Publications (79)282.12 Total impact
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ABSTRACT: We consider a mixture of interacting bosons and fermions in optical lattices described by the Bose–Fermi Hubbard Hamiltonian. To treat bosonic degrees of freedom, we use a generalized dynamical mean field theory (GDMFT). By combining the GDMFT with the numerical renormalization group method, we revisit the zerotemperature phase diagram with particular emphasis on manybody effects in a supersolid state and discuss the origin of an anomalous peak structure emerging in the density of states for fermions.  [Show abstract] [Hide abstract]
ABSTRACT: We study ultracold fermionic atoms trapped in a three dimensional optical lattice by combining the realspace dynamical meanfield approach with continuoustime quantum Monte Carlo simulations. For a spinunpolarized system we show results the density and pair potential profile in the trap for a range of temperatures. We discuss how a polarized superfluid state is spatially realized in the spinpolarized system with harmonic confinement at low temperatures and present the local particle density, local magnetization, and pair potential.  [Show abstract] [Hide abstract]
ABSTRACT: We study the effects of the exchange interaction between an adsorbed magnetic atom with easyaxis magnetic anisotropy and the conductionband electrons from the substrate. We model the system using an anisotropic Kondo model and we compute the impurity spectral function, which is related to the differential conductance (dI/dV) spectra measured using a scanning tunneling microscope. To make contact with the known experimental results for iron atoms on the CuN/Cu(100) surface (Hirjibehedin et al 2007 Science 317 1199), we calculated the spectral functions in the presence of an external magnetic field of varying strength applied along all three spatial directions. It is possible to establish an upper bound on the coupling constant J: in the range of the magnetic fields for which the experimental results are currently known (up to 7 T), the lowenergy features in the calculated spectra agree well with the measured dI/dV spectra if the exchange coupling constant J is at most half as large as that for cobalt atoms on the same surface. We show that for an even higher magnetic field (between 8 and 9 T) applied along the 'hollow direction', the impurity energy states cross, giving rise to a Kondo effect which takes the form of a zerobias resonance. The coupling strength J could be determined experimentally by performing tunneling spectroscopy in this range of magnetic fields. On the technical side, the paper introduces an approach for calculating the expectation values of global spin operators and all the components of the impurity magnetic susceptibility tensor (including the outofdiagonal ones) in numerical renormalization group (NRG) calculations with no spin symmetry. An appendix contains a density functional theory (DFT) study of the Co and Fe adsorbates on the CuN/Cu(100) surface: we compare magnetic moments, as well as orbital energies, occupancies, centers and spreads, by calculating the maximally localized Wannier orbitals of the adsorbates.  [Show abstract] [Hide abstract]
ABSTRACT: We present an extension of the concepts of the functional renormalization group approach to quantum manybody problems in nonequilibrium situations. The approach is completely general and allows calculations for both stationary and timedependent situations. As a specific example we study the stationary state transport through a quantum dot with local Coulomb correlations. We discuss the influence of finite bias voltage and temperature on the current and conductance.  [Show abstract] [Hide abstract]
ABSTRACT: Using inhomogeneous dynamical meanfield theory, we show that the normalmetal proximity effect could force any finite number of Mottinsulating "barrier" planes sandwiched between semiinfinite metallic leads to become "fragile" Fermi liquids. They are fully Fermiliquidlike at T=0, leading to a restoration of lattice periodicity at zero frequency, with a welldefined Fermi surface, and perfect (ballistic) conductivity. However, the Fermiliquid character can rapidly disappear at finite omega, V, T, disorder, or magnetism, all of which restore the expected quantum tunneling regime, leading to fascinating possibilities for nonlinear response in devices.  [Show abstract] [Hide abstract]
