Faculty Profiles - JESCHKE HARALD OLAF

Papers - JESCHKE HARALD OLAF

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Mott correlated states in the underdoped two-dimensional Hubbard model: Variational Monte Carlo versus a dynamical cluster approximation Reviewed

Tocchio Luca F, Lee Hunpyo, Jeschke Harald O, Valenti Roser, Gros Claudius

PHYSICAL REVIEW B 87 ( 4 ) 2013.1

Fermi Surface Topology of LaFePO and LiFeP Reviewed

Johannes Ferber, Harald O. Jeschke, Roser Valenti

PHYSICAL REVIEW LETTERS 109 ( 23 ) 2012.12

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

Ab initio study of the two-dimensional metallic state at the surface of SrTiO3: Importance of oxygen vacancies Reviewed

Juan Shen, Hunpyo Lee, Roser Valenti, Harald O. Jeschke

PHYSICAL REVIEW B 86 ( 19 ) 2012.11

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

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

Simulation of structural and electronic properties of amorphous tungsten oxycarbides Reviewed

Kaliappan Muthukumar, Roser Valenti, Harald O. Jeschke

NEW JOURNAL OF PHYSICS 14 2012.11

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

Na2IrO3 as a Molecular Orbital Crystal Reviewed

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

PHYSICAL REVIEW LETTERS 109 ( 19 ) 2012.11

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

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

Momentum spectrometer for electron-electron coincidence studies on superconductors Reviewed

Robert Wallauer, Stefan Voss, Lutz Foucar, Tobias Bauer, Deborah Schneider, Jasmin Titze, Birte Ulrich, Katharina Kreidi, Nadine Neumann, Tilo Havermeier, Markus Schoeffler, Till Jahnke, Achim Czasch, Lothar Schmidt, Amit Kanigel, Juan Carlos Campuzano, Harald Jeschke, Roser Valenti, Andreas Mueller, Goetz Berner, Michael Sing, Ralph Claessen, Horst Schmidt-Boecking, Reinhard Doerner

REVIEW OF SCIENTIFIC INSTRUMENTS 83 ( 10 ) 2012.10

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

Multiferroic FeTe 2O 5Br: Alternating spin chains with frustrated interchain interactions Reviewed

M. Pregelj, H. O. Jeschke, H. Feldner, R. Valentí, A. Honecker, T. Saha-Dasgupta, H. Das, S. Yoshii, T. Morioka, H. Nojiri, H. Berger, A. Zorko, O. Zaharko, D. Arčon

Physical Review B - Condensed Matter and Materials Physics 86 ( 5 ) 2012.8

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

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

Multiferroic FeTe2O5Br: Alternating spin chains with frustrated interchain interactions Reviewed

M. Pregelj, H. O. Jeschke, H. Feldner, R. Valenti, A. Honecker, T. Saha-Dasgupta, H. Das, S. Yoshii, T. Morioka, H. Nojiri, H. Berger, A. Zorko, O. Zaharko, D. Arcon

PHYSICAL REVIEW B 86 ( 5 ) 2012.8

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

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

Physisorption of an organometallic platinum complex on silica: an ab initio study Reviewed

Juan Shen, Kaliappan Muthukumar, Harald O. Jeschke, Roser Valenti

NEW JOURNAL OF PHYSICS 14 2012.7

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

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

Spontaneous dissociation of Co-2(CO)(8) and autocatalytic growth of Co on SiO2: A combined experimental and theoretical investigation Reviewed

Kaliappan Muthukumar, Harald O. Jeschke, Roser Valenti, Evgeniya Begun, Johannes Schwenk, Fabrizio Porrati, Michael Huth

BEILSTEIN JOURNAL OF NANOTECHNOLOGY 3 546 - 555 2012.7

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

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

Dynamical cluster approximation within an augmented plane wave framework: Spectral properties of SrVO3 Reviewed

Hunpyo Lee, Kateryna Foyevtsova, Johannes Ferber, Markus Aichhorn, Harald O. Jeschke, Roser Valenti

PHYSICAL REVIEW B 85 ( 16 ) 2012.4

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

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

Uniaxial versus hydrostatic pressure-induced phase transitions in CaFe2As2 and BaFe2As2 Reviewed

Milan Tomic, Roser Valenti, Harald O. Jeschke

PHYSICAL REVIEW B 85 ( 9 ) 2012.3

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

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

Orbital-Resolved Partial Charge Transfer from the Methoxy Groups of Substituted Pyrenes in Complexes with Tetracyanoquinodimethane-A NEXAFS Study Reviewed

Katerina Medjanik, Dennis Chercka, Peter Nagel, Michael Merz, Stefan Schuppler, Martin Baumgarten, Klaus Muellen, Sergej A. Nepijko, Hans-Joachim Elmers, Gerd Schoenhense, Harald O. Jeschke, Roser Valenti

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 134 ( 10 ) 4694 - 4699 2012.3

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

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

LDA plus DMFT study of the effects of correlation in LiFeAs Reviewed

Johannes Ferber, Kateryna Foyevtsova, Roser Valenti, Harald O. Jeschke

PHYSICAL REVIEW B 85 ( 9 ) 2012.3

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

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

General mechanism for orbital selective phase transitions Reviewed

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

PHYSICAL REVIEW B 85 ( 3 ) 2012.1

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

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

Temperature dependence of structural and electronic properties of the spin-liquid candidate kappa-(BEDT-TTF)(2)Cu-2(CN)(3) Reviewed

Harald O. Jeschke, Mariano de Souza, Roser Valenti, Rudra Sekhar Manna, Michael Lang, John A. Schlueter

PHYSICAL REVIEW B 85 ( 3 ) 2012.1

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

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

Dynamical cluster approximation within an augmented plane wave framework: Spectral properties of SrVO <inf>3</inf>

Lee, H., Foyevtsova, K., Ferber, J., Aichhorn, M., Jeschke, H.O., Valentí, R.

Physical Review B - Condensed Matter and Materials Physics 85 ( 16 ) 2012

Interaction of W(CO)(6) with SiO2 surfaces: A density functional study Reviewed

Kaliappan Muthukumar, Ingo Opahle, Juan Shen, Harald O. Jeschke, Roser Valenti

PHYSICAL REVIEW B 84 ( 20 ) 2011.11

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

Anisotropic two-orbital Hubbard model: Single-site versus cluster Dynamical Mean-Field Theory Reviewed

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

ANNALEN DER PHYSIK 523 ( 8-9 ) 689 - 697 2011.8

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

Orbital-selective phase transition induced by different magnetic states: A dynamical cluster approximation study Reviewed

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

PHYSICAL REVIEW B 84 ( 2 ) 2011.7

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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