Publications (112)376.49 Total impact
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ABSTRACT: We generalize the recently introduced dual fermion (DF) formalism for disordered fermion systems by including the effect of interactions. For an interacting disordered system the contributions to the full vertex function have to be separated into elastic and inelastic scattering processes, and addressed differently when constructing the DF diagrams. By applying our approach to the AndersonFalicovKimball model and systematically restoring the nonlocal correlations in the DF lattice calculation, we show a significant improvement over the Dynamical MeanField Theory and the Coherent Potential Approximation for both oneparticle and twoparticle quantities.Physical Review B 05/2014; 89:195116. · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Superconductivity in solids usually arises due to the generation of an attractive effective interaction between fermions close to the Fermi energy by some bosonic fluctuations. In the conventional theory, these are phonons, but in correlated electron systems like the cuprates or heavy fermions, one believes that the relevant bosonic degrees of freedom are the spin fluctuations. In this context, one usually argues that standard swave superconductivity cannot be formed as these spin fluctuations in general lead to a repulsive local interaction. Recently, we observed swave superconductivity in the Kondo lattice model using the dynamical meanfield approach. We can indeed show that this superconducting (SC) solution is due to local spin fluctuations arising from the Kondo effect. The reason for these fluctuations mediating an effective attractive interaction lies in the special properties of the heavy electron ground state, i.e., the formation of hybridized bands. Using a simple model, we can show that it is indeed an interband coupling that is largely responsible for the observed SC state. Such an observation is possibly rather interesting also concerning the situation in the pnictide superconductors.IEEE Transactions on Magnetics 01/2014; 50(6):15. · 1.42 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Superconductivity in solids usually arises due to the generation of an attractive effective interaction between fermions close to the Fermi energy by some bosonic fluctuations. In the conventional theory, these are phonons, but in correlated electron systems like the cuprates or heavy fermions, one believes that the relevant bosonic degrees of freedom are the spin fluctuations. In this context, one usually argues that standard swave superconductivity cannot be formed as these spin fluctuations in general lead to a repulsive local interaction. Recently, we observed swave superconductivity in the Kondo lattice model using the dynamical meanfield approach. We can indeed show that this superconducting (SC) solution is due to local spin fluctuations arising from the Kondo effect. The reason for these fluctuations mediating an effective attractive interaction lies in the special properties of the heavy electron ground state, i.e., the formation of hybridized bands. Using a simple model, we can show that it is indeed an interband coupling that is largely responsible for the observed SC state. Such an observation is possibly rather interesting also concerning the situation in the pnictide superconductors.IEEE Transactions on Magnetics 01/2014; 50(6):15. · 1.42 Impact Factor 
Article: Double Occupancy and Magnetic Susceptibility of the Anderson Impurity Model out of Equilibrium
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ABSTRACT: We use different numerical approaches to calculate the double occupancy and mag netic susceptibility as a function of a bias voltage in an Anderson impurity model. Specifically, we compare results from the Matsubaravoltage quantum MonteCarlo approach (MVQMC), the scatteringstates numerical renormalization group (SNRG), and realtime quantum MonteCarlo (RTQMC), covering Coulomb repulsions ranging from the weakcoupling well into the strong coupling regime. We observe a distinctly different behavior of the double occupancy and the magnetic response. The former measures charge fluctuations and thus only indirectly exhibits the Kondo scale, while the latter exhibits structures on the scale of the equilibrium Kondo tempera ture. The Matsubaravoltage approach and the scatteringstates numerical renormalization group yield consistent values for the magnetic susceptibility in the Kondo limit. On the other hand, all three numerical methods produce different results for the behavior of charge fluctuations in strongly interacting dots out of equilibrium.EPL (Europhysics Letters) 02/2013; 102(3). · 2.26 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In order to study the interplay between Kondo and RudermanKittelKasuyaYosida (RKKY) interaction, we calculate the spinspin correlation functions between two Kondo impurities coupled to different sites of a halffilled open Hubbard chain. Using the densitymatrix renormalization group (DMRG), we reexamine the exponents for the powerlaw decay of the correlation function between the two impurity spins as a function of the antiferromagnetic coupling J, the Hubbard interaction U, and the distance R between the impurities. The exponents for finite systems obtained in this work deviate from previously published DMRG calculations. We furthermore show that the longdistance behavior of the exponents is the same for impurities coupled to the bulk or to both ends of the chain. We note that a universal exponent for the asymptotic behavior cannot be extracted from these finitesize systems with open boundary conditions.Physical review. B, Condensed matter 12/2012; 87(7). · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The densitymatrix renormalization group (DMRG) algorithm can be adapted to the calculation