/temperature scales based on kagome lattices of transition metal ions.

DOI： 10.1038/srep25988

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Motivated by recent spin-and angular-resolved photoemission (SARPES) measurements of the two-dimensional electronic states confined near the (001) surface of oxygen-deficient SrTiO3, we explore their spin structure by means of ab initio density functional theory (DFT) calculations of slabs. Relativistic nonmagnetic DFT calculations display Rashba-like spin winding with a splitting of a few meV and when surface magnetism on the Ti ions is included, bands become spin-split with an energy difference similar to 100 meV at the G point, consistent with SARPES findings. While magnetism tends to suppress the effects of the relativistic Rashba interaction, signatures of it are still clearly visible in terms of complex spin textures. Furthermore, we observe an atomic specialization phenomenon, namely, two types of electronic contributions: one is from Ti atoms neighboring the oxygen vacancies that acquire rather large magnetic moments and mostly create in-gap states; another comes from the partly polarized t(2g) itinerant electrons of Ti atoms lying further away from the oxygen vacancy, which form the two-dimensional electron system and are responsible for the Rashba spin winding and the spin splitting at the Fermi surface.

DOI： 10.1103/PhysRevLett.116.157203

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In a common paradigm, the electronic structure of condensed matter is divided into weakly and strongly correlated compounds. While conventional band theory usually works well for the former class, many-body effects are essential for the latter. Materials such as the familiar SrTiO3 (STO) compound that bridge or even abandon this characterization scheme are highly interesting. Here, it is shown, by means of combining density functional theory with dynamical mean-field theory, that oxygen vacancies on the STO (001) surface give rise to a dichotomy of weakly correlated t(2g) low-energy quasiparticles and localized "in-gap" states of dominant e(g) character with a subtle correlation signature. We furthermore touch base with recent experimental work and study the surface instability towards magnetic order.

DOI： 10.1103/PhysRevB.93.121103

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The layered honeycomb magnet alpha-RuCl3 has been proposed as a candidate to realize a Kitaev spin model with strongly frustrated, bond-dependent, anisotropic interactions between spin-orbit entangled j(eff) = 1/2 Ru3+ magnetic moments. Here, we report a detailed study of the three-dimensional crystal structure using x-ray diffraction on untwinned crystals combined with structural relaxation calculations. We consider several models for the stacking of honeycomb layers and find evidence for a parent crystal structure with a monoclinic unit cell corresponding to a stacking of layers with a unidirectional in-plane offset, with occasional in-plane sliding stacking faults, in contrast with the currently assumed trigonal three-layer stacking periodicity. We report electronic band-structure calculations for the monoclinic structure, which find support for the applicability of the j(eff) = 1/2 picture once spin-orbit coupling and electron correlations are included. Of the three nearest-neighbor Ru-Ru bonds that comprise the honeycomb lattice, the monoclinic structure makes the bond parallel to the b axis nonequivalent to the other two, and we propose that the resulting differences in the magnitude of the anisotropic exchange along these bonds could provide a natural mechanism to explain the previously reported spin gap in powder inelastic neutron scattering measurements, in contrast to spin models based on the three-fold symmetric trigonal structure, which predict a gapless spectrum within linear spin wave theory. Our susceptibility measurements on both powders and stacked crystals, as well as magnetic neutron powder diffraction, show a single magnetic transition upon cooling below T-N approximate to 13 K. The analysis of our neutron powder diffraction data provides evidence for zigzag magnetic order in the honeycomb layers with an antiferromagnetic stacking between layers. Magnetization measurements on stacked single crystals in pulsed field up to 60 T show a single transition around 8 T for in-plane fields followed by a gradual, asymptotic approach to magnetization saturation, as characteristic of strongly anisotropic exchange interactions.

DOI： 10.1103/PhysRevB.92.235119

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Frustrated magnetism on the kagome lattice has been a fertile ground for rich and fascinating physics, ranging from experimental evidence of a spin liquid to theoretical predictions of exotic superconductivity. Among experimentally realized spin-1/2 kagome magnets, herbertsmithite, kapellasite, and haydeeite [(Zn,Mg)Cu-3(OH)(6)Cl-2] are all well described by a three-parameter Heisenberg model, but they exhibit distinctly different physics. We address the problem using a pseudofermion functional renormalization-group approach and analyze the low-energy physics in the experimentally accessible parameter range. Our analysis places kapellasite and haydeeite near the boundaries between magnetically ordered and disordered phases, implying that slight modifications could dramatically affect their magnetic properties. Inspired by this, we perform ab initio density functional theory calculations of (Zn,Mg,Cd)Cu-3 (OH)(6)Cl-2 at various pressures. Our results suggest that by varying pressure and composition one can traverse a paramagnetic regime between different magnetically ordered phases.

DOI： 10.1103/PhysRevB.92.220404

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Over the past few years iron chalcogenides have been intensively studied as part of the wider family of iron-based superconductors, with many intriguing results reported so far on intercalated and monolayer Fe Se. Nevertheless, bulk Fe Se itself remains an unusual case when compared with pnictogen-based iron superconductors, and may hold clues to understanding the more exotic derivatives of the Fe Se system. The Fe Se phase diagram is distinct from the pnictides: the orthorhombic distortion, which is likely to be of a 'spin-nematic' nature in numerous pnictides, is not accompanied by magnetic order in Fe Se, and the superconducting transition temperature T-c rises significantly with pressure before decreasing. Here we show that the magnetic interactions in Fe Se, as opposed to most pnictides, demonstrate an unusual and unanticipated frustration, which suppresses magnetic (but not nematic) order, triggers ferro-orbital order in the nematic phase and can naturally explain the non-monotonic pressure dependence of the superconducting critical temperature T-c(P).

DOI： 10.1038/NPHYS3434

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We investigate via LDA+DMFT (local density approximation combined with dynamical mean field theory) the manifestation of correlation effects in a wide range of binding energies in the hole-doped family of Fe pnictides AFe(2)As(2) (A = K, Rb, Cs) as well as the fictitious FrFe2As2 and a-axis stretched CsFe2As2. This choice of systems allows for a systematic analysis of the interplay of Hund's coupling J(H) and on-site Coulomb repulsion U in multiorbital Fe pnictides under negative pressure. With increasing ionic size of the alkali metal, we observe a nontrivial change in the iron 3d hoppings, an increase of orbitally-selective correlations, and the presence of incoherent weight at high binding energies that do not show the typical lower Hubbard-band behavior but rather characteristic features of a Hund's metal. This is especially prominent in ab-stretched CsFe2As2. We also find that the coherent/incoherent electronic behavior of the systems is, apart from temperature, strongly dependent on JH, and we provide estimates of the coherence scale T*. We discuss these results in the framework of reported experimental observations.

DOI： 10.1103/PhysRevB.92.195128

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We present a comprehensive experimental and theoretical study of the electronic and magnetic properties of two quasi-two-dimensional (2D) honeycomb lattice monoclinic compounds A(3)Ni(2)SbO(6) (A = Li, Na). Magnetic susceptibility and specific heat data are consistent with the onset of antiferromagnetic long-range order with Neel temperatures of similar to 14 and 16 K for Li3Ni2SbO6 and Na3Ni2SbO6, respectively. The effective magnetic moments of 4.3 mu(B)/f.u. (Li3Ni2SbO6) and 4.4 mu(B)/f.u. (Na3Ni2SbO6), where f.u. is formula units, indicate that Ni2+ is in a high-spin configuration (S = 1). The temperature dependence of the inverse magnetic susceptibility follows the Curie-Weiss law in the high-temperature region and shows positive values of the Weiss temperature, similar to 8 K (Li3Ni2SbO6) and similar to 12 K (Na3Ni2SbO6), pointing to the presence of nonnegligible ferromagnetic interactions, although the system orders antiferromagnetically at lowtemperatures. In addition, themagnetization curves reveal a field-induced (spin-flop type) transition below T-N that can be related to the magnetocrystalline anisotropy in these systems. These observations are in agreement with density functional theory calculations, which show that both antiferromagnetic and ferromagnetic intralayer spin exchange couplings between Ni2+ ions are present in the honeycomb planes, supporting a zigzag antiferromagnetic ground state. Based on our experimental measurements and theoretical calculations, we propose magnetic phase diagrams for the two compounds.

DOI： 10.1103/PhysRevB.92.144401

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We investigate the structural, electronic, and magnetic properties of the newly synthesized mineral barlowite Cu-4(OH)(6)FBr which contains Cu2+ ions in a perfect kagome arrangement. In contrast to the spin-liquid candidate herbertsmithite ZnCu3(OH)(6)Cl-2, kagome layers in barlowite are perfectly aligned due to the different bonding environments adopted by F- and Br- compared to Cl-. With the synthesis of this material we unveil a design strategy for layered kagome systems with possible exotic magnetic states. Density functional theory calculations and effective model considerations for Cu-4(OH)(6)FBr, which has a Cu2+ site coupling the kagome layers, predict a three-dimensional network of exchange couplings, which together with a substantial Dzyaloshinskii-Moriya coupling lead to canted antiferromagnetic ordering of this compound in excellent agreement with magnetic susceptibility measurements on single crystals yielding T-N = 15 K.

DOI： 10.1103/PhysRevB.92.094417

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We report ab initio calculations for the electronic structure of organic charge transfer salts kappa-(ET)(2)Cu[N(CN)(2)]Br, kappa-(ET)(2)Cu[N(CN)(2)]I, kappa ''-(ET)(2)Cu[N(CN)(2)]Cl, and kappa-(ET)(2)Cu-2(CN)(3). These materials show an ordering of the relative orientation of terminal ethylene groups in the bis-ethylenedithio-tetrathiafulvalene molecules at finite temperature and our calculations correctly predict the experimentally observed ground state molecular conformations (eclipsed or staggered). Further, it was recently demonstrated that the ethylene end group relative orientations can be used to reversibly tune kappa-(ET)(2)Cu[N(CN)(2)]Br through a metal-insulator transition. Using a tight-binding analysis, we show that the molecular conformations of ethylene end groups are intimately connected to the electronic structure and significantly influence hopping and Hubbard repulsion parameters. Our results place kappa-(ET)(2)Cu[N(CN)(2)]Br in eclipsed and staggered configurations on opposite sides of the metal-insulator transition.

DOI： 10.1103/PhysRevB.92.081109

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The charge-transfer compound (BEDT-TTF)(2)Ag(CF3)(4)(TCE) crystallizes in three polymorphs with different alternating layers: While a phase with a kappa packing motif has a low superconducting transition temperature of T-c = 2.6 K, two phases with higher T-c of 9.5 and 11 K are multilayered structures consisting of alpha' and kappa layers. We investigate these three systems within density functional theory and find that the alpha' layer shows different degrees of charge order for the two kappa-alpha' systems and directly influences the electronic behavior of the conducting. layer. We discuss the origin of the distinct behavior of the three polymorphs and propose a minimal tight-binding Hamiltonian for the description of these systems based on projective molecular Wannier functions.