ABSTRACT: In this work, we theoretically examine recent pump/probe photoemission experiments on the strongly correlated chargedensitywave insulator TaS2.We describe the general nonequilibrium manybody formulation of timeresolved photoemission in the sudden approximation, and then solve the problem using dynamical meanfield theory with the numerical renormalization group and a bare density of states calculated from density functional theory including the chargedensitywave distortion of the ion cores and spinorbit coupling. We find a number of interesting results: (i) the bare band structure actually has more dispersion in the perpendicular direction than in the twodimensional planes; (ii) the DMFT approach can produce upper and lower Hubbard bands that resemble those in the experiment, but the upper bands will overlap in energy with other higher energy bands; (iii) the effect of the finite width of the probe pulse is minimal on the shape of the photoemission spectra; and (iv) the quasiequilibrium approximation does not fully describe the behavior in this system (© 2009 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim)  [Show abstract] [Hide abstract]
ABSTRACT: The theory for timeresolved, pumpprobe, photoemission spectroscopy and other pumpprobe experiments is developed. The formal development is completely general, incorporating all of the nonequilibrium effects of the pump pulse and the finite time width of the probe pulse, and including possibilities for taking into account band structure and matrix element effects, surface states, and the interaction of the photoexcited electrons with the system leading to corrections to the sudden approximation. We also illustrate the effects of windowing that arise from the finite width of the probe pulse in a simple model system by assuming the quasiequilibrium approximation.  [Show abstract] [Hide abstract]
ABSTRACT: Using numerical renormalizationgroup techniques, we study static and dynamic properties of a family of singlechannel Kondo impurity models with axial magnetic anisotropy DSz2 terms; such models are appropriate to describe magnetic impurity atoms adsorbed on nonmagnetic surfaces, which may exhibit surface Kondo effect. We show that for positive anisotropy D and for any spin S, the systems behave at low temperatures as regular Fermi liquids with fully compensated impurity spin. The approach to the stable fixed point depends on the value of the spin S and on the ratio D/TK(0), where TK(0) is the Kondo temperature in the absence of the anisotropy. For S=1, the screening occurs in two stages if D<TK(0); the second Kondo temperature is exponentially reduced in this case. More generally, there is an effective spin1/2 Kondo effect for any integer S if D<TK(0) and for any halfinteger S if D>TK(0). For negative anisotropy D, the system is a nonFermi liquid with residual anisotropic exchange interaction. However, the presence of transverse magnetic anisotropy E(Sx2−Sy2) restores Fermiliquid behavior in real systems.  [Show abstract] [Hide abstract]
ABSTRACT: An exact formalism for calculating the retarded and advanced Green's functions of strongly correlated lattice models in a uniform electric field is derived within dynamical meanfield theory. To illustrate the method, we solve for the nonequilibrium density of states of the Hubbard model in both the metallic and Mottinsulating phases at halffilling (with an arbitrary strength electric field) by employing the approximate numerical renormalization group as the impurity solver. This general approach can be applied to any strongly correlated lattice model in the limit of large dimensions. 
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ABSTRACT: Using a dynamical cluster quantum Monte Carlo approximation we investigate the dwave superconducting transition temperature $T_c$ in the doped 2D repulsive Hubbard model with a weak inhomogeneity. The inhomogeneity is introduced in the hoppings $\tp$ and $t$ in the form of a checkerboard pattern where $t$ is the hopping within a $2\times2$ plaquette and $\tp$ is the hopping between the plaquettes. We find inhomogeneity suppresses $T_c$. The characteristic spin excitation energy and the strength of dwave pairing interaction decrease with decreasing $T_c$ suggesting a strong correlation between these quantities. Comment: Five pages, four figures. Accepted for Phys. Rev. B (Rapid Com.)  [Show abstract] [Hide abstract]
ABSTRACT: We use the Matsubara functional renormalization group (FRG) to describe electronic correlations within the single impurity Anderson model. In contrast to standard FRG calculations, we account for the frequencydependence of the twoparticle vertex in order to address finiteenergy properties (e.g, spectral functions). By comparing with data obtained from the numerical renormalization group (NRG) framework, the FRG approximation is shown to work well for arbitrary parameters (particularly finite temperatures) provided that the electronelectron interaction U is not too large. We demonstrate that aspects of (large U) Kondo physics which are described well by a simpler frequencyindependent truncation scheme are no longer captured by the 'higherorder' frequencydependent approximation. In contrast, at small to intermediate U the results obtained by the more elaborate scheme agree better with NRG data. We suggest to parametrize the twoparticle vertex not by three independent energy variables but by introducing three functions each of a single frequency. This considerably reduces the numerical effort to integrate the FRG flow equations. Comment: accepted by J. Phys.: Condensed Matter 