of dynamical correlation functions in various ways which all represent compromises between computational efficiency and physical accuracy. In this paper we reconsider the oldest approach based on a suitable Lanczosgenerated approximate basis and implement it using matrix product states (MPS) for the representation of the basis states. The direct use of matrix product states combined with an expost reorthogonalization method allows to avoid several shortcomings of the original approach, namely the multitargeting and the approximate representation of the Hamiltonian inherent in earlier Lanczosmethod implementations in the DMRG framework, and to deal with the ghost problem of Lanczos methods, leading to a much better convergence of the spectral weights and poles. We present results for the dynamic spin structure factor of the spin1/2 antiferromagnetic Heisenberg chain. A comparison to Bethe ansatz results in the thermodynamic limit reveals that the MPSbased Lanczos approach is much more accurate than earlier approaches at minor additional numerical cost.Physical Review B 05/2012; 85(20):205119. · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We present Quantum MonteCarlo simulations of an exchangeanisotropic spin1/2 Heisenberg model on a square lattice with nearest and nextnearest neighbor interactions. The ground state phase diagram shows two classical magnetically ordered phases for dominating antiferromagnetic S^zinteractions and for large quantum fluctuations a ferromagnetic order in the xy plane. In between a finite region is detected where neither classical nor quantum mechanical order, e.g. longranged dimer correlations, are found.Journal of Physics Conference Series 01/2012; 391:012156.  [Show abstract] [Hide abstract]
ABSTRACT: We analyze the antiferromagnetic Kondo lattice model for the twodimensional square lattice including frustration. The Kondo lattice model is especially interesting in the context of heavy fermion physics, as it possibly exhibits a quantum phase transition between an antiferromagnetically ordered state and a paramagnetic heavy Fermion state. The latter can, depending on the electronic properties be either a metallic state with dynamically generated flat bands at the Fermi energy, or an insulator, the socalled Kondo insulator. We analyze the effects of next nearest neighbor (NNN) hopping upon the properties of the Kondo lattice model, especially focusing on the paramagnetic and antiferromagnetic properties around half filling. To this end we use the dynamical mean field theory (DMFT) with the Numerical Renormalization Group (NRG) as an impurity solver. Whereas the paramagnetic results remain qualitatively unchanged when introducing the NNN hopping, the Néel state at half filling is replaced by a spindensity wave. Also the transition between the magnetic ordered state and the Kondo insulator depends on the NNN hopping. An interesting effect of the NNN hopping is the stabilization of Néel states away from half filling.Journal of Physics Conference Series 09/2011; 320(1):012057.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the phase diagram of hardcore bosons on a square lattice with competing interactions. The hardcore bosons can be represented also by spin1/2 operators and the model can therefore be mapped onto an anisotropic $J_1$$J_2$Heisenberg model. We find the N\'eel state and a collinear antiferromagnetic state as classical ordered phases to be suppressed for small ferromagnetic exchange terms $J_{1,2}^{x,y}$ and a ferromagnetic phase which orders in the xyplane for large $J_{1,2}^{x,y}$. For an intermediate regime the emergence of new quantum states like valence bond crystals or supersolids is predicted for similar models. We do not observe any signal for longrange order in terms of conventional order or dimer correlations in our model and find an exponential decay in the spin correlations. Hence, all evidence is pointing towards a quantum disordered ground state for a small region in the phase diagram.Physical Review B 05/2011; 83(17):174519. · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Current widelyused approaches to calculate spectral functions using the densitymatrix renormalization group in frequency space either necessarily include an artificial broadening (correctionvector method) or have limited resolution (timedomain densitymatrix renormalization group with Fourier transform method). Here we propose an adaptive Lanczosvector method to calculate the coefficients of a continued fraction expansion of the spectral function iteratively. We show that one can obtain a very accurate representation of the spectral function very efficiently, and that one can also directly extract the spectral weights and poles for the discrete system. As a test case, we study spinless fermions in one dimension and compare our approach to the correction vector method.Physical Review B 04/2011; 83(16):161104(R). · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We have designed a new multiscale approach for Strongly Correlated Systems by combining the Dynamical Cluster Approximation (DCA) and the recently introduced dualfermion formalism. This approach employs an exact mapping from a real lattice to a DCA cluster of linear size Lc embedded in a dual fermion lattice. Shortlengthscale physics is addressed by the DCA cluster calculation, while longerlengthscale physics is addressed diagrammatically using dual fermions. The bare and dressed dual Fermionic Green functions scale as O(1/Lc) so perturbation theory on the dual lattice converges very quickly. E.g., the dual Fermion selfenergy calculated with simple second order perturbation theory is of order O(1/Lc^3), with third order and three body corrections down by an additional factor of O(1/Lc^2).Physical review. B, Condensed matter 04/2011; 84. · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The