DOI： 10.1103/PhysRevB.91.245137

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Recently, KFe2As2 was shown to exhibit a structural phase transition from a tetragonal to a collapsed tetragonal phase under an applied pressure of about 15 GPa. Surprisingly, the collapsed tetragonal phase hosts a superconducting state with T-c similar to 12 K, while the tetragonal phase is a T-c &lt;= 3.4 K superconductor. We show that the key difference between the previously known nonsuperconducting collapsed tetragonal phase in AFe(2)As(2) (A = Ba, Ca, Eu, Sr) and the superconducting collapsed tetragonal phase in KFe2As2 is the qualitatively distinct electronic structure. While the collapsed phase in the former compounds features only electron pockets at the Brillouin zone boundary and no hole pockets are present in the Brillouin zone center, the collapsed phase in KFe2As2 has almost nested electron and hole pockets. Within a random phase approximation spin fluctuation approach we calculate the superconducting order parameter in the collapsed tetragonal phase. We propose that a Lifshitz transition associated with the structural collapse changes the pairing symmetry from d wave (tetragonal) to s(+/-) (collapsed tetragonal). Our density functional theory combined with dynamical mean-field theory calculations show that effects of correlations on the electronic structure of the collapsed tetragonal phase are minimal. Finally, we argue that our results are compatible with a change of sign of the Hall coefficient with pressure, as observed experimentally.

DOI： 10.1103/PhysRevB.91.140503

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We present results for the optical conductivity of Na2IrO3 within density functional theory by including spin-orbit and correlation effects as implemented in the generalized gradient approximation. We identify the various interband transitions and show that the underlying quasimolecular-orbital nature of the electronic structure in Na2IrO3 translates into distinct features in the optical conductivity. Most importantly, the parity of the quasimolecular orbitals appears to be the main factor in determining strong and weak optical transitions. We also present optical conductivity calculations for alpha-Li2IrO3 and discuss the similarities and differences with Na2IrO3.

DOI： 10.1103/PhysRevB.91.161101

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The unusual temperature dependence of the resistivity and its in-plane anisotropy observed in the Fe-based superconducting materials, particularly Ba(Fe1-xCox)(2)As-2, has been a long-standing puzzle. Here, we consider the effect of impurity scattering on the temperature dependence of the average resistivity within a simple two-band model of a dirty spin density wave metal. The sharp drop in resistivity below the Neel temperature T-N in the parent compound can only be understood in terms of a Lifshitz transition following Fermi surface reconstruction upon magnetic ordering. We show that the observed resistivity anisotropy in this phase, arising from nematic defect structures, is affected by the Lifshitz transition as well.

DOI： 10.1103/PhysRevLett.114.097003

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Oxygen vacancies in strontium titanate surfaces (SrTiO3) have been linked to the presence of a two-dimensional electron gas with unique behavior. We perform a detailed density functional theory study of the lattice and electronic structure of SrTiO3 slabs with multiple oxygen vacancies, with a main focus on two vacancies near a titanium dioxide terminated SrTiO3 surface. We conclude based on total energies that the two vacancies preferably inhabit the first two layers, i.e. they cluster vertically, while in the direction parallel to the surface, the vacancies show a weak tendency towards equal spacing. Analysis of the nonmagnetic electronic structure indicates that oxygen defects in the surface TiO2 layer lead to population of Tit(2g) states and thus itinerancy of the electrons donated by the oxygen vacancy. In contrast, electrons from subsurface oxygen vacancies populate Ti e(g) states and remain localized on the two Ti ions neighboring the vacancy. We find that both the formation of a bound oxygen-vacancy state composed of hybridized Ti 3e(g) and 4p states neighboring the oxygen vacancy as well as the elastic deformation after extracting oxygen contribute to the stabilization of the in-gap state.

DOI： 10.1088/1367-2630/17/2/023034

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In the recently synthesized Li-x(NH2)(y)(NH3)(z)Fe2Se2 family of iron chalcogenides, a molecular spacer consisting of lithium ions, lithium amide, and ammonia separates the layers of FeSe. It has been shown that upon variation of the chemical composition of the spacer layer, superconducting transition temperatures can reach T-c similar to 44 K, but the relative importance of the layer separation and effective doping to the T-c enhancement is currently unclear. Using state of the art band structure unfolding techniques, we construct eight-orbital models from ab initio density functional theory calculations for these materials. Within an RPA spin-fluctuation approach, we show that the electron doping enhances the superconducting pairing, which is of s(+/-) symmetry and explain the experimentally observed limit to T-c in the molecular spacer intercalated FeSe class of materials.

DOI： 10.1103/PhysRevB.91.041112

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DOI： 10.1103/PhysRevB.92.195128

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DOI： 10.1103/PhysRevB.91.245137

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

In the search for novel organic charge transfer salts with variable degrees of charge transfer we have studied the effects of two modifications of the recently synthesized donor-acceptor system [tetra-methoxypyrene (TMP)]-[tetracyanoquinodimethane (TCNQ)]. One is of chemical nature by substituting the acceptor TCNQ molecules by F(4)TCNQ molecules. The second consists in simulating the application of uniaxial pressure along the stacking axis of the system. In order to test the chemical substitution, we have grown single crystals of the TMP-F(4)TCNQ complex and analyzed its electronic structure via electronic transport measurements, ab initio density functional theory (DFT) calculations and UV/VIS/IR absorption spectroscopy. This system shows an almost ideal geometrical overlap of nearly planar molecules stacked alternately (mixed stack) and this arrangement is echoed by a semiconductor-like transport behavior with an increased conductivity along the stacking direction. This is in contrast to TMP-TCNQ which shows a less pronounced anisotropy and a smaller conductivity response. Our band structure calculations confirm the one-dimensional behavior of TMP-F(4)TCNQ with pronounced dispersion only along the stacking axis. Infrared measurements illustrating the C N vibration frequency shift in F(4)TCNQ suggest however no improvement in the degree of charge transfer in TMP-F(4)TCNQ with respect to TMP-TCNQ. In both complexes about 0.1e is transferred from TMP to the acceptor. Concerning the pressure effect, our DFT calculations on the designed TMP-TCNQ and TMP-F(4)TCNQ structures under different pressure conditions show that application of uniaxial pressure along the stacking axis of TMP-TCNQ may be the route to follow in order to obtain a much more pronounced charge transfer.

DOI： 10.1039/c4cp04461d

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We revisit the problem that relevant parts of band structures for a given cell choice can reflect exact or approximate higher symmetries of subsystems in the cell and can therefore be significantly simplified by an unfolding procedure that recovers the higher symmetry. We show that band-structure unfolding can be understood as projection onto induced irreducible representations of a group obtained by extending the original group of translations with a number of additional symmetry operations. The resulting framework allows us to define a generalized unfolding procedure that includes the point group operations and can be applied to any quantity in the reciprocal space. The unfolding of the Brillouin zone follows naturally from the properties of the induced irreducible representations. In this context, we also introduce a procedure to derive tight-binding models of reduced dimensionality by making use of point group symmetries. Further, we show that careful consideration of unfolding has important consequences on the interpretation of angle-resolved photoemission experiments. Finally, we apply the unfolding procedure to various representative examples of Fe-based superconductor compounds and show that the one-iron picture arises as an irreducible representation of the glide-reflection group, and we comment on the consequences for the interpretation of one-iron versus two-iron Brillouin zone representations.

DOI： 10.1103/PhysRevB.90.195121

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Two-dimensional (2D) systems with continuous symmetry lack conventional long-range order because of thermal fluctuations. Instead, as pointed out by Berezinskii, Kosterlitz and Thouless (BKT), 2D systems may exhibit so-called topological order driven by the binding of vortex-antivortex pairs. Signatures of the BKT mechanism have been observed in thin films, specially designed heterostructures, layered magnets and trapped atomic gases. Here we report on an alternative approach for studying BKT physics by using a chemically constructed multilayer magnet. The novelty of this approach is to use molecular-based pairs of spin S = 1/2 ions, which, by the application of a magnetic field, provide a gas of magnetic excitations. On the basis of measurements of the magnetic susceptibility and specific heat on a so-designed material, combined with density functional theory and quantum Monte Carlo calculations, we conclude that these excitations have a distinct 2D character, consistent with a BKT scenario, implying the emergence of vortices and antivortices.

DOI： 10.1038/ncomms6169

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We investigate the role of correlations in the tetragonal and collapsed tetragonal phases of CaFe2As2 by performing charge self-consistent DFT+DMFT (density functional theory combined with dynamical mean-field theory) calculations. While the topology of the Fermi surface is basically unaffected by the inclusion of correlation effects, we find important orbital-dependent mass renormalizations which show good agreement with recent angle-resolved photoemission experiments. Moreover, we observe a markedly different behavior of these quantities between the low-pressure tetragonal and the high-pressure collapsed tetragonal phase. We attribute these effects to the increased hybridization between the iron and arsenic orbitals as one enters the collapsed tetragonal phase.

DOI： 10.1103/PhysRevB.90.085110

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Recent density functional theory (DFT) calculations for KFe2As2 have been shown to be insufficient to satisfactorily describe angle-resolved photoemission (ARPES) measurements as well as observed de Haas-van Alphen (dHvA) frequencies. In the present work, we extend DFT calculations based on the full-potential linear augmented plane-wave method by dynamical mean field theory (DFT+DMFT) to include correlation effects beyond the local density approximation. We present results for two sets of reported crystal structures. Our calculations indicate that KFe2As2 is a moderately correlated metal with a mass renormalization factor of the Fe 3d orbitals between 1.6 and 2.7. Furthermore, the obtained shape and size of the Fermi surface are in good agreement with ARPES measurements and we observe some topological changes with respect to DFT calculations such as the opening of an inner hole cylinder at the Z point. As a result, our calculated dHvA frequencies differ greatly from existing DFT results and qualitatively agree with experimental data. On this basis, we argue that correlation effects are important to understand the -presently under debate-nature of the superconducting state in KFe2As2.

DOI： 10.1088/1367-2630/16/8/083025

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Organic charge-transfer salts show a variety of complex phases ranging from antiferromagnetic long-range order, spin liquid, bad metal, or even superconductivity. A powerful method to investigate magnetism is spin-polarized inelastic neutron scattering. However, such measurements have often been hindered in the past by the small size of available crystals as well as by the fact that the spin in these materials is distributed over molecular rather than atomic orbitals, and good estimates for the magnetic form factors are missing. By considering Wannier functions obtained from density-functional theory calculations, we derive magnetic form factors for a number of representative organic molecules. Compared to Cu2+, the form factors vertical bar F(q)vertical bar(2) fall off more rapidly as function of q, reflecting the fact that the spin density is very extended in real space. Form factors vertical bar F(q)vertical bar(2) for TMTTF, BEDT-TTF, and (BEDT-TTF)(2) have anisotropic and nonmonotonic structures.

DOI： 10.1103/PhysRevB.90.041101

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Recently, the most intensely studied objects in the electronic theory of solids have been strongly correlated systems and graphene. However, the fact that the Dirac bands in graphene are made up of sp(2) electrons, which are subject to neither strong Hubbard repulsion U nor strong Hund's rule coupling J, creates certain limitations in terms of novel, interaction-induced physics that could be derived from Dirac points. Here we propose GaCu3(OH)(6)Cl-2 (Ga-substituted herbertsmithite) as a correlated Dirac-Kagome metal combining Dirac electrons, strong interactions and frustrated magnetic interactions. Using density functional theory, we calculate its crystallographic and electronic properties, and observe that it has symmetry-protected Dirac points at the Fermi level. Its many-body physics is diverse, with possible charge, magnetic and superconducting instabilities. Through a combination of various many-body methods we study possible symmetry-lowering phase transitions such as Mott-Hubbard, charge or magnetic ordering, and unconventional superconductivity, which in this compound assumes an f-wave symmetry.

