Faculty Profiles - JESCHKE HARALD OLAF

Papers - JESCHKE HARALD OLAF

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Effects of Lifshitz Transitions on Charge Transport in Magnetic Phases of Fe-Based Superconductors (vol 114, 097003, 2015)

Y. Wang, Maria N. Gastiasoro, Brian M. Andersen, M. Tomic, Harald O. Jeschke, Roser Valenti, Indranil Paul, P. J. Hirschfeld

PHYSICAL REVIEW LETTERS 117 ( 25 ) 2016.12

Hubbard band versus oxygen vacancy states in the correlated electron metal SrVO3 Reviewed

S. Backes, T. C. Rodel, F. Fortuna, E. Frantzeskakis, P. Le Fevre, F. Bertran, M. Kobayashi, R. Yukawa, T. Mitsuhashi, M. Kitamura, K. Horiba, H. Kumigashira, R. Saint-Martin, A. Fouchet, B. Berini, Y. Dumont, A. J. Kim, F. Lechermann, H. O. Jeschke, M. J. Rozenberg, R. Valenti, A. F. Santander-Syro

PHYSICAL REVIEW B 94 ( 24 ) 2016.12

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

Pressure-Induced Conductivity in a Neutral Nonplanar Spin-Localized Radical Reviewed

Manuel Souto, HengBo Cui, Miriam Pena-Alvarez, Valentin G. Baonza, Harald O. Jeschke, Milan Tomic, Roser Valenti, Davide Blasi, Imma Ratera, Concepcio Rovira, Jaume Veciana

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 138 ( 36 ) 11517 - 11525 2016.9

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

Reduction of magnetic interlayer coupling in barlowite through isoelectronic substitution Reviewed

Daniel Guterding, Roser Valenti, Harald O. Jeschke

PHYSICAL REVIEW B 94 ( 12 ) 2016.9

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

Ab initio determination of spin Hamiltonians with anisotropic exchange interactions: The case of the pyrochlore ferromagnet Lu2V2O7 Reviewed

Kira Riedl, Daniel Guterding, Harald O. Jeschke, Michel J. P. Gingras, Roser Valenti

PHYSICAL REVIEW B 94 ( 1 ) 2016.7

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

Near-degeneracy of extended s + d(x)(-y)(2)(2) and d(xy) order parameters in quasi-two-dimensional organic superconductors Reviewed

Daniel Guterding, Michaela Altmeyer, Harald O. Jeschke, Roser Valenti

PHYSICAL REVIEW B 94 ( 2 ) 2016.7

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

Microscopic origin of the charge transfer in single crystals based on thiophene derivatives: A combined NEXAFS and density functional theory approach Reviewed

A. Chernenkaya, A. Morherr, S. Backes, W. Popp, S. Witt, X. Kozina, S. A. Nepijko, M. Bolte, K. Medjanik, G. Ohrwall, C. Krellner, M. Baumgarten, H. J. Elmers, G. Schoenhense, H. O. Jeschke, R. Valent

JOURNAL OF CHEMICAL PHYSICS 145 ( 3 ) 2016.7

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

Challenges in design of Kitaev materials: Magnetic interactions from competing energy scales Reviewed

Stephen M. Winter, Ying Li, Harald O. Jeschke, Roser Valenti

PHYSICAL REVIEW B 93 ( 21 ) 2016.6

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

Evidence for Eight-Node Mixed-Symmetry Superconductivity in a Correlated Organic Metal Reviewed

Daniel Guterding, Sandra Diehl, Michaela Altmeyer, Torsten Methfessel, Ulrich Tutsch, Harald Schubert, Michael Lang, Jens Mueller, Michael Huth, Harald O. Jeschke, Roser Valenti, Martin Jourdan, Hans-Joachim Elmers

PHYSICAL REVIEW LETTERS 116 ( 23 ) 7001 - 7001 2016.6

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

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

Hydrostatic pressure response of an oxide-based two-dimensional electron system Reviewed

J. Zabaleta, V. S. Borisov, R. Wanke, H. O. Jeschke, S. C. Parks, B. Baum, A. Teker, T. Harada, K. Syassen, T. Kopp, N. Pavlenko, R. Valenti, J. Mannhart