effect of electronic interactions in graphene with vacancies or resonant scatterers is investigated. We apply dynamical meanfield theory in combination with quantum Monte Carlo simulations, which allow us to treat nonperturbatively quantum fluctuations beyond HartreeFock approximations. The interactions narrow the width of the resonance and induce a Curie magnetic susceptibility, signaling the formation of local moments. The absence of saturation of the susceptibility at low temperatures suggests a ferromagnetic Kondo effect.Physical review. B, Condensed matter 04/2011; 83(24). · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The effect of electronic interactions in graphene with vacancies or resonant scatterers is investigated. We apply dynamical meanfield theory in combination with quantum Monte Carlo simulations, which allow us to treat nonperturbatively quantum fluctuations beyond HartreeFock approximations. The interactions narrow the width of the resonance and induce a Curie magnetic susceptibility, signaling the formation of local moments. The absence of saturation of the susceptibility at low temperatures suggests that the coupling between the local moment and the conduction electrons is ferromagnetic.04/2011;  [Show abstract] [Hide abstract]
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.Journal of Physics Conference Series 02/2011; 273(1):012146.  [Show abstract] [Hide abstract]
ABSTRACT: We study the zerotemperature properties of the Kondo lattice model within the dynamical meanfield theory. As an impurity solver we use the numerical renormalization group. We present results for the paramagnetic case showing the anticipated heavyfermion physics, including direct evidence for the appearance of a large Fermi surface for antiferromagnetic exchange interaction. Allowing for the formation of a Néel state, we observe at finite doping an antiferromagnetic metal below a critical exchange interaction, which shows a crossover from a local moment antiferromagnet with a small Fermi surface for weak exchange coupling to a heavyfermion antiferromagnet with a large Fermi surface for increasing exchange. Including lattice degrees of freedom via an additional Holstein term we observe a significant suppression of the Kondo effect, leading to a strongly reduced lowenergy scale. For too large electronphonon coupling we find a complete collapse of the heavy Fermi liquid and the formation of polarons.Journal of Physics Condensed Matter 02/2011; 23(9):094212. · 2.22 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We study the Kondo lattice model with an additional Einstein phonon mode coupled via a Holstein term to the electrons within the dynamical meanfield theory at T = 0. Asimpurity solver we use the numerical renormalization group. We present results for the paramagnetic case showing the anticipated heavy Fermion physics, including direct evidence for the appearance of a large Fermi surface for antiferromagnetic exchange interaction.By introducing a Nambu notation, we find that increasing electronphonon coupling favorssuperconductivity, which however is not BCS like but shows additional structures in the density of states and the gap function.Physical Properties of Nanosystems. 01/2011;  [Show abstract] [Hide abstract]
ABSTRACT: We study ultracold fermionic atoms trapped in a threedimensional 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.Physica E Lowdimensional Systems and Nanostructures 01/2011; · 1.86 Impact Factor  [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.New Journal of Physics 06/2010; 12(6):063040. · 4.06 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We examine the two orbital Hubbard model within dynamical mean field theory. The two orbital Hubbard model is the basic model for treating strongly correlated electrons in degenerate egbands, as found for example in the manganites. We have calculated the magnetic phases of the model for different interaction values and fillings of the system at T = 0.Journal of Physics Conference Series 02/2010; 200(1):012158.  [Show abstract] [Hide abstract]
ABSTRACT: We set up a new approach to study the physics of spin systems which uses the resolvent method originally proposed by Keiter and Kuramoto. In analogy to the BaymKadanoff formalism the latter introduced the partition sum of a system as a functional of the resolvent of the Hamiltonian and its so called generalized selfenergy. This functional is the starting point for a variational cluster method which is based on Potthoffs selfenergy functional approach. In a first step we apply our ansatz to a Heisenberg spin chain.Journal of Physics Conference Series 02/2010; 200(2):022007.
Publication Stats
3k  Citations  
376.49  Total Impact Points  
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Institutions

2013

University at Buffalo, The State University of New York
 Department of Physics
Buffalo, New York, United States


2003–2012

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


1990–2011

Tokyo Institute of Technology
 Department of Physics
Tokyo, Tokyoto, Japan


2009

Louisiana State University
 Department of Physics & Astronomy
Baton Rouge, LA, United States 
Jawaharlal Nehru Centre for Advanced Scientific Research
Bengalūru, Karnātaka, India


2008–2009

Georgetown University
 Department of Physics
Washington, D. C., DC, United States


2002–2006

Universität Augsburg
 Institute of Physics
Augsburg, Bavaria, Germany


2004

Osaka University
 Department of Electronics and Materials Physics
Ōsakashi, Osakafu, Japan


1992–2002

Universität Regensburg
 Institut für Theoretische Physik
Regensburg, Bavaria, Germany


1996–2000

University of Cincinnati
 Department of Physics
Cincinnati, OH, United States


1993

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


1985–1990

Darmstadt University of Applied Sciences
Darmstadt, Hesse, Germany