DOI： 10.1038/ncomms5261

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We have investigated, experimentally and theoretically, the series (Na1-x Li-x)(2)IrO3. Contrary to what has been believed so far, only for x &lt;= 0.25 does the system form uniform solid solutions where Li preferentially goes to the Ir2Na planes, as observed in our density functional theory calculations and consistent with x-ray diffraction analysis. For larger Li content, as evidenced by powder x-ray diffraction, scanning electron microscopy, and density functional theory calculations, the system shows a miscibility gap and a phase separation into an ordered Na3LiIr2O6 phase with alternating Na-3 and LiIr2O6 planes, and a Li-rich phase close to pure Li2IrO3. For x &lt;= 0.25 we observe (1) an increase of c/a with Li doping up to x = 0.25, despite the fact that c/a in pure Li2IrO3 is smaller than in Na2IrO3, and (2) a gradual reduction of the antiferromagnetic ordering temperature T-N and ordered moment. In view of our results showing clear evidence for phase separation for 0.25 &lt;= x &lt;= 0.6, more detailed studies are needed to confirm the presence or absence of phase separation at the higher doping x similar to 0.7, where a continuum quantum phase transition has been proposed previously.

DOI： 10.1103/PhysRevB.89.245113

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A few organic conductors show a diversity of exciting properties like Mott insulating behavior, spin liquid, antiferromagnetism, bad metal, or unconventional superconductivity controlled by small changes in temperature, pressure, or chemical substitution. While such a behavior can be technologically relevant for functional switches, a full understanding of its microscopic origin is still lacking and poses a challenge in condensed matter physics since these phases may be a manifestation of electronic correlation. Here we determine from first principles the microscopic nature of the electronic phases in the family of organic systems kappa-(ET)(2)Cu[N(CN)(2)]BrxCl1- x by a combination of density functional theory calculations and the dynamical mean field theory approach in a form adapted for organic systems. By computing spectral and optical properties we are able to disentangle the origin of the various optical transitions in these materials and prove that correlations are responsible for relevant features. Remarkably, while some transitions are inherently affected by correlations, others are completely uncorrelated. We discuss the consequences of our findings for the phase diagram in these materials.

DOI： 10.1103/PhysRevB.89.205106

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Tungsten and cobalt carbonyls adsorbed on a substrate are typical starting points for the electron beam induced deposition of tungsten or cobalt based metallic nanostructures. We employ first principles molecular dynamics simulations to investigate the dynamics and vibrational spectra of W(CO)(6) and W(CO)(5) as well as Co-2(CO)(8) and Co(CO)(4) precursor molecules on fully and partially hydroxylated silica surfaces. Such surfaces resemble the initial conditions of electron beam induced growth processes. We find that both W(CO)(6) and Co-2(CO)(8) are stable at room temperature and mobile on a silica surface saturated with hydroxyl groups (OH), moving up to half an Angstrom per picosecond. In contrast, chemisorbed W(CO)(5) or Co(CO)(4) ions at room temperature do not change their binding site. These results contribute to gaining fundamental insight into how the molecules behave in the simulated time window of 20 ps and our determined vibrational spectra of all species provide signatures for experimentally distinguishing the form in which precursors cover a substrate. (C) 2014 AIP Publishing LLC.

DOI： 10.1063/1.4873584

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We report a high magnetic field study up to 55 T of the parent and the nearly optimally doped iron-pnictide superconductor Ca-10(Pt3As8)((Fe1-xPtx)(2)As-2)(5) [x = 0 and 0.078(6)] using magnetic torque, tunnel diode oscillator technique, and transport measurements. We determine the superconducting phase diagram, revealing an anisotropy of the irreversibility field up to a factor of 10 near T-c and signatures of multiband superconductivity. Unexpectedly, we find a prominent anomaly in magnetic torque close to 22 T, when the magnetic field is applied perpendicular to the (ab) planes, which becomes significantly more pronounced as the temperature is lowered to 0.33 K. We suggest that this field-induced transition, observed both in the magnetically ordered parent compound and a nonordered superconducting sample, is a signature of a spin-flop-like transition associated not with long-range order but driven by antiferromagnetic fluctuations of magnetic moments aligned preferentially out of the conducting planes at low temperatures.

DOI： 10.1103/PhysRevB.89.205136

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATL ACAD SCIENCES  

Dramatic volume collapses under pressure are fundamental to geochemistry and of increasing importance to fields as diverse as hydrogen storage and high-temperature superconductivity. In transition metal materials, collapses are usually driven by so-called spin-state transitions, the interplay between the single-ion crystal field and the size of the magnetic moment. Here we show that the classical S=5/2 mineral hauerite (MnS2) undergoes an unprecedented (Delta V similar to 22 %) collapse driven by a conceptually different magnetic mechanism. Using synchrotron X-ray diffraction we show that cold compression induces the formation of a disordered intermediate. However, using an evolutionary algorithm we predict a new structure with edge-sharing chains. This is confirmed as the thermodynamic ground state using in situ laser heating. We show that magnetism is globally absent in the new phase, as low-spin quantum S=1/2 moments are quenched by dimerization. Our results show how the emergence of metal-metal bonding can stabilize giant spin-lattice coupling in Earth's minerals.

DOI： 10.1073/pnas.1318543111

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The honeycomb lattice material Li2RuO3 undergoes a dimerization of Ru4+ cations on cooling below 270 degrees C, where the magnetic susceptibility vanishes. We use density functional theory calculations to show that this reflects the formation of a valence bond crystal, with a strong structural bond disproportionation. On warming, x-ray diffraction shows that discrete threefold symmetry is regained on average, and the dimerization apparently disappears. In contrast, local structural measurements using high-energy x rays show that disordered dimers survive at the nanoscale up to at least 650 degrees C. The high-temperature phase of Li2RuO3 is thus an example of a valence bond liquid, where thermal fluctuations drive resonance between different dimer coverages, a classic analog of the resonating valence bond state often discussed in connection with high-T-c cuprates.

DOI： 10.1103/PhysRevB.89.081408

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We investigate the nature of the insulating phases in a bilayer Hubbard model with intralayer coupling t and interlayer coupling t(perpendicular to) at large interaction strength U/t and half-filling. We consider a dynamical cluster approximation with a cluster size of N-c = 2 x 4, where short-range spatial fluctuations as well as on-site dynamical fluctuations are emphasized. By varying the band splitting (t(perpendicular to)/t), we find that at t(perpendicular to)/t similar or equal to 1.5 the Mott behavior is rapidly suppressed in the momentum sectors (pi,0) and (0,pi). At t(perpendicular to)/t similar or equal to 2.5, Mott features dominate in the momentum sectors (pi,pi) of the bonding band and (0,0) of the antibonding band, and at t(perpendicular to)/t similar or equal to 3.0, a tiny scattering rate is observed in all momentum sectors at the Fermi level, indicating a transition from a Mott to a band insulator. We attribute such a momentum-dependent evolution of the insulating behavior to the competition and cooperation between short-range spatial fluctuations and interlayer coupling t(perpendicular to) with the help of the Coulomb interaction U. Finally, we also discuss the possible appearance of non-Fermi liquid behavior away from half-filling.

DOI： 10.1103/PhysRevB.89.035139

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DOI： 10.1103/PhysRevB.89.205136

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DOI： 10.1103/PhysRevB.88.224304

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：BEILSTEIN-INSTITUT  

We present a numerical investigation of energy and charge distributions during electron-beam-induced growth of tungsten nanostructures on SiO2 substrates by using a Monte Carlo simulation of the electron transport. This study gives a quantitative insight into the deposition of energy and charge in the substrate and in the already existing metallic nanostructures in the presence of the electron beam. We analyze electron trajectories, inelastic mean free paths, and the distribution of backscattered electrons in different compositions and at different depths of the deposit. We find that, while in the early stages of the nanostructure growth a significant fraction of electron trajectories still interacts with the substrate, when the nanostructure becomes thicker the transport takes place almost exclusively in the nanostructure. In particular, a larger deposit density leads to enhanced electron backscattering. This work shows how mesoscopic radiation-transport techniques can contribute to a model that addresses the multi-scale nature of the electron-beam-induced deposition (EBID) process. Furthermore, similar simulations can help to understand the role that is played by backscattered electrons and emitted secondary electrons in the change of structural properties of nanostructured materials during post-growth electron-beam treatments.

DOI： 10.3762/bjnano.4.89

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We propose that a combination of the semiclassical approximation with Monte Carlo simulations can be an efficient and reliable impurity solver for dynamical mean field theory equations and their cluster extensions with large cluster sizes. In order to show the reliability of the method, we consider two test cases: (i) the single-band Hubbard model within the dynamical cluster approximation with four-and eight-site clusters and (ii) the anisotropic two-orbital Hubbard model with orbitals of different bandwidth within the single-site dynamical mean field theory. We compare the critical interaction Uc/t with those obtained from solving the dynamical mean field equations with the continuous-time and Hirsch-Fye quantum Monte Carlo approaches. In both test cases we observe reasonable values of the metal-insulator critical interaction strength Uc/t and the nature of Mott physics in the self-energy behavior. While some details of the spectral functions cannot be captured by the semiclassical approximation due to the freezing of dynamical fluctuations, the main features are reproduced by the approach.

DOI： 10.1103/PhysRevB.88.165126

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Organic charge-transfer salts based on the molecule Pd(dmit)(2) display strong electronic correlations and geometrical frustration, leading to spin-liquid, valence bond solid, and superconducting states, among other interesting phases. The low-energy electronic degrees of freedom of these materials are often described by a single band model: a triangular lattice with a molecular orbital representing a Pd(dmit)(2) dimer on each site. We use ab initio electronic structure calculations to construct and parametrize low-energy effective model Hamiltonians for a class of Me4-n EtnX[Pd(dmit)(2)](2) (X = As, P, N, Sb) salts and investigate how best to model these systems by using variational Monte Carlo simulations. Our findings suggest that the prevailing model of these systems as a t - t' triangular lattice is incomplete and that a fully anisotropic triangular lattice description produces importantly different results, including a significant lowering of the critical U of the spin-liquid phase.

DOI： 10.1103/PhysRevB.88.155139

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We study the temperature and pressure dependence of the structural and electronic properties of the iron pnictide superconductor BaFe2As2. We use density functional theory-based Born-Oppenheimer molecular dynamics simulations to investigate the system at temperatures from T = 5 to 150 K and pressures from P = 0 to 30 GPa. When increasing the pressure at low temperature, we find the two transitions from an orthorhombic to a tetragonal and to a collapsed tetragonal structure that are also observed in zero temperature structure relaxations and in experiment. These transitions are considerably smeared out at finite temperature, with the critical pressure for the first transition increasing with temperature. We also analyze the electronic structure of BaFe2As2 at finite temperature and the effect of the structural oscillations on the band structure and Fermi surface in comparison to known zero-temperature results. Our results should be helpful for resolving some open issues in experimental reports for BaFe2As2 under high pressure.

DOI： 10.1103/PhysRevB.88.075111

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We studied experimentally and theoretically the structural transition of diamond under an irradiation with an intense femtosecond extreme ultraviolet laser (XUV) pulse of 24-275 eV photon energy provided by free-electron lasers. Experimental results obtained show that the irradiated diamond undergoes a solid-to-solid phase transition to graphite, and not to an amorphous state. Our theoretical findings suggest that the nature of this transition is nonthermal, stimulated by a change of the interatomic potential triggered by the excitation of valence electrons. Ultrashort laser pulse duration enables to identify the subsequent steps of this process: electron excitation, band gap collapse, and the following atomic motion. A good agreement between the experimentally measured and theoretically calculated damage thresholds for the XUV range supports our conclusions.