Article: Suppression of dwave superconductivity in the weakly inhomogeneous checkerboard Hubbard Model
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ABSTRACT: Using a dynamical cluster quantum Monte Carlo approximation we investigate the dwave superconducting transition temperature Tc of the doped 2D Hubbard model with a weak inhomogeneity in the form of checkerboard pattern in the hoppings. The hopping within a 2 x 2 cluster (plaquette) is t and the hopping between the plaquettes is t' ( 0.8t 
Article: Reply to "Comment on 'Evidence for strong electronic correlations in the spectra of Sr(2)RuO(4)' "
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ABSTRACT: In a recent publication [Phys. Rev. B 75, 035122 (2007)], we presented clear evidence that Coulomb correlations play a key role in the electronic structure of Sr(2)RuO(4). First, we obtained a mass enhancement with respect to the band mass of about 2.5 for Ru 4d states. Second, a Hubbard satellite formed by the Ru 4d states was found at 3 eV. These two facts perfectly agree with known experimental results. Here, we show that our linear muffintin orbital band structure is very similar to the linearized augmented plane wave data presented by Singh in his Comment [Phys. Rev. B 77, 046101 (2008)]. Moreover, we argue that the cross section of O 2p states in the photoemission experiments with high photon energy is much smaller than that of the Ru 4d states. Therefore, although the oxygen 2p nonbonding states as pointed out by Singh are dominant in the band structure of Sr(2)RuO(4) for energies around 3 eV, they almost do not contribute to the spectra measured with high photon energies. Also, the mass enhancement can not be obtained within plain band structure calculations. We, thus, believe that a proper inclusion of Coulomb correlations is of crucial importance for a correct description of the electronic structure of Sr(2)RuO(4). 
Article: Functional Renormalization Group Approach To NonEquilibrium Properties Of Mesoscopic Systems
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ABSTRACT: We present an extension of the concepts of the functional renormalization group approach to quantum manybody problems in nonequilibrium situations. The approach is completely general and allows calculations for both stationary and timedependent situations. As a specific example we study the stationary state transport through a quantum dot with local Coulomb correlations. We discuss the influence of finite bias voltage as well as magnetic field and temperature on the current and conductance. For finite bias and magnetic fields we compare our results to recent experimental observations on a quantum dot in an external magnetic field. 
Article: Reply to “Comment on ‘Evidence for strong electronic correlations in the spectra of Sr2RuO4’ ”
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ABSTRACT: The Silences of the Archives, the Reknown of the Story. The Martin Guerre affair has been told many times since Jean de Coras and Guillaume Lesueur published their stories in 1561. It is in many ways a perfect intrigue with uncanny resemblance, persuasive deception and a surprizing end when the two Martin stood face to face, memory to memory, before captivated judges and a guilty feeling Bertrande de Rols. The historian wanted to go beyond the known story in order to discover the world of the heroes. This research led to disappointments and surprizes as documents were discovered concerning the environment of Artigat’s inhabitants and bearing directly on the main characters thanks to notarial contracts. Along the way, study of the works of Coras and Lesueur took a new direction. Coming back to the affair a quarter century later did not result in finding new documents (some are perhaps still buried in Spanish archives), but by going back over her tracks, the historian could only be struck by the silences of the archives that refuse to reveal their secrets and, at the same time, by the possible openings they suggest, by the intuition that almost invisible threads link here and there characters and events.  [Show abstract] [Hide abstract]
