Faculty Profiles - JESCHKE HARALD OLAF

Papers - JESCHKE HARALD OLAF

Division display 141 - 160 of about 192 ／ All the affair displays >>

Importance of itinerancy and quantum fluctuations for the magnetism in iron pnictides Reviewed

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

JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 72 ( 5 ) 324 - 328 2011.5

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

Multistep Approach to Microscopic Models for Frustrated Quantum Magnets: The Case of the Natural Mineral Azurite Reviewed

Harald Jeschke, Ingo Opahle, Hem Kandpal, Roser Valenti, Hena Das, Tanusri Saha-Dasgupta, Oleg Janson, Helge Rosner, Andreas Bruehl, Bernd Wolf, Michael Lang, Johannes Richter, Shijie Hu, Xiaoqun Wang, Robert Peters, Thomas Pruschke, Andreas Honecker

PHYSICAL REVIEW LETTERS 106 ( 21 ) 2011.5

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

Determination of effective microscopic models for the frustrated antiferromagnets Cs2CuCl4 and Cs2CuBr4 by density functional methods Reviewed

Kateryna Foyevtsova, Ingo Opahle, Yu-Zhong Zhang, Harald O. Jeschke, Roser Valenti

PHYSICAL REVIEW B 83 ( 12 ) 2011.3

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

Distinct magnetic regimes through site-selective atom substitution in the frustrated quantum antiferromagnet Cs2CuCl4-xBrx Reviewed

P. T. Cong, B. Wolf, M. de Souza, N. Krueger, A. A. Haghighirad, S. Gottlieb-Schoenmeyer, F. Ritter, W. Assmus, I. Opahle, K. Foyevtsova, H. O. Jeschke, R. Valenti, L. Wiehl, M. Lang

PHYSICAL REVIEW B 83 ( 6 ) 2011.2

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 <= x <= 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

Thermally induced crystal-to-crystal transformations accompanied by changes in the magnetic properties of a Cu-II-p-hydroquinonate polymer Reviewed

Ngoc Hien Phan, Ivan Halasz, Ingo Opahle, Edith Alig, Lothar Fink, Jan W. Bats, Pham Thanh Cong, Hans-Wolfram Lerner, Biprajit Sarkar, Bernd Wolf, Harald O. Jeschke, Michael Lang, Roser Valenti, Robert Dinnebier, Matthias Wagner

CRYSTENGCOMM 13 ( 2 ) 391 - 395 2011

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

Analysis of spin-density wave conductivity spectra of iron pnictides in the framework of density functional theory Reviewed

Johannes Ferber, Yu-Zhong Zhang, Harald O. Jeschke, Roser Valenti

PHYSICAL REVIEW B 82 ( 16 ) 2010.10

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

Local moments and symmetry breaking in metallic PrMnSbO Reviewed

Simon A. J. Kimber, Adrian H. Hill, Yu-Zhong Zhang, Harald O. Jeschke, Roser Valenti, Clemens Ritter, Inga Schellenberg, Wilfred Hermes, Rainer Poettgen, Dimitri N. Argyriou

PHYSICAL REVIEW B 82 ( 10 ) 2010.9

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

Modulation of pairing interaction in Bi2Sr2CaCu2O8+delta by an O dopant: A density functional theory study Reviewed

Kateryna Foyevtsova, H. C. Kandpal, Harald O. Jeschke, S. Graser, H. -P. Cheng, Roser Valenti, P. J. Hirschfeld

PHYSICAL REVIEW B 82 ( 5 ) 2010.8

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

First principles determination of the model parameters in kappa-(ET)(2)Cu-2(CN)(3) Reviewed

Harald O. Jeschke, Hem C. Kandpal, Ingo Opahle, Yu-Zhong Zhang, Roser Valenti

PHYSICA B-CONDENSED MATTER 405 ( 11 ) S224 - S228 2010.6

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

Possible origin of the reduced ordered magnetic moment in iron pnictides: A dynamical mean-field theory study Reviewed

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

PHYSICAL REVIEW B 81 ( 22 ) 2010.6

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

Can the Mott Insulator TiOCl be Metallized by Doping? A First-Principles Study Reviewed

Yu-Zhong Zhang, Kateryna Foyevtsova, Harald O. Jeschke, Martin U. Schmidt, Roser Valenti

PHYSICAL REVIEW LETTERS 104 ( 14 ) 2010.4

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

Pressure-driven phase transitions in TiOCl and the family (Ca, Sr, Ba)Fe2As2 Reviewed

Yu-Zhong Zhang, Ingo Opahle, Harald O. Jeschke, Roser Valenti

JOURNAL OF PHYSICS-CONDENSED MATTER 22 ( 16 ) 2010.4

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

Orbital character variation of the Fermi surface and doping dependent changes of the dimensionality in BaFe2-xCoxAs2 from angle-resolved photoemission spectroscopy Reviewed

S. Thirupathaiah, S. de Jong, R. Ovsyannikov, H. A. Duerr, A. Varykhalov, R. Follath, Y. Huang, R. Huisman, M. S. Golden, Yu-Zhong Zhang, H. O. Jeschke, R. Valenti, A. Erb, A. Gloskovskii, J. Fink

PHYSICAL REVIEW B 81 ( 10 ) 2010.3

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

Itinerant nature of magnetism in iron pnictides: A first-principles study Reviewed

Yu-Zhong Zhang, Ingo Opahle, Harald O. Jeschke, Roser Valenti

PHYSICAL REVIEW B 81 ( 9 ) 2010.3

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

Dynamical Cluster Approximation Study of the Anisotropic Two-Orbital Hubbard Model Reviewed

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

PHYSICAL REVIEW LETTERS 104 ( 2 ) 2010.1

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

Microscopic origin of pressure-induced phase transitions in the iron pnictide superconductors AFe(2)As(2): An ab initio molecular dynamics study Reviewed

Yu-Zhong Zhang, Hem C. Kandpal, Ingo Opahle, Harald O. Jeschke, Roser Valenti

PHYSICAL REVIEW B 80 ( 9 ) 2009.9

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

Revision of Model Parameters for kappa-Type Charge Transfer Salts: An Ab Initio Study Reviewed

Hem C. Kandpal, Ingo Opahle, Yu-Zhong Zhang, Harald O. Jeschke, Roser Valenti

PHYSICAL REVIEW LETTERS 103 ( 6 ) 2009.8

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' description are discussed.

DOI： 10.1103/PhysRevLett.103.067004

Molecular dynamics simulations of laser-induced damage of nanostructures and solids Reviewed

Harald O. Jeschke, Momar S. Diakhate, Martin E. Garcia

APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING 96 ( 1 ) 33 - 42 2009.7

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

Fast impurity solver based on equations of motion and decoupling Reviewed

Qingguo Feng, Yu-Zhong Zhang, Harald O. Jeschke

PHYSICAL REVIEW B 79 ( 23 ) 2009.6

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

Similarities between structural distortions under pressure and chemical doping in superconducting BaFe2As2 Reviewed

Simon A. J. Kimber, Andreas Kreyssig, Yu-Zhong Zhang, Harald O. Jeschke, Roser Valenti, Fabiano Yokaichiya, Estelle Colombier, Jiaqiang Yan, Thomas C. Hansen, Tapan Chatterji, Robert J. McQueeney, Paul C. Canfield, Alan I. Goldman, Dimitri N. Argyriou

NATURE MATERIALS 8 ( 6 ) 471 - 475 2009.6

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

　 PREV - NEXT 　

1
2
3
4
5
6
7
8
9
10