DOI： 10.1103/PhysRevB.88.060101

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WORLD SCIENTIFIC PUBL CO PTE LTD  

In this paper, we review theoretical investigations on the origin of the orbital selective phase where localized and itinerant electrons coexist in the d shell at intermediate strength of the on-site Coulomb interactions between electrons. In particular, the effect of spatial fluctuations on the phase diagram of the two-orbital Hubbard model with unequal bandwidths is discussed. And different band dispersions in different orbitals as well as different magnetically ordered states in different orbitals which are responsible for orbital selective phase transitions are emphasized. This is due to the fact that these two mechanisms are independent of the Hund's rule coupling, and are completely distinct from other well-known mechanisms like orbitals of unequal bandwidths and orbitals with different degeneracies. Moreover, crystal field splitting is not required in these two recently proposed mechanisms.

DOI： 10.1142/S0217984913300159

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Herbertsmithite [ZnCu3(OH)(6)Cl-2] is often discussed as the best realization of the highly frustrated antiferromagnetic kagome lattice known so far. We employ density functional theory (DFT) calculations to determine eight exchange coupling constants of the underlying Heisenberg Hamiltonian. We find the nearest-neighbor coupling J(1) to exceed all other couplings by far. However, next-nearest-neighbor kagome layer couplings of 0.019J(1) and interlayer couplings of up to -0.035J(1) slightly modify the perfect antiferromagnetic kagome Hamiltonian. Interestingly, the largest interlayer coupling is ferromagnetic, even without Cu impurities in the Zn layer. In addition, we validate our DFT approach by applying it to kapellasite, a polymorph of herbertsmithite, which is known experimentally to exhibit competing exchange interactions.

DOI： 10.1103/PhysRevB.88.075106

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By means of density functional theory (DFT) calculations [with and without inclusion of spin-orbit (SO) coupling] we present a detailed study of the electronic structure and corresponding microscopic Hamiltonian parameters of Na2IrO3. In particular, we address the following aspects: (i) We investigate the role of the various structural distortions and show that the electronic structure of Na2IrO3 is exceptionally sensitive to structural details. (ii) We discuss both limiting descriptions for Na2IrO3-quasimolecular orbitals (small SO limit, itinerant) versus relativistic orbitals (large SO limit, localized)-and show that the description of Na2IrO3 lies in an intermediate regime. (iii) We investigate whether the nearest neighbor Kitaev-Heisenberg model is sufficient to describe the electronic structure and magnetism in Na2IrO3. In particular, we verify the recent suggestion of an antiferromagnetic Kitaev interaction and show that it is not consistent with actual or even plausible electronic parameters. Finally, (iv) we discuss correlation effects in Na2IrO3. We conclude that while the Kitaev-Heisenberg Hamiltonian is the most general expression of the quadratic spin-spin interaction in the presence of spin-orbit coupling (neglecting single-site anisotropy), the itinerant character of the electrons in Na2IrO3 makes other terms beyond this model (including, but not limited to, 2nd and 3rd neighbor interactions) essential.

DOI： 10.1103/PhysRevB.88.035107

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burning question in the emerging field of spin-orbit driven insulating iridates, such as Na2IrO3 and Li2IrO3, is whether the observed insulating state should be classified as a Mott-Hubbard insulator derived from a half-filled relativistic j(eff) = 1/2 band or as a band insulator where the gap is assisted by spin-orbit interaction or Coulomb correlations or both. The difference between these two interpretations is that only for the former strong spin-orbit coupling (lambda greater than or similar to W, whereW is the bandwidth) is essential. We have synthesized the isostructural and isoelectronic Li2RhO3 and report its electrical resistivity and magnetic susceptibility. Remarkably, it shows insulating behavior together with fluctuating effective S = 1/2 moments, similar to Na2IrO3 and Li2IrO3, although in Rh4+ (4d(5)) the spin-orbit coupling is greatly reduced. We show that this behavior has a nonrelativistic one-electron origin (although Coulomb correlations assist in opening the gap) and can be traced to the formation of quasimolecular orbitals, similar to those in Na2IrO3.

DOI： 10.1103/PhysRevB.88.035115

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DOI： 10.1103/PhysRevB.87.241105

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Starting from the orthorhombic magnetically ordered phase, we investigate the effects of uniaxial tensile and compressive stresses along a, b, and the diagonal a + b directions in BaFe2As2 and CaFe2As2 in the framework of ab initio density functional theory (DFT) and a phenomonological Ginzburg-Landau model. While-contrary to the application of hydrostatic or c-axis uniaxial pressure-both systems remain in the orthorhombic phase with a pressure-dependent nonzero magnetic moment, we observe a sign-changing jump in the orthorhombicity at a critical uniaxial pressure, accompanied by a reversal of the orbital occupancy and a switch between the ferromagnetic and antiferromagnetic directions. Our Ginzburg-Landau analysis reveals that this behavior is a direct consequence of the competition between the intrinsic magneto-elastic coupling of the system and the applied compressive stress, which helps the system to overcome the energy barrier between the two possible magneto-elastic ground states. Our results shed light on the mechanisms involved in the detwinning process of an orthorhombic iron-pnictide crystal and on the changes in the magnetic properties of a system under uniaxial stress.

DOI： 10.1103/PhysRevB.87.174503

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Potassium-doped picene (K(x)picene) has recently been reported to be a superconductor at x 3 with critical temperatures up to 18 K. Here we study the electronic structure of K-doped picene films by photoelectron spectroscopy and ab initio density functional theory combined with dynamical mean-field theory (DFT + DMFT). Experimentally we observe that, except for spurious spectral weight due to the lack of a homogeneous chemical potential at low K concentrations (x approximate to 1), the spectra always display a finite energy gap. This result is supported by our DFT + DMFT calculations which provide clear evidence that K(x)picene is a Mott insulator for integer doping concentrations x 1, 2, and 3. We discuss various scenarios to understand the discrepancies with previous reports of superconductivity and metallic behavior.

DOI： 10.1103/PhysRevLett.110.216403

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DOI： 10.1103/PhysRevB.87.155139

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DOI： 10.1088/1367-2630/15/1/015016

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DOI： 10.1103/PhysRevB.87.045111

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We perform charge self-consistent density functional theory combined with dynamical mean field theory calculations to study correlation effects on the Fermi surfaces of the iron pnictide superconductors LaFePO and LiFeP. We find a distinctive change in the topology of the Fermi surface in both compounds where a hole pocket with Fe d(z)(2) orbital character changes its geometry from a closed shape in the local-density approximation to an open shape upon inclusion of correlations. The opening of the pocket occurs in the vicinity of the Gamma (Z) point in LaFePO ( LiFeP). We discuss the relevance of these findings for the low superconducting transition temperature and the nodal gap observed in these materials.

DOI： 10.1103/PhysRevLett.109.236403

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Motivated by recent angle-resolved photoemission spectroscopy observations of a highly metallic two-dimensional electron gas (2DEG) at the (001) vacuum-cleaved surface of SrTiO3 and the subsequent discussion on the possible role of oxygen vacancies for the appearance of such a state [Santander-Syro et al., Nature (London) 469, 189 (2011)], we analyze by means of density functional theory the electronic structure of various oxygen-deficient SrTiO3 surface slabs. We find a significant surface reconstruction after introducing oxygen vacancies and we show that the charges resulting from surface-localized oxygen vacancies-independent of the oxygen concentration-redistribute in the surface region and deplete rapidly within a few layers from the surface suggesting the formation of a 2DEG. We discuss the underlying model emerging from such observations.

DOI： 10.1103/PhysRevB.86.195119

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Electron beam-induced deposition with tungsten hexacarbonyl W(CO)(6) as precursors leads to granular deposits with varying compositions of tungsten, carbon and oxygen. Depending on the deposition conditions, the deposits are insulating or metallic. We employ an evolutionary algorithm to predict the crystal structures starting from a series of chemical compositions that were determined experimentally. We show that this method leads to better structures than structural relaxation based on estimated initial structures. We approximate the expected amorphous structures by reasonably large unit cells that can accommodate local structural environments that resemble the true amorphous structure. Our predicted structures show an insulator-to-metal transition close to the experimental composition at which this transition is actually observed and they also allow comparison with experimental electron diffraction patterns.

DOI： 10.1088/1367-2630/14/11/113028

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Contrary to previous studies that classify Na2IrO3 as a realization of the Heisenberg-Kitaev model with a dominant spin-orbit coupling, we show that this system represents a highly unusual case in which the electronic structure is dominated by the formation of quasimolecular orbitals (QMOs), with substantial quenching of the orbital moments. The QMOs consist of six atomic orbitals on an Ir hexagon, but each Ir atom belongs to three different QMOs. The concept of such QMOs in solids invokes very different physics compared to the models considered previously. Employing density functional theory calculations and model considerations we find that both the insulating behavior and the experimentally observed zigzag antiferromagnetism in Na2IrO3 naturally follow from the QMO model.

DOI： 10.1103/PhysRevLett.109.197201

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We present a new experimental setup to study electron-electron coincidences from superconducting surfaces. In our approach, electrons emitted from a surface are projected onto a time-and position-sensitive microchannel plate detector with delayline position readout. Electrons that are emitted within 2 pi solid angle with respect to the surface are detected in coincidence. The detector used is a hexagonal delayline detector with enhanced multiple hit capabilities. It is read out with a Flash analog-to-digital converter. The three-dimensional momentum vector is obtained for each electron. The intrinsic dead time of the detector has been greatly reduced by implementing a new algorithm for pulse analysis. The sample holder has been matched to fit the spectrometer while being capable of cooling down the sample to 4.5 K during the measurement and heating it up to 420 K for the cleaning procedure. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4754470]

DOI： 10.1063/1.4754470

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A combination of density functional theory calculations, many-body model considerations, and magnetization and electron-spin-resonance measurements shows that the multiferroic FeTe 2O 5Br should be described as a system of alternating antiferromagnetic S=5/2 chains with strong Fe-O-Te-O-Fe bridges weakly coupled by two-dimensional frustrated interactions, rather than the previously reported tetramer model. The peculiar temperature dependence of the incommensurate magnetic vector can be explained in terms of interchain exchange striction being responsible for the emergent net electric polarization. © 2012 American Physical Society.

DOI： 10.1103/PhysRevB.86.054402

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A combination of density functional theory calculations, many-body model considerations, and magnetization and electron-spin-resonance measurements shows that the multiferroic FeTe2O5Br should be described as a system of alternating antiferromagnetic S = 5/2 chains with strong Fe-O-Te-O-Fe bridges weakly coupled by two-dimensional frustrated interactions, rather than the previously reported tetramer model. The peculiar temperature dependence of the incommensurate magnetic vector can be explained in terms of interchain exchange striction being responsible for the emergent net electric polarization.

DOI： 10.1103/PhysRevB.86.054402

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The interaction of trimethyl methylcyclopentadienyl platinum (MeCpPtMe3) with a fully hydroxylated SiO2 surface has been explored by means of ab initio calculations. A large slab model (3x3x4 supercell) cut out from the hydroxylated beta-cristobalite SiO2 (111) surface was chosen to simulate a silica surface. Density functional theory calculations were performed to evaluate the energies of MeCpPtMe3 adsorption to the SiO2 surface. Our results show that the physisorption of the molecule is dependent on both (i) the orientation of the adsorbate and (ii) the adsorption site on the substrate. The most stable configuration was found with the MeCp and Me-3 groups of the molecule oriented toward the surface. Finally, we observe that van der Waals corrections are crucial for the stabilization of the molecule on the surface. We discuss the relevance of our results for the growth of Pt-based nanostructured materials via deposition processes such as electron beam-induced deposition.