ABSTRACT: The properties of condensed matter are determined by singleparticle and collective excitations and their interactions. These quantummechanical excitations are characterized by an energy E and a momentum \hbar k which are related through their dispersion E_k. The coupling of two excitations may lead to abrupt changes (kinks) in the slope of the dispersion. Such kinks thus carry important information about interactions in a manybody system. For example, kinks detected at 4070 meV below the Fermi level in the electronic dispersion of hightemperature superconductors are taken as evidence for phonon or spinfluctuation based pairing mechanisms. Kinks in the electronic dispersion at binding energies ranging from 30 to 800 meV are also found in various other metals posing questions about their origins. Here we report a novel, purely electronic mechanism yielding kinks in the electron dispersions. It applies to strongly correlated metals whose spectral function shows well separated Hubbard subbands and central peak as, for example, in transition metaloxides. The position of the kinks and the energy range of validity of Fermiliquid (FL) theory is determined solely by the FL renormalization factor and the bare, uncorrelated band structure. Angleresolved photoemission spectroscopy (ARPES) experiments at binding energies outside the FL regime can thus provide new, previously unexpected information about strongly correlated electronic systems.  [Show abstract] [Hide abstract]
ABSTRACT: We extend the concept of the functional renormalization for quantum manybody problems to nonequilibrium situations. Using a suitable generating functional based on the Keldysh approach, we derive a system of coupled differential equations for the mparticle vertex functions. The approach is completely general and allows calculations for both stationary and timedependent situations. As a specific example we study the stationary state transport through a quantum dot with local Coulomb correlations at finite bias voltage employing two different truncation schemes for the infinite hierarchy of equations arising in the functional renormalization group scheme.  [Show abstract] [Hide abstract]
ABSTRACT: Conventional band structure calculations in the local density approximation (LDA) [1–3] are highly successful for many materials, but miss important aspects of the physics and energetics of strongly correlated electron systems, such as transition metal oxides and felectron systems displaying, e.g., Mott insulating and heavy quasiparticle behavior. In this respect, the LDA + DMFT approach which merges LDA with a modern manybody approach, the dynamical meanfield theory (DMFT), has proved to be a breakthrough for the realistic modeling of correlated materials. Depending on the strength of the electronic correlation, a LDA + DMFT calculation yields the weakly correlated LDA results, a strongly correlated metal, or a Mott insulator. In this paper, the basic ideas and the setup of the LDA + DMFT(X) approach, where X is the method used to solve the DMFT equations, are discussed. Results obtained with X = QMC (quantum Monte Carlo) and X = NCA (noncrossing approximation) are presented and compared, showing that the method X matters quantitatively. We also discuss LDA + DMFT results for two prime examples of correlated materials, i.e., V2O3 and Ce which undergo a Mott–Hubbard metal–insulator and volume collapse transition, respectively. (© 2006 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim)  [Show abstract] [Hide abstract]
ABSTRACT: The importance of electronic correlation effects in the layered perovskite Sr$_2$RuO$_4$ is evidenced. To this end we use stateoftheart LDA+DMFT (Local Density Approximation + Dynamical MeanField Theory) in the basis of Wannier functions to compute spectral functions and the quasiparticle dispersion of Sr$_2$RuO$_4$. The spectra are found to be in good agreement with various spectroscopic experiments. We also calculate the $\textbf{k}$dependence of the quasiparticle bands and compare the results with new angle resolved photoemission (ARPES) data. Two typical manifestations of strong Coulomb correlations are revealed: (i) the calculated quasiparticle mass enhancement of $m^*/m \approx2.5$ agrees with various experimental results, and (ii) the satellite structure at about 3 eV binding energy observed in photoemission experiments is shown to be the lower Hubbard band. For these reasons Sr$_2$RuO$_4$ is identified as a strongly correlated 4$d$ electron material.
Publication Stats
4k  Citations  
282.12  Total Impact Points  
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Institutions

20032011

GeorgAugustUniversität Göttingen
 Institute for Theoretical Physics
Göttingen, Lower Saxony, Germany


20082009

Universitätsmedizin Göttingen
Göttingen, Lower Saxony, Germany


20022006

Universität Augsburg
 Institute of Physics
Augsburg, Bavaria, Germany


19922002

Universität Regensburg
 Intitute of Theoretical Physics
Ratisbon, Bavaria, Germany


1993

The Ohio State University
 Department of Physics
Columbus, OH, United States


19851990

Darmstadt University of Applied Sciences
Darmstadt, Hesse, Germany
