Faculty Profiles - JESCHKE HARALD OLAF

Papers - JESCHKE HARALD OLAF

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Electronic structure studies of BaFe2As2 by angle-resolved photoemission spectroscopy Reviewed

J. Fink, S. Thirupathaiah, R. Ovsyannikov, H. A. Duerr, R. Follath, Y. Huang, S. de Jong, M. S. Golden, Yu-Zhong Zhang, H. O. Jeschke, R. Valenti, C. Felser, S. Dastjani Farahani, M. Rotter, D. Johrendt

PHYSICAL REVIEW B 79 ( 15 ) 2009.4

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

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

DOI： 10.1103/PhysRevB.79.155118

First principles perspective on the microscopic model for Cs(2)CuCl(4) and Cs(2)CuBr(4) Reviewed

K. Foyevtsova, Y. Zhang, H. O. Jeschke, R. Valenti

HIGHLY FRUSTRATED MAGNETISM 2008 (HFM 2008) 145 2009

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

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

DOI： 10.1088/1742-6596/145/1/012038

Microscopic model for transitions from Mott to spin-Peierls insulator in TiOCl Reviewed

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

PHYSICAL REVIEW B 78 ( 20 ) 2008.11

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

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

DOI： 10.1103/PhysRevB.78.205104

Two pressure-induced transitions in TiOCl: Mott insulator to anisotropic metal Reviewed

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

PHYSICAL REVIEW LETTERS 101 ( 13 ) 2008.9

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

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

DOI： 10.1103/PhysRevLett.101.136406

Microscopic modeling of a spin crossover transition Reviewed

Harald O. Jeschke, L. Andrea Salguero, Badiur Rahaman, Christian Buchsbaum, Volodymyr Pashchenko, Martin U. Schmidt, Tanusri Saha-Dasgupta, Roser Valenti

NEW JOURNAL OF PHYSICS 9 2007.12

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

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

DOI： 10.1088/1367-2630/9/12/448

Comparative investigation of the coupled-tetrahedra quantum spin systems Cu2Te2O5X2, X = Cl, Br and Cu4Te5O12Cl4 Reviewed

R. Valenti, T. Saha-Dasgupta, H. O. Jeschke, B. Rahaman, H. Rosner, P. Lemmens, R. Takagi, M. Johnsson

PHYSICA C-SUPERCONDUCTIVITY AND ITS APPLICATIONS 460 ( SPEC. ISS. ) 462 - 463 2007.9

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

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

DOI： 10.1016/j.physc.2007.03.255

Microscopic investigation of laser-induced structural changes in single-wall carbon nanotubes Reviewed

Harald O. Jeschke, Aldo H. Romero, Martin E. Garcia, Angel Rubio

PHYSICAL REVIEW B 75 ( 12 ) 2007.3

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

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

DOI： 10.1103/PhysRevB.75.125412

Cu-based metalorganic systems: an ab initio study of the electronic structure Reviewed

L. Andrea Salguero, Harald O. Jeschke, Badiur Rahaman, Tanusri Saha-Dasgupta, Christian Buchsbaum, Martin U. Schmidt, Roser Valenti

NEW JOURNAL OF PHYSICS 9 2007.2

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

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

DOI： 10.1088/1367-2630/9/2/026

Microscopic model for the frustrated CuII-spin tetrahedron-based Cu4Te5O12X4 (X = Cl, Br) systems Reviewed

Badiur Rahaman, Harald O. Jeschke, Roser Valenti, T. Saha-Dasgupta

PHYSICAL REVIEW B 75 ( 2 ) 2007.1

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

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

DOI： 10.1103/PhysRevB.75.024404

Classical and ab initio preparation of reliable structures for polymeric coordination compounds Reviewed

Harald O. Jeschke, L. Andrea Salguero, Roser Valenti, Christian Buchsbaum, Martin U. Schmidt, Matthias Wagner

COMPTES RENDUS CHIMIE 10 ( 1-2 ) 82 - 88 2007.1

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

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

DOI： 10.1016/j.crci.2006.06.007

Breathing coherent phonons and caps fragmentation in carbon nanotubes following ultrafast laser pulses Reviewed

Traian Dumitrica, Martin E. Garcia, Harald O. Jeschke, Boris I. Yakobson

PHYSICAL REVIEW B 74 ( 19 ) 2006.11

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

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

DOI： 10.1103/PhysRevB.74.193406

Large-amplitude coherent phonons and inverse Stone-Wales transitions in graphitic systems with defects interacting with ultrashort laser pulses Reviewed

Felipe Valencia, Aldo H. Romero, Harald O. Jeschke, Martin E. Garcia

PHYSICAL REVIEW B 74 ( 7 ) 2006.8

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

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

DOI： 10.1103/PhysRevB.74.075409

Laser manipulation of nanodiamonds Reviewed

AH Romero, HO Jeschke, ME Garcia

COMPUTATIONAL MATERIALS SCIENCE 35 ( 3 ) 179 - 182 2006.3

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

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

DOI： 10.1016/j.commatsci.2004.09.051

Femtosecond laser nanosurgery of defects in carbon nanotubes Reviewed

AH Romero, ME Garcia, F Valencia, H Terrones, M Terrones, HO Jeschke

NANO LETTERS 5 ( 7 ) 1361 - 1365 2005.7

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

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

DOI： 10.1021/nl050626t

Decoupling method for dynamical mean-field theory calculations Reviewed

HO Jeschke, G Kotliar

PHYSICAL REVIEW B 71 ( 8 ) 2005.2

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

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

DOI： 10.1103/PhysRevB.71.085103

Laser-induced coherent phonons in graphite and carbon nanotubes: model and simulations Reviewed

ME Garcia, T Dumitrica, HO Jeschke

APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING 79 ( 4-6 ) 855 - 857 2004.9

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

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

DOI： 10.1007/s00339-004-2690-2

Atomistic simulation of the laser induced damage in single wall carbon nanotubes: Diameter and chirality dependence Reviewed

AH Romero, HO Jeschke, A Rubio, ME Garcia

APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING 79 ( 4-6 ) 899 - 901 2004.9

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

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

DOI： 10.1007/s00339-004-2801-0

Selective cap opening in carbon nanotubes driven by laser-induced coherent phonons Reviewed

T Dumitrica, ME Garcia, HO Jeschke, BI Yakobson

PHYSICAL REVIEW LETTERS 92 ( 11 ) 2004.3

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

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

DOI： 10.1103/PhysRevLett.92.117401

Properties of liquid silicon observed by time-resolved x-ray absorption spectroscopy Reviewed

SL Johnson, PA Heimann, AM Lindenberg, HO Jeschke, ME Garcia, Z Chang, RW Lee, JJ Rehr, RW Falcone

PHYSICAL REVIEW LETTERS 91 ( 15 ) 2003.10

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

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

DOI： 10.1103/PhysRevLett.91.157403

Theoretical approach to the laser-induced melting of graphite under different pressure conditions Reviewed

ME Garcia, HO Jeschke

APPLIED SURFACE SCIENCE 208 ( 1 ) 61 - 70 2003.3

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

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

DOI： 10.1016/S0169-4332(02)01337-5

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