DOI： 10.1088/1367-2630/14/7/073040

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We present experimental results and theoretical simulations of the adsorption behavior of the metal-organic precursor Co-2(CO)(8) on SiO2 surfaces after application of two different pretreatment steps, namely by air plasma cleaning or a focused electron beam pre-irradiation. We observe a spontaneous dissociation of the precursor molecules as well as autodeposition of cobalt on the pretreated SiO2 surfaces. We also find that the differences in metal content and relative stability of these deposits depend on the pretreatment conditions of the substrate. Transport measurements of these deposits are also presented. We are led to assume that the degree of passivation of the SiO2 surface by hydroxyl groups is an important controlling factor in the dissociation process. Our calculations of various slab settings, using dispersion-corrected density functional theory, support this assumption. We observe physisorption of the precursor molecule on a fully hydroxylated SiO2 surface (untreated surface) and chemisorption on a partially hydroxylated SiO2 surface (pretreated surface) with a spontaneous dissociation of the precursor molecule. In view of these calculations, we discuss the origin of this dissociation and the subsequent autocatalysis.

DOI： 10.3762/bjnano.3.63

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We present a combination of local-density approximation (LDA) with the dynamical cluster approximation (LDA + DCA) in the framework of the full-potential linear augmented plane wave method, and compare our LDA + DCA results for SrVO3 to LDA with the dynamical mean-field theory (LDA + DMFT) calculations as well as experimental observations on SrVO3. We find a qualitative agreement of the momentum resolved spectral function with angle-resolved photoemission spectra (ARPES) and former LDA + DMFT results. As a correction to LDA + DMFT, we observe more pronounced coherent peaks below the Fermi level, as indicated by ARPES experiments. In addition, we resolve the spectral functions in the K-0 = (0,0,0) and K-1 = (pi, pi, pi) sectors of DCA, where band insulating and metallic phases coexist. Our approach can be applied to correlated compounds where not only local quantum fluctuations but also spatial fluctuations are important.

DOI： 10.1103/PhysRevB.85.165103

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We present uniaxial pressure structural relaxations for CaFe2As2 and BaFe2As2 within density functional theory and compare them with calculations under hydrostatic pressure as well as available experimental results. We find that CaFe2As2 shows a unique phase transition from a magnetic orthorhombic phase to a nonmagnetic collapsed tetragonal phase for both pressure conditions and no indication of a tetragonal phase is observed at intermediate uniaxial pressures. In contrast, BaFe2As2 shows for both pressure conditions two phase transitions from a magnetic orthorhombic to a collapsed tetragonal phase through an intermediate nonmagnetic tetragonal phase. We find that the critical transition pressures under uniaxial conditions are much lower than those under hydrostatic conditions manifesting the high sensitivity of the systems to uniaxial stress. We discuss the origin of this sensitivity and its relation to experimental observations.

DOI： 10.1103/PhysRevB.85.094105

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It is demonstrated that the near-edge X-ray absorption fine structure (NEXAFS) provides a powerful local probe of functional groups in novel charge transfer (CT) compounds and their electronic properties. Microcrystals of tetra-/hexamethoxypyrene as donors with the strong acceptor tetracyano-p-quinodimethane (TMP/HMP-TCNQ) were grown by vapor diffusion. The oxygen and nitrogen K-edge spectra are spectroscopic fingerprints of the functional groups in the donor and acceptor moieties, respectively. The orbital selectivity of the NEXAFS pre-edge resonances allows us to precisely elucidate the participation of specific orbitals in the charge transfer process. Upon complex formation, the intensities of several resonances change substantially and a new resonance occurs in the oxygen K-edge spectrum. This gives evidence of a corresponding change of hybridization of specific orbitals in the functional groups of the donor (those derived from the frontier orbitals 2e and 6a(1) of the isolated methoxy group) and acceptor (orbitals b(3g), a(w), b(1g), and b(2w), all located at the cyano group) with pi*-orbitals of the ring systems. Along with this intensity effect, the resonance positions associated with the oxygen K-edge (donor) and nitrogen K-edge (acceptor) shift to higher and lower photon energies in the complex, respectively. A calculation based on density functional theory qualitatively explains the experimental results. NEXAFS measurements shine light on the action of the functional groups and elucidate charge transfer on a submolecular level.

DOI： 10.1021/ja2100802

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We discuss the role of electronic correlations in the iron-based superconductor LiFeAs by studying the effects on band structure, mass enhancements, and Fermi surface in the framework of density functional theory combined with dynamical mean field theory calculations. We conclude that LiFeAs shows characteristics of a moderately correlated metal and that the strength of correlations is mainly controlled by the value of the Hund's rule coupling J. The hole pockets of the Fermi surface show a distinctive change in form and size with implications for the nesting properties. Our calculations are in good agreement with recent angle-resolved photoemission spectroscopy and de Haas-van Alphen experiments.

DOI： 10.1103/PhysRevB.85.094505

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Based on the analysis of a two-orbital Hubbard model within a mean-field approach, we propose a mechanism for an orbital selective phase transition (OSPT) where coexistence of localized and itinerant electrons can be realized. We show that this OSPT exists both at and near half-filling even in the absence of crystal-field splittings or when bandwidths, orbital degeneracies, and magnetic states are equal for both orbitals, provided the orbitals have different band dispersions. Such conditions should generally be satisfied in many materials. We find that this OSPT is not sensitive to the strength of Hund's rule coupling and that heavy doping favors the collinear antiferromagnetic state over the OSPT. We discuss our results in relation to the iron pnictides.

DOI： 10.1103/PhysRevB.85.035123

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We investigate the effect that the temperature dependence of the crystal structure of a two-dimensional organic charge-transfer salt has on the low-energy Hamiltonian representation of the electronic structure. For that, we determine the crystal structure of kappa-(BEDT-TTF)(2)Cu-2(CN)(3) for a series of temperatures between T = 5 and 300 K by single crystal X-ray diffraction and analyze the evolution of the electronic structure with temperature by using density functional theory and tight binding methods. We find a considerable temperature dependence of the corresponding triangular lattice Hubbard Hamiltonian parameters. We conclude that even in the absence of a change of symmetry, the temperature dependence of quantities like frustration and interaction strength can be significant and should be taken into account.

DOI： 10.1103/PhysRevB.85.035125

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DOI： 10.1103/PhysRevB.85.165103

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The interaction of tungsten hexacarbonyl [W(CO)(6)] precursor molecules with SiO2 substrates is investigated by means of density functional theory calculations with and without inclusion of long-range van der Waals interactions. We consider two different surface models, a fully hydroxylated and a partially hydroxylated SiO2 surface, corresponding to substrates under different experimental conditions. For the fully hydroxylated surface, we observe only a weak interaction between the precursor molecule and the substrate, with physisorption of W(CO)(6). Inclusion of van der Waals corrections results in a stabilization of the molecules on this surface, but does not lead to significant changes in the chemical bonding. In contrast, we find a spontaneous dissociation of the precursor molecule on the partially hydroxylated SiO2 surface, where chemisorption of a W(CO)(5) fragment is observed upon removal of one of the CO ligands from the precursor molecule. Irrespective of the hydroxylation, the precursor molecule prefers binding of more than one of its CO ligands. In light of these results, implications for the initial growth stage of tungsten nanodeposits on SiO2 in an electron-beam-induced deposition process are discussed.

DOI： 10.1103/PhysRevB.84.205442

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

The anisotropic two-orbital Hubbard model with different bandwidths and degrees of frustration in each orbital is investigated in the framework of both single-site dynamical mean-field theory (DMFT) as well as its cluster extension (DCA) for clusters up to four sites combined with a continuous-time quantum Monte Carlo algorithm. This model shows a rich phase diagram which includes the appearance of orbital selective phase transitions, non-Fermi liquid behavior as well as antiferromagnetic metallic states. We discuss the advantages and drawbacks of employing the single-site DMFT with respect to DCA and the consequences for the physical picture obtained out of these calculations. Finally, we argue that such a minimal model may be of relevance to understand the nature of the antiferromagnetic metallic state in the iron-pnictide superconductors as well as the origin of the small staggered magnetization observed in these systems. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/andp.201100021

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Motivated by the unexplored complexity of phases present in the multiorbital Hubbard model, we analyze in this work the behavior of a degenerate two-orbital anisotropic Hubbard model at half-filling where both orbitals have equal bandwidths and one orbital is constrained to be paramagnetic (PM), while the second one is allowed to have an antiferromagnetic (AF) solution. Such a model may be relevant for a large class of correlated materials with competing magnetic states in different orbitals such as the recently discovered Fe-based superconductors. Using a dynamical cluster approximation we observe that unique orbital selective phase transitions appear regardless of the strength of the Ising Hund's rule coupling J(z). Moreover, the PM orbital undergoes a transition from a Fermi liquid (FL) to a Mott insulator through a non-FL phase while the AF orbital shows a transition from a FL to an AF insulator through an AF metallic phase. We discuss the implications of the results in the context of the Fe-based superconductors.

DOI： 10.1103/PhysRevB.84.020401

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

By applying density functional theory, we find strong evidence for an itinerant nature of magnetism in two families of iron pnictides. Furthermore, by employing dynamical mean field theory with continuous time quantum Monte Carlo as an impurity solver, we observe that the antiferromagnetic metal with small magnetic moment naturally arises out of coupling between unfrustrated and frustrated bands. Our results point to a possible scenario for magnetism in iron pnictides where magnetism originates from a strong instability at the momentum vector (pi,pi,pi) while it is reduced by quantum fluctuations due to the coupling between weakly and strongly frustrated bands. (C) 2010 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.jpcs.2010.10.005

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The natural mineral azurite Cu-3(CO3)(2)(OH)(2) is a frustrated magnet displaying unusual and controversially discussed magnetic behavior. Motivated by the lack of a unified description for this system, we perform a theoretical study based on density functional theory as well as state-of-the-art numerical many-body calculations. We propose an effective generalized spin-1/2 diamond chain model which provides a consistent description of experiments: low-temperature magnetization, inelastic neutron scattering, nuclear magnetic resonance measurements, magnetic susceptibility as well as new specific heat measurements. With this study we demonstrate that the balanced combination of first principles with powerful many-body methods successfully describes the behavior of this frustrated material.

DOI： 10.1103/PhysRevLett.106.217201

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We investigate the electronic and magnetic properties of the frustrated triangular-lattice antiferromagnets Cs2CuCl4 and Cs2CuBr4 in the framework of density functional theory. Analysis of the exchange couplings J and J' using the available x-ray structural data corroborates the values obtained from experimental results for Cs2CuBr4 but not for Cs2CuCl4. In order to understand this discrepancy, we perform a detailed study of the effect of structural optimization on the exchange couplings of Cs2CuCl4 employing different exchange-correlation functionals. We find that the exchange couplings depend on rather subtle details of the structural optimization and that only when the insulating state (mediated through spin polarization) is present in the structural optimization, we do have good agreement between the calculated and the experimentally determined exchange couplings. Finally, we discuss the effect of interlayer couplings as well as longer-ranged couplings in both systems.