PHYSICAL REVIEW B 93 ( 23 ) 5117 - 5117 2016.6

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

Competition between disorder and Coulomb interaction in a two-dimensional plaquette Hubbard model Reviewed

Hunpyo Lee, Harald O. Jeschke, Roser Valenti

PHYSICAL REVIEW B 93 ( 22 ) 2016.6

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

Stabilization of the tetragonal structure in (Ba1-xSrx)CuSi2O6 Reviewed

Pascal Puphal, Denis Sheptyakov, Natalija van Well, Lars Postulka, Ivo Heinmaa, Franz Ritter, Wolf Assmus, Bernd Wolf, Michael Lang, Harald O. Jeschke, Roser Valenti, Raivo Stern, Christian Rueegg, Cornelius Krellner

PHYSICAL REVIEW B 93 ( 17 ) 4121 - 4121 2016.5

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

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

Prospect of quantum anomalous Hall and quantum spin Hall effect in doped kagome lattice Mott insulators Reviewed

Daniel Guterding, Harald O. Jeschke, Roser Valenti

SCIENTIFIC REPORTS 6 2016.5

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

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

Magnetism, Spin Texture, and In-Gap States: Atomic Specialization at the Surface of Oxygen-Deficient SrTiO3 Reviewed

Michaela Altmeyer, Harald O. Jeschke, Oliver Hijano-Cubelos, Cyril Martins, Frank Lechermann, Klaus Koepernik, Andres F. Santander-Syro, Marcelo J. Rozenberg, Roser Valenti, Marc Gabay

PHYSICAL REVIEW LETTERS 116 ( 15 ) 7203 - 7203 2016.4

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

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

Electron dichotomy on the SrTiO3 defect surface augmented by many-body effects Reviewed

Frank Lechermann, Harald O. Jeschke, Aaram J. Kim, Steffen Backes, Roser Valenti

PHYSICAL REVIEW B 93 ( 12 ) 2016.3

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

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

Monoclinic crystal structure of alpha-RuCl3 and the zigzag antiferromagnetic ground state Reviewed

R. D. Johnson, S. C. Williams, A. A. Haghighirad, J. Singleton, V. Zapf, P. Manuel, I. I. Mazin, Y. Li, H. O. Jeschke, R. Valenti, R. Coldea

PHYSICAL REVIEW B 92 ( 23 ) 2015.12

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

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

Paramagnetism in the kagome compounds (Zn,Mg,Cd)Cu3(OH)6Cl2 Reviewed

Yasir Iqbal, Harald O. Jeschke, Johannes Reuther, Roser Valenti, I. I. Mazin, Martin Greiter, Ronny Thomale

PHYSICAL REVIEW B 92 ( 22 ) 2015.12

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

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

Effect of magnetic frustration on nematicity and superconductivity in iron chalcogenides Reviewed

J. K. Glasbrenner, I. I. Mazin, Harald O. Jeschke, P. J. Hirschfeld, R. M. Fernandes, Roser Valenti

NATURE PHYSICS 11 ( 11 ) 953 - 958 2015.11

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

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

Microscopic nature of correlations in multiorbital AFe(2)As(2) (A = K, Rb, Cs): Hund's coupling versus Coulomb repulsion Reviewed

Steffen Backes, Harald O. Jeschke, Roser Valenti

PHYSICAL REVIEW B 92 ( 19 ) 2015.11

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

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

Zigzag antiferromagnetic quantum ground state in monoclinic honeycomb lattice antimonates A(3)Ni(2)SbO(6) (A = Li, Na) Reviewed

E. A. Zvereva, M. I. Stratan, Y. A. Ovchenkov, V. B. Nalbandyan, J. -Y. Lin, E. L. Vavilova, M. F. Iakovleva, M. Abdel-Hafiez, A. V. Silhanek, X. -J. Chen, A. Stroppa, S. Picozzi, H. O. Jeschke, R. Valenti, A. N. Vasiliev

PHYSICAL REVIEW B 92 ( 14 ) 2015.10

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

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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