Faculty Profiles - JESCHKE HARALD OLAF

Papers - JESCHKE HARALD OLAF

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Field-induced magnetic transitions in Ca-10(Pt3As8)((Fe1-xPtx)(2)As-2)(5) compounds Reviewed

M. D. Watson, A. McCollam, S. F. Blake, D. Vignolles, L. Drigo, I. I. Mazin, D. Guterding, H. O. Jeschke, R. Valenti, N. Ni, R. Cava, A. I. Coldea

PHYSICAL REVIEW B 89 ( 20 ) 2014.5

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

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

Giant pressure-induced volume collapse in the pyrite mineral MnS2 Reviewed

Simon A. J. Kimber, Ashkan Salamat, Shaun R. Evans, Harald O. Jeschke, Kaliappan Muthukumar, Milan Tomic, Francesc Salvat-Pujol, Roser Valenti, Maria V. Kaisheva, Ivo Zizak, Tapan Chatterji

PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA 111 ( 14 ) 5106 - 5110 2014.4

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

Valence bond liquid phase in the honeycomb lattice material Li2RuO3 Reviewed

Simon A. J. Kimber, I. I. Mazin, Juan Shen, Harald O. Jeschke, Sergey V. Streltsov, Dimitri N. Argyriou, Roser Valenti, Daniel I. Khomskii

PHYSICAL REVIEW B 89 ( 8 ) 2014.2

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

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

Competition between band and Mott insulators in the bilayer Hubbard model: A dynamical cluster approximation study Reviewed

Hunpyo Lee, Yu-Zhong Zhang, Harald O. Jeschke, Roser Valenti

PHYSICAL REVIEW B 89 ( 3 ) 2014.1

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

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

Field-induced magnetic transitions in Ca10(Pt3As8)((Fe1 xPtx)2As2)5 compounds

Watson, M.D., McCollam, A., Blake, S.F., Vignolles, D., Drigo, L., Mazin, I.I., Guterding, D., Jeschke, H.O., Valentí, R., Ni, N., Cava, R., Coldea, A.I.

Physical Review B - Condensed Matter and Materials Physics 89 ( 20 ) 2014

Nonthermal graphitization of diamond induced by a femtosecond x-ray laser pulse Reviewed

Medvedev N, Jeschke H. O, Ziaja B

PHYSICAL REVIEW B 88 ( 22 ) 2013.12

Simulation of electron transport during electron-beam-induced deposition of nanostructures Reviewed

Francesc Salvat-Pujol, Harald O. Jeschke, Roser Valenti

BEILSTEIN JOURNAL OF NANOTECHNOLOGY 4 781 - 792 2013.11

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

Semiclassical approximation solved by Monte Carlo integration as an efficient impurity solver for dynamical mean field theory and its cluster extensions Reviewed

Hunpyo Lee, Yu-Zhong Zhang, Hoonkyung Lee, Yongkyung Kwon, Harald O. Jeschke, Roser Valenti

PHYSICAL REVIEW B 88 ( 16 ) 2013.10

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

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

Importance of anisotropy in the spin-liquid candidate Me3EtSb[Pd(dmit)(2)](2) Reviewed

A. C. Jacko, Luca F. Tocchio, Harald O. Jeschke, Roser Valenti

PHYSICAL REVIEW B 88 ( 15 ) 2013.10

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

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

Finite temperature and pressure molecular dynamics for BaFe2As2 Reviewed

Steffen Backes, Harald O. Jeschke

PHYSICAL REVIEW B 88 ( 7 ) 2013.8

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

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

Photon energy dependence of graphitization threshold for diamond irradiated with an intense XUV FEL pulse Reviewed

J. Gaudin, N. Medvedev, J. Chalupsky, T. Burian, S. Dastjani-Farahani, V. Hajkova, M. Harmand, H. O. Jeschke, L. Juha, M. Jurek, D. Klinger, J. Krzywinski, R. A. Loch, S. Moeller, M. Nagasono, C. Ozkan, K. Saksl, H. Sinn, R. Sobierajski, P. Sovak, S. Toleikis, K. Tiedtke, M. Toufarova, T. Tschentscher, V. Vorlicek, L. Vysin, H. Wabnitz, B. Ziaja

PHYSICAL REVIEW B 88 ( 6 ) 2013.8

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

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

ORBITAL SELECTIVE PHASE TRANSITION Reviewed

Yao Yao, Yu-Zhong Zhang, Hunpyo Lee, Harald O. Jeschke, Roser Valenti, Hai-Qing Lin, Chang-Qin Wu

MODERN PHYSICS LETTERS B 27 ( 20 ) 2013.8

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

First-principles determination of Heisenberg Hamiltonian parameters for the spin-1/2 kagome antiferromagnet ZnCu3(OH)(6)Cl-2 Reviewed

Harald O. Jeschke, Francesc Salvat-Pujol, Roser Valenti

PHYSICAL REVIEW B 88 ( 7 ) 2013.8

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

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

Ab initio analysis of the tight-binding parameters and magnetic interactions in Na2IrO3 Reviewed

Kateryna Foyevtsova, Harald O. Jeschke, I. I. Mazin, D. I. Khomskii, Roser Valenti

PHYSICAL REVIEW B 88 ( 3 ) 2013.7

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

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

Origin of the insulating state in honeycomb iridates and rhodates Reviewed

I. I. Mazin, S. Manni, K. Foyevtsova, Harald O. Jeschke, P. Gegenwart, Roser Valenti

PHYSICAL REVIEW B 88 ( 3 ) 2013.7

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

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

Why MgFeGe is not a superconductor Reviewed

Jeschke Harald O, Mazin I. I, Valenti Roser

PHYSICAL REVIEW B 87 ( 24 ) 2013.6

In-plane uniaxial stress effects on the structural and electronic properties of BaFe2As2 and CaFe2As2 Reviewed

Milan Tomic, Harald O. Jeschke, Rafael M. Fernandes, Roser Valenti

PHYSICAL REVIEW B 87 ( 17 ) 2013.5

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

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

Absence of Metallicity in K-doped Picene: Importance of Electronic Correlations Reviewed

Andreas Ruff, Michael Sing, Ralph Claessen, Hunpyo Lee, Milan Tomic, Harald O. Jeschke, Roser Valenti

PHYSICAL REVIEW LETTERS 110 ( 21 ) 2013.5

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

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

Electronic properties of Fabre charge-transfer salts under various temperature and pressure conditions Reviewed

Jacko A. C, Feldner H, Rose E, Lissner F, Dressel M, Valenti Roser, Jeschke Harald O

PHYSICAL REVIEW B 87 ( 15 ) 2013.4

Nonthermal phase transitions in semiconductors induced by a femtosecond extreme ultraviolet laser pulse Reviewed

Medvedev Nikita, Jeschke Harald O, Ziaja Beata

NEW JOURNAL OF PHYSICS 15 2013.1

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