DOI： 10.1103/PhysRevB.83.125126

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We report on a systematic study of the magnetic properties on single crystals of the solid solution Cs2CuCl4-xBrx (0 &lt;= x &lt;= 4), which include the two known end-member compounds Cs2CuCl4 and Cs2CuBr4, classified as quasi-two-dimensional quantum antiferromagnets with different degrees of magnetic frustration. By comparative measurements of the magnetic susceptibility chi(T) on as many as 18 different Br concentrations, we found that the in-plane and out-of-plane magnetic correlations, probed by the position and height of a maximum in the magnetic susceptibility, respectively, do not show a smooth variation with x. Instead, three distinct concentration regimes can be identified, which are separated by critical concentrations x(c1) = 1 and x(c2) = 2. This unusual magnetic behavior can be explained by considering the structural peculiarities of the materials, especially the distorted Cu-halide tetrahedra, which support a site-selective replacement of Cl- by Br- ions. Consequently, the critical concentrations x(c1) (x(c2)) mark particularly interesting systems, where one (two) halide-sublattice positions are fully occupied.

DOI： 10.1103/PhysRevB.83.064425

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

In the Cu-II-p-hydroquinonate coordination polymer 1, the major pathway for antiferromagnetic exchange coupling runs along the hydroquinonate linker. Upon heating, 1 looses its supporting DMF ligands in a two-step sequence; the antiferromagnetic Cu-II-Cu-II interaction in the final product 3 is now mediated by two bridging oxygen atoms which results in an increase of the J value by two orders of magnitude.

DOI： 10.1039/c0ce00367k

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The optical conductivity of LaFeAsO, BaFe2As2, SrFe2As2, and EuFe2As2 in the spin-density wave (SDW) state is investigated within density functional theory (DFT) in the framework of spin-polarized generalized gradient approximation (GGA) and GGA+U. We find a strong dependence of the optical features on the Fe magnetic moments. In order to recover the small Fe magnetic moments observed experimentally, GGA+U-eff with a suitable choice of negative on-site interaction U-eff=U-J was considered. Such an approach may be justified in terms of an overscreening which induces a relatively small U compared to the Hund's rule coupling J, as well as a strong Holstein-type electron-phonon interaction. Moreover, reminiscent of the fact that GGA+U-eff with a positive U-eff is a simple approximation for reproducing a gap with correct amplitude in correlated insulators, a negative U-eff can also be understood as a way to suppress magnetism and mimic the effects of quantum fluctuations ignored in DFT calculations. With these considerations, the resulting optical spectra reproduce the SDW gap and a number of experimentally observed features related to the antiferromagnetic order. We find electronic contributions to excitations that so far have been attributed to purely phononic modes. Also, an orbital-resolved analysis of the optical conductivity reveals significant contributions from all Fe 3d orbitals. Finally, we observe that there is an important renormalization of kinetic energy in these SDW metals, implying that the effects of correlations cannot be neglected.

DOI： 10.1103/PhysRevB.82.165102

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We report a combined experimental and theoretical investigation of the layered antimonide PrMnSbO which is isostructural to the parent phase of the iron pnictide superconductors. We find linear resistivity near room temperature and Fermi liquidlike T(2) behavior below 150 K. Neutron powder diffraction shows that unfrustrated C-type Mn magnetic order develops below similar to 230 K, followed by a spin flop coupled to induced Pr order. At T similar to 35 K, we find a tetragonal to orthorhombic (T-O) transition. First-principles calculations show that the large magnetic moments observed in this metallic compound are of local origin. Our results are thus inconsistent with either the itinerant or frustrated models proposed for symmetry breaking in the iron pnictides. We show that PrMnSbO is instead a rare example of a metal where structural distortions are driven by f-electron degrees of freedom.

DOI： 10.1103/PhysRevB.82.100412

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Scanning tunneling spectroscopy measurements on the high-temperature superconductor Bi2Sr2CaCu2O8+delta have reported an enhanced spectral gap in the neighborhood of O dopant atoms. We calculate, within density functional theory (DFT), the change in electronic structure due to such a dopant. We then construct and discuss the validity of several tight-binding (TB) fits to the DFT bands with and without an O dopant. With the doping-modulated TB parameters, we finally evaluate the spin susceptibility and pairing interaction within spin-fluctuation theory. The d-wave pairing eigenvalues are enhanced above the pure system without O dopant, supporting the picture of enhanced local pairing around such a defect.

DOI： 10.1103/PhysRevB.82.054514

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We present a detailed study of the derivation of the Hubbard model parameters for kappa-(ET)(2)Cu-2(CN)(3) in the framework of ab initio Density Functional Theory We show that calculations with different (i) wavefunction basis (ii) exchange correlation functionals and (iii) tight-binding models provide a reliable benchmark for the parameter values We compare our results with available extended Huckel molecular orbital calculations and discuss its implications for the description of the properties of kappa-(ET)(2)Cu-2(CN)(3) The electronic properties of kappa-(ET)(2)Cu(SCN)(2) are also briefly discussed (C) 2010 Elsevier B V All rights reserved

DOI： 10.1016/j.physb.2009.12.055

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We investigate the phase diagram of a two-band frustrated Hubbard model in the framework of dynamical mean-field theory. While a first-order phase transition occurs from a paramagnetic (PM) metal to an antiferromagnetic (AF) insulator when both bands are equally frustrated, an intermediate AF metallic phase appears in each band at different U(c) values if only one of the two bands is frustrated, resulting in continuous orbital-selective phase transitions from PM metal to AF metal and AF metal to AF insulator, regardless of the strength of the Ising Hund's coupling. We argue that our minimal-model calculations capture the frustration behavior in the undoped iron-pnictide superconductors as well as local quantum-fluctuation effects and that the intermediate phases observed in our results are possibly related to the puzzling AF metallic state with small staggered magnetization observed in these systems as well as to the pseudogap features observed in optical experiments.

DOI： 10.1103/PhysRevB.81.220506

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We investigate the effect of Na intercalation in the layered Mott insulator TiOCl within the framework of density functional theory. We show that the system remains always insulating for all studied Na concentrations, and the evolution of the spectral weight upon Na doping is consistent with recent photoemission experiments. We predict the Na-doped superlattice structures, and show that substitutions of O by F, Cl by S, or Ti by V (or Sc), respectively, fail to metallize the system. We propose a description in terms of a multiorbital ionic Hubbard model in a quasi-two-dimensional lattice and discuss the nature of the insulating state under doping. Finally, a likely route for metallizing TiOCl by doping is proposed.

DOI： 10.1103/PhysRevLett.104.146402

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Motivated by recent experimental measurements on pressure-driven phase transitions in Mott insulators as well as the new iron pnictide superconductors, we show that first principles Car-Parrinello molecular dynamics calculations are a powerful method to describe the microscopic origin of such transitions. We present results for (i) the pressure-induced insulator to metal phase transition in the prototypical Mott insulator TiOCl as well as (ii) the pressure-induced structural and magnetic phase transitions in the family of correlated metals AFe(2)As(2) (A = Ca, Sr, Ba). Comparison of our predictions with existing experimental results yields very good agreement.

DOI： 10.1088/0953-8984/22/16/164208

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Within the framework of density-functional theory, we investigate the nature of magnetism in various families of Fe-based superconductors. (i) We show that magnetization of stripe-type antiferromagnetic order always becomes stronger when As is substituted by Sb in LaOFeAs, BaFe2As2 , and LiFeAs. By calculating Pauli susceptibilities, we attribute the magnetization increase obtained after replacing As by Sb to the enhancement of an instability at (pi,pi). This points to a strong connection between Fermi-surface nesting and magnetism, which supports the theory of the itinerant nature of magnetism in various families of Fe-based superconductors. (ii) We find that within the family LaOFePn ( Pn=P , As, Sb, and Bi), the absence of an antiferromagnetic phase in LaOFeP and its presence in LaOFeAs can be attributed to the competition of instabilities in the Pauli susceptibility at (pi,pi) and (0,0), which further strengthens the close relation between Fermi-surface nesting and experimentally observed magnetization. (iii) Finally, based on our relaxed structures and Pauli susceptibility results, we predict that LaOFeSb upon doping or application of pressure should be a candidate for a superconductor with the highest transition temperature among the hypothetical compounds LaOFeSb, LaOFeBi, ScOFeP, and ScOFeAs while the parent compounds LaOFeSb and LaOFeBi should show, at ambient pressure, a stripe-type antiferromagnetic metallic state.

DOI： 10.1103/PhysRevB.81.094505

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

From a combination of high resolution angle-resolved photoemission spectroscopy and density functional calculations, we derive information on the dimensionality and the orbital character of the electronic states of BaFe2-xCoxAs2. Upon increasing Co doping, the electronic states in the vicinity of the Fermi level take on increasingly three-dimensional character. Both the orbital variation with k(z) and the more three-dimensional nature of the doped compounds have important consequences for the nesting conditions and thus possibly also for the appearance of antiferromagnetic and superconducting phases.

DOI： 10.1103/PhysRevB.81.104512

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We investigate the properties of a two-orbital Hubbard model with unequal bandwidths on the square lattice in the framework of the dynamical cluster approximation (DCA) combined with a continuous-time quantum Monte Carlo algorithm. We explore the effect of short-range spatial fluctuations on the nature of the metal-insulator transition and the possible occurrence of an orbital-selective Mott transition (OSMT) as a function of cluster size N-c. We observe that for N-c = 2 no OSMT is present, instead a band insulator state for both orbitals is stabilized at low temperatures due to the appearance of an artificial local ordered state. For N-c = 4 the DCA calculations suggest the presence of five different phases which originate out of the cooperation and competition between spatial fluctuations and orbitals of different bandwidths and an OSMT phase is stabilized. Based on our results, we discuss the nature of the gap opening.

DOI： 10.1103/PhysRevLett.104.026402

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Using ab initio molecular dynamics we investigate the electronic and lattice structure of AFe(2)As(2) (A=Ca,Sr,Ba) under pressure. We find that the structural phase transition (orthorhombic to tetragonal symmetry) is always accompanied by a magnetic phase transition in all the compounds while the nature of the transitions is different for the three systems. Our calculations explain the origin of the existence of a collapsed tetragonal phase in CaFe2As2 and its absence in BaFe2As2. We argue that changes in the Fermi-surface nesting features dominate the phase transitions under pressure rather than spin frustration or a Kondo scenario. The consequences for superconductivity are discussed.

DOI： 10.1103/PhysRevB.80.094530

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Intense experimental and theoretical studies have demonstrated that the anisotropic triangular lattice as realized in the kappa-(BEDT-TTF)(2)X family of organic charge transfer salts yields a complex phase diagram with magnetic, superconducting, Mott insulating, and even spin liquid phases. With extensive density functional theory calculations we refresh the link between many body theory and experiment by determining hopping parameters of the underlying Hubbard model. This leads us to revise the widely used semiempirical parameters in the direction of less frustrated, more anisotropic triangular lattices. The implications of these results on the systems&apos; description are discussed.

DOI： 10.1103/PhysRevLett.103.067004

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

A theoretical approach to treat laser induced femtosecond structural changes in covalently bonded nanostructures and solids is described. Our approach consists in molecular dynamic simulations performed on the basis of time-dependent, many-body potential energy surfaces derived from tight-binding Hamiltonians. The shape and spectral composition of the laser pulse is explicitly taking into account in a non-perturbative way. We show a few examples of the application of this approach to describe the laser damage and healing of defects in carbon nanotubes with different chiralities and the ultrafast nonequilibrium melting of bulk germanium, initiated by the laser-induced softening and destabilization of transversal acoustic phonon modes.

DOI： 10.1007/s00339-009-5178-2

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

In this paper a fast impurity solver is proposed for dynamical mean-field theory (DMFT) based on a decoupling of the equations of motion for the impurity Green's function. The resulting integral equations are solved efficiently with a method based on genetic algorithms. The Hubbard and periodic Anderson models are studied with this impurity solver. The method describes the Mott metal-insulator transition and works for a large range of parameters at finite temperature on the real frequency axis. This makes it useful for the exploration of real materials in the framework of local-density approximation+DMFT.

DOI： 10.1103/PhysRevB.79.235112

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

The discovery of a new family of high-T-C materials(1), the iron arsenides (FeAs), has led to a resurgence of interest in superconductivity. Several important traits of these materials are now apparent: for example, layers of iron tetrahedrally coordinated by arsenic are crucial structural ingredients. It is also now well established that the parent non-superconducting phases are itinerant magnets(2-5), and that superconductivity can be induced by either chemical substitution(6) or application of pressure(7), in sharp contrast to the cuprate family of materials. The structure and properties of chemically substituted samples are known to be intimately linked(8,9); however, remarkably little is known about this relationship when high pressure is used to induce superconductivity in undoped compounds. Here we show that the key structural features in BaFe2As2, namely suppression of the tetragonal-to-orthorhombic phase transition and reduction in the As-Fe-As bond angle and Fe-Fe distance, show the same behaviour under pressure as found in chemically substituted samples. Using experimentally derived structural data, we show that the electronic structure evolves similarly in both cases. These results suggest that modification of the Fermi surface by structural distortions is more important than charge doping for inducing superconductivity in BaFe2As2.

DOI： 10.1038/NMAT2443

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We report high resolution angle-resolved photoemission spectroscopy (ARPES) studies of the electronic structure of BaFe2As2, which is one of the parent compounds of the Fe-pnictide superconductors. ARPES measurements have been performed at 20 and 300 K, corresponding to the orthorhombic antiferromagnetic phase and the tetragonal paramagnetic phase, respectively. Photon energies between 30 and 175 eV and polarizations parallel and perpendicular to the scattering plane have been used. Measurements of the Fermi surface yield two hole pockets at the Gamma point and an electron pocket at each of the X points. The topology of the pockets has been concluded from the dispersion of the spectral weight as a function of binding energy. Changes in the spectral weight at the Fermi level upon variation in the polarization of the incident photons yield important information on the orbital character of the states near the Fermi level. No differences in the electronic structure between 20 and 300 K could be resolved. The results are compared with density functional theory band structure calculations for the tetragonal paramagnetic phase.

DOI： 10.1103/PhysRevB.79.155118

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

We investigate the microscopic model for the frustrated layered antiferromagnets Cs(2)CuCl(4) and Cs(2)CuBr(4) by performing ab initio density functional theory (DFT) calculations and with the help of the tight-binding method. The combination of both methods provide the relevant interaction paths in these materials, and we estimate the corresponding exchange constants. We find for Cs(2)CuCl(4) that the calculated ratio of the strongest in-plane exchange constants J&apos;=J between the spin-1/2 Cu ions agrees well with neutrons cattering experiments. The microscopic model based on the derived exchange constants is tested by comparing the magnetic susceptibilities obtained from exact diagonalization with experimental data. The electronic structure differences between Cs(2)CuCl(4) and Cs(2)CuBr(4) are also analyzed.

DOI： 10.1088/1742-6596/145/1/012038

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

On the basis of ab initio density-functional-theory calculations, we derive the underlying microscopic model Hamiltonian for TiOCl, a unique system that shows two consecutive phase transitions from a Mott insulator to a spin-Peierls insulator through a structurally incommensurate phase. We show with our model that the presence of magnetic frustration in TiOCl leads to a competition with the spin-Peierls distortion, which results in the unusual incommensurate phase. In addition, our calculations indicate that the spin-Peierls state is triggered by adiabatic phonons, which is essential for understanding the nature of the phase transition.

DOI： 10.1103/PhysRevB.78.205104

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Using Car-Parrinello molecular dynamics we investigate the behavior of the low-dimensional multi-orbital Mott insulator TiOCl under pressure. We show that the system undergoes two consecutive phase transitions, first at Pc from a Mott-insulator to a metallic phase in the ab plane with a strong Ti-Ti dimerization along b. At a pressure P(c)(1) &gt; Pc the dimerization disappears and the system behaves as a uniform metal. This second transition has not yet been reported experimentally. We show that the insulator-to-metal transition at P(c) is driven by the widening of the bandwidth rather than structural changes or reduction of crystal field splittings and it shows a redistribution of the electronic occupation within the t(28) bands. Our computed pressure-dependent lattice parameters are consistent with experimental observations and the existing controversy on the change of crystal symmetry at high pressures is discussed.

DOI： 10.1103/PhysRevLett.101.136406

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In spin crossover materials, an abrupt phase transition between a low-spin state and a high-spin (HS) state can be driven by temperature, pressure or by light irradiation. Of special relevance are Fe(II) based coordination polymers where, in contrast to molecular systems, the phase transition between a spin S = 0 and 2 state shows a pronounced hysteresis which is desirable for technical applications. A satisfactory microscopic explanation of this large cooperative phenomenon has been sought for a long time. The lack of x-ray data has been one of the reasons for the absence of microscopic studies. In this work, we present an efficient route to prepare reliable model structures and within an ab initio density functional theory analysis and effective model considerations we show that in polymeric spin crossover compounds magnetic exchange between HS Fe(II) centers is as important as elastic couplings for understanding the phase transition. We discuss the relevance of these interactions for the cooperative behavior in these materials.

DOI： 10.1088/1367-2630/9/12/448

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We present a comparative study of the coupled-tetrahedra quantum spin Systems Cu2Te2O5X2, X = Cl, Br (Cu-2252(X)) and the newly synthesized Cu4Te5O12Cl4 (Cu-45124(Cl)) based on ab initio density functional theory calculations. The magnetic behavior of Cu-45124(Cl) with a phase transition to an ordered state at a lower critical temperature T-c = 13.6 K than in Cu-2252(Cl) (T-c = 18.2 K) can be well understood in terms of the modified interaction paths. We identify the relevant structural changes between the two systems and discuss the hypothetical behavior of the not yet synthesized Cu-45124(Br) with an ab initio relaxed structure using Car-Parrinello molecular dynamics. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.03.255

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Extensive excited-state molecular dynamics simulations of femtosecond laser-induced structural transformation in single-walled carbon nanotubes (SWNTs) are presented. We considered in the simulation two limiting cases; one where only a short portion of the tube is irradiated and the other where the whole tube is affected by the laser pulse. We have analyzed the role of chirality (zigzag versus armchair and some chiral tube cases) in the damage threshold as a function of tube diameter. Nontrivial dependence of the damage threshold as a function of diameter has been found. We find that for equal laser parameters, zigzag SWNTs are on average equally stable with respect to laser excitation as armchair SWNTs, but their stabilities show different dependencies on diameter. Due to the higher stiffness of the (n,n) tubes in the direction perpendicular to its axis as compared to the (n,0) tubes, we find the formation of standing waves in the nanotube wall for zigzag and not for armchair tubes. We also studied the role of the laser pulse duration and show that in general longer laser pulses increase the damage threshold. This result is rationalized in terms of electron-ion relaxation times. Implications of laser-induced structural transformations are analyzed.

DOI： 10.1103/PhysRevB.75.125412

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Within a first principles framework, we study the electronic structure of the recently synthesized polymeric coordination compound Cu(II)-2,5-bis(pyrazol-1-yl)-1,4-dihydroxybenzene (CuCCP), which has been suggested to be a good realization of a Heisenberg spin-1/2 chain with antiferromagnetic coupling. By using a combination of classical with ab initio quantum mechanical methods, we design on the computer reliable modified structures of CuCCP aimed at studying effects of Cu-Cu coupling strength variations on this spin-1/2 system. For this purpose, we performed two types of modifications on CuCCP. In one case, we replaced H in the linker by (i) an electron donating group (NH2) and (ii) an electron withdrawing group (CN), while the other modification consisted of adding H2O and NH3 molecules in the structure which change the local coordination of the Cu( II) ions. With the Nth order muffin tin orbital (NMTO) downfolding method, we provide a quantitative analysis of the modified electronic structure and the nature of the Cu-Cu interaction paths in these new structures and discuss its implications for the underlying microscopic model.

DOI： 10.1088/1367-2630/9/2/026

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We present a microscopic study of the electronic and magnetic properties of the recently synthesized spin tetrahedron system Cu4Te5O12Cl4 based on density functional calculations and on ab initio-derived effective models. In view of these results, we discuss the origin of the observed differences in behavior between this system and the structurally similar Cu2Te2O5Cl2. Since the Br analog of the title compound has not been synthesized yet, we derive the crystal structure of Cu4Te5O12Br4 by geometry optimization in an ab initio molecular dynamics calculation and investigate the effect of substituting Cl by Br. The possible magnetic behavior of Cu4Te5O12Br4 in comparison with the recently studied Cu2Te2O5Br2 is also discussed.

DOI： 10.1103/PhysRevB.75.024404

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER FRANCE-EDITIONS SCIENTIFIQUES MEDICALES ELSEVIER  

The detailed investigation of electronic and magnetic properties of polymeric coordination materials with accurate ab initio quantum mechanical methods is often computationally extremely demanding because of the large number of atoms in the unit cell. Moreover, usually the available structural data are insufficient or poorly determined, especially when the structure contains hydrogen atoms. In order to be able to perform controlled ab initio calculations on reliable structures, we use a two-step approach to systematically prepare model structures for polymeric coordination compound systems and to relax them to their equilibrium configuration. First, a structure is constructed on the basis of a crystallographic database and optimized by force field methods; in the second step, the structure is relaxed by ab initio quantum mechanical molecular dynamics. With this structure, we per-form accurate electronic structure calculations. We will apply this procedure to a Fe(II) triazole compound and to a coordination polymer of Cu(II) ions with 2,5-bis(pyrazol-1-yl)-1,4-dihydroxybenzene.

DOI： 10.1016/j.crci.2006.06.007

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The response of carbon nanotubes to femtosecond laser pulses is studied with a nonadiabatic simulation technique, which accounts for the evolution of electronic and ionic degrees of freedom, and for the coupling with the external electromagnetic field. As a direct result of electronic excitation, three coherent breathing phonon modes are excited: two radial vibrations localized in the caps and cylindrical body, and one longitudinal vibration coupled to the nanotube length. Under high absorbed energies (but below 2.9 eV/atom, the graphite's ultrafast fragmentation threshold), the resulting oscillatory motion leads to the opening of nanotube caps. Following the cap photofragmentation the nanotube body remains intact for the rest of the 2 ps simulation time.

DOI： 10.1103/PhysRevB.74.193406

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The mechanical response of a defective graphene layer to an ultrafast laser pulse is investigated through nonadiabatic molecular dynamics simulations. The defects are pentagon-heptagon pairs introduced by a single Stone-Wales transformation in the simulation cell. We found that when the fraction of excited electrons xi is below 6%, the layer exhibits strong transversal displacements in the neighborhood of the defect. The amplitude of these movements increases with the amount of energy absorbed until the threshold of xi=6% is reached. Under this condition the layer undergoes a subpicosecond inverse Stone-Wales transition, healing the defect. The absorbed energy per atom required to induce this mechanism is approximately 1.3 eV, a value that is below the laser damage thresholds for the pristine layers. The transition is lead by the electronic entropy and follows a path with strong out-of-plane contributions; it differs from the predicted path for thermally activated transitions, as calculated using standard transition state approaches. The same phenomenon is observed in defective zig-zag and armchair nanotubes. In contrast, for a defective C-60 fullerene the mechanism is hindered by the presence of edge-sharing pentagons.

DOI： 10.1103/PhysRevB.74.075409

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We present atomistic simulations of laser induced structural modifications in nanodiamonds. The method used is based on a microscopic approach for the interaction between femtosecond laser pulses and nanostructured materials. It allows us to describe the ultrafast change of bonding type produced in diamond-like nanostructures as a consequence of the laser excitation. In contrast to a thermal processing of nanodiamonds, an ultrashort laser pulse can eliminate all sp(3) bonds in the system on a sub-picosecond time scale, opening the possibility for the production of graphitic nanostructures and the creation of nanocontacts between carbon nanoclusters and other nanostructures. (c) 2005 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.commatsci.2004.09.051

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CHEMICAL SOC  

All self-assembled nanostructures, like carbon nanotubes, exhibit structural imperfections that affect their electronic and mechanical properties and constitute a serious problem for the development of novel electronic nanodevices. Very common defects in nanotubes are pentagon-heptagon pairs, in which the replacement of four hexagons by two pentagons and two heptagons disrupts the perfect hexagonal lattice. In this work, we demonstrate that these defects can be eliminated efficiently with the help of femtosecond laser pulses. By performing nonadiabatic molecular dynamics simulations, we show that in the laser-induced electronic nonequilibrium the pentagon-heptagon pair is transformed back into four hexagons without producing any irreversible damage to the rest of the nanotube.

DOI： 10.1021/nl050626t

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

In this paper we explore the use of an equation of motion decoupling method as an impurity solver to be used in conjunction with the dynamical mean field self-consistency condition for the solution of lattice models. We benchmark the impurity solver against exact diagonalization, and apply the method to study the infinite U Hubbard model, the periodic Anderson model and the pd model. This simple and numerically efficient approach yields the spectra expected for strongly correlated materials, with a quasiparticle peak and a Hubbard band. It works in a large range of parameters, and therefore can be used for the exploration of real materials using the local density approximation and dynamical mean field theory.

DOI： 10.1103/PhysRevB.71.085103

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

We present a microscopic description of the excitation of coherent phonons as a response to femtosecond laser excitation. Using molecular dynamics simulations based on a tight-binding electronic Hamiltonian we discuss two examples of laser-induced coherent phonons: (1) excitation of the E-2g1 phonon mode in graphite under high external pressure and (2) the displacive excitation of two perpendicular phonon modes in carbon nanotubes. We discuss the influence of these coherent phonons on the ablation mechanisms.

DOI： 10.1007/s00339-004-2690-2

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

The effect of high energy laser pulses on single wall carbon nanotubes (SWNT) is studied by a non-equilibrium quantum mechanical model. For the studied laser parameters, we find ablation thresholds that vary between 1.9 eV/atom and 2.3 eV/atom. For zigzag tubes a linear increase of damage thresholds as function of diameter is observed. For armchair tubes, a stability maximum is found at the (10,10) SWNT. We find that below but close to the damage threshold the nanotubes show the presence of standing waves.

DOI： 10.1007/s00339-004-2801-0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

We demonstrate the possibility of a selective nonequilibrium cap opening of carbon nanotubes as a response to femtosecond laser excitation. By performing molecular dynamics simulations based on a microscopic electronic model we show that the laser-induced ultrafast structural changes differ dramatically from the thermally induced dimer emission. Ultrafast bond weakening and simultaneous excitation of two coherent phonon modes of different frequencies, localized in the spherical caps and cylindrical nanotube body, are responsible for the selective cap opening.

DOI： 10.1103/PhysRevLett.92.117401

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Time-resolved x-ray spectroscopy at the Si L edges is used to probe the electronic structure of an amorphous Si foil as it melts following absorption of an ultrafast laser pulse. Picosecond temporal resolution allows observation of the transient liquid phase before vaporization and before the liquid breaks up into droplets. The melting causes changes in the spectrum that match predictions of molecular dynamics and ab initio x-ray absorption codes.

DOI： 10.1103/PhysRevLett.91.157403

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

We present a theoretical study of the laser-induced femtosecond melting of (1) graphite under high external pressure and (2) ultrathin graphite films under normal conditions. Our approach consists of molecular dynamic simulations performed on the basis of a time-dependent, many-body potential energy surface derived from a tight-binding Hamiltonian. Our results show that the laser-induced melting process occurs in two steps: (i) destruction of the graphite sheets via bond breaking, and (ii) merging of the melted layers. The separation of the two steps is more evident for graphite under pressure (10 GPa), but is also present in graphite films at normal pressure. The melting product is a low-density carbon phase, which remains stable under high pressure, but is unstable with an ultrashort life-time under normal pressure. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0169-4332(02)01337-5

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPRINGER-VERLAG BERLIN  

Ultrafast time-resolved x-ray absorption spectroscopy has been developed based on synchrotron radiation and a dispersive spectrometer. Liquid silicon and carbon have been studied.

DOI： 10.1007/978-3-642-59319-2_11

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

The ultrafast laser ablation of silicon has been investigated experimentally and theoretically. The theoretical description is based on molecular dynamics (MD) simulations combined with a microscopic electronic model. We determine the thresholds of melting and ablation for two different pulse durations tau = 20 and 500 fs. Experiments have been performed using 100 Ti:Sapphire laser pulses per spot in air environment. The ablation thresholds were determined for pulses with a duration of 25 and 400 fs, respectively. Good agreement is obtained between theory and experiment. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0169-4332(02)00458-0

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

A theoretical description of the ultrafast ablation of diamond and graphite is presented. Laser induced lattice deformations and melting are described with the help of molecular dynamics simulations on time dependent potential energy surfaces derived from a microscopic electronic Hamiltonian. Thermalization effects are explicitly taken into account. We calculate the ablation thresholds as a function of the pulse duration for femtosecond pulses. For both materials we obtain smoothly increasing thresholds for increasing duration. The damage and ablation mechanisms are discussed. (C) 2002 Elsevier Science B.V All rights reserved.

DOI： 10.1016/S0169-4332(02)00312-4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The ultrafast time dependence of the energy absorption of covalent solids upon excitation with femtosecond laser pulses is theoretically analyzed. We use a microscopic theory to describe laser induced structural changes and their influence on the electronic properties. We show that from the time evolution of the energy absorbed by the system important information on the electronic and atomic structure during ultrafast phase transitions can be gained. Our results reflect how structural changes affect the capability of the system to absorb external energy. (C) 2002 American Institute of Physics.

DOI： 10.1063/1.1423397

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

A theoretical study of the subpicosecond fragmentation of C-60 clusters in response to ultrafast laser pulses is presented. We simulate the laser excitation and the consequent nonequilibrium relaxation dynamics of the electronic and nuclear degrees of freedom. The first stages of the nonequilibrium dynamics are dominated by a breathing mode followed by the cold ejection of single C atoms, in contrast to the dimer emission which characterizes the thermal relaxation. We also determine the nonequilibrium damage thresholds as a function of the pulse duration. 0 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0009-2614(01)01453-1

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The physical mechanisms for damage formation in graphite films induced by femtosecond laser pulses are analyzed using a microscopic electronic theory. We describe the nonequilibrium dynamics of electrons and lattice by performing molecular dynamics simulations on time-dependent potential energy surfaces. We show that graphite has the unique property of exhibiting two distinct laser-induced structural instabilities. For high absorbed energies (&gt;3.3 eV/atom) we find nonequilibrium melting followed by fast evaporation. For low intensities above the damage threshold (&gt;2.0 eV/atom) ablation occurs via removal of intact graphite sheets.

DOI： 10.1103/PhysRevLett.87.015003

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPRINGER-VERLAG BERLIN  

The dynamics of fs-laser ablation of graphite has been investigated experimentally and theoretically. The experimental observation of two different ablation mechanisms is supported by molecular dynamics calculations, which incorporate the changes of the interatomic potentials due to electronic excitation.

DOI： 10.1007/978-3-642-56546-5_124

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER-VERLAG  

We present a theoretical study of the short-time relaxation of clusters in response to ultrafast excitations using femtosecond laser pulses. We analyze the excitation of different types of clusters (Hg-n, Ag-n, Si-n C-60 and Xe-n) and classify the relaxation dynamics in three different regimes, depending on the intensity of the exciting laser pulse.
For low-intensity pulses (I &lt; 10(12) W/cm(2)) we determine the time-dependent structural changes of clusters upon ultrashort ionization and photodetachment. We also study the laser-induced non-equilibrium fragmentation and melting of Si-n and C-60 clusters, which occurs for moderate laser intensities, as a function of the pulse duration and energy.
As an example for the case of high intensities (I &gt; 10(15) W/cm(2)), the explosion of clusters under the action of very intense ultrashort laser fields is described.

DOI： 10.1007/s003400000356

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER VERLAG  

The response of carbon to femtosecond laser pulses of arbitrary form, different durations, and different intensities is studied theoretically. We perform molecular dynamics simulations based on a microscopic electronic Hamiltonian. We include in our model the theoretical description of the pulse form, the electron thermalization, and diffusion effects explicitly. We apply our method to diamond and C-60 crystals. For the diamond case, we show that a femtosecond laser pulse induces a nonequilibrium transition to graphite, which takes place for a wide range of pulse durations and intensities. This ultrafast collective motion of the atoms occurs within a time scale shorter than 100 fs. The laser-induced melting of a C-60 crystal under pressure is also analyzed. In this case, an ultrafast melting of the system occurs. We discuss the mechanisms underlying these nonequilibrium phase transitions.

DOI： 10.1007/s003390051354

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

We present a theoretical study of ultrafast phase transitions induced by femtosecond laser pulses of arbitrary form. Molecular-dynamics simulations on time dependent potential-energy surfaces derived from a microscopic Hamiltonian are performed. Applying this method to diamond, we show that a nonequilibrium transition to graphite takes place for a wide range of laser pulse durations and intensities. This ultrafast transition (similar to 100 fs) is driven by the suppression of the diamond minimum in the potential-energy surface of the laser excited system. [S0163-1829(99)50730-3].

DOI： 10.1103/PhysRevB.60.R3701

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Combining an electronic theory with molecular-dynamics simulations we present results for the ultrafast structural changes in small clusters. We determine the time scale for the change from the linear to a triangular structure after the photodetachment process Ag-3(-) --&gt; Ag-3. We show that the time-dependent change of the ionization potential reflects in detail the internal degrees of freedom, in particular coherent and incoherent motion, and that it is sensitive to the initial temperature. We compare with experiment and point out the general significance of our results.

DOI： 10.1103/PhysRevA.54.R4601

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We study the ultrafast dynamics of Ag-3 clusters immediately after the photodetachment of Ag-3(-) by performing molecular dynamics simulations based on a microscopic electronic theory. We determine the time scale for the change from the linear (initial situation) to a triangular structure (potential minimum) of the ground state of Ag-3. Our results are in good agreement with experiment. We show how the time dependence of the internal degrees of freedom is reflected in the 'pump and probe' signal. Furthermore, we discuss the application of our results to magnetic systems and chemical reactions.

DOI： 10.1088/0953-4075/29/14/007

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