arXiv subjects

Thomas Pruschke

Publications and source records attributed to Thomas Pruschke.

At least 55 records · Page 3Linked to original sources

arXiv2008

Anomaly in the heat capacity of Kondo superconductors

Using numerical renormalization group we study thermodynamic properties of a magnetic impurity described by the Anderson impurity model in a superconducting host material described by the BCS Hamiltonian. When the Kondo temperature in the normal state, T_K, is comparable to the critical temperature of the superconducting transition, T_c, the magnetic doublet state may become degenerate with the Kondo singlet state, leading to a ln3 peak in the temperature dependence of the impurity contribution to the entropy. This entropy increase translates into an anomalous feature in the heat capacity which might have already been experimentally observed.

cond-mat.supr-con↗

arXiv2008

Energy resolution and discretization artefacts in the numerical renormalization group

We study the limits of the energy resolution that can be achieved in the calculations of spectral functions of quantum impurity models using the numerical renormalization group (NRG) technique with interleaving (z-averaging). We show that overbroadening errors can be largely eliminated, that higher-moment spectral sum rules are satisfied to a good accuracy, and that positions, heights and widths of spectral features are well reproduced; the NRG approximates very well the spectral-weight distribution. We find, however, that the discretization of the conduction-band continuum nevertheless introduces artefacts. We present a new discretization scheme which removes the band-edge discretization artefacts of the conventional approach and significantly improves the convergence to the continuum (Lambda -> 1) limit. Sample calculations of spectral functions with high energy resolution are presented. We follow in detail the emergence of the Kondo resonance in the Anderson impurity model as the electron-electron repulsion is increased, and the emergence of the phononic side peaks and the transition from the spin Kondo effect to the charge Kondo effect in the Anderson-Holstein impurity model as the electron-phonon coupling is increased. We also compute the spectral function of the Hubbard model within the dynamical mean-field theory (DMFT), confirming the presence of fine structure in the Hubbard bands.

cond-mat.str-el↗

arXiv2008

Properties of anisotropic magnetic impurities on surfaces

Using numerical renormalization group techniques, we study static and dynamic properties of a family of single-channel Kondo impurity models with axial magnetic anisotropy $DS_z^2$ terms; such models are appropriate to describe magnetic impurity atoms adsorbed on non-magnetic 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 $D/T_K^{(0)}$, where $T_K^{(0)}$ is the Kondo temperature in the absence of the anisotropy. For S=1, the screening occurs in two stages if $D T_K^{(0)}$. For negative anisotropy $D$, the system is a non-Fermi liquid with residual anisotropic exchange interaction. However, the presence of transverse magnetic anisotropy $E(S_x^2-S_y^2)$ restores Fermi-liquid behavior in real systems.

cond-mat.str-el↗

arXiv2008

Influence of disorder on the transport properties of heavy-fermion systems

The influence of substitutional disorder on the transport properties of heavy-fermion systems is investigated. We extend the dynamical mean-field theory treatment of the periodic Anderson model (PAM) to a coherent-potential approximation for disordered strongly correlated electron systems. Considering two distinct local environments of a binary alloy $A_c B_{1-c}$ with arbitrary concentration $c$, we explore two types of disorder: on the f site and on the ligand sites. We calculate the spectral functions and self-energies for the disordered PAM as well as the temperature dependence of the resistivity and the thermoelectric power. The characteristic concentration dependence as well as the order of magnitude of transport properties are reproduced for metallic heavy-fermion systems and Kondo insulators. In particular, sign changes of the Seebeck coefficient as function of temperature and concentration are observed.

cond-mat.str-el↗

arXiv2008

Magnetic properties of the extended periodic Anderson model

We study magnetic properties of the extended periodic Anderson model, which includes electron correlations within and between itinerant and localized bands. By combining dynamical mean-field theory with the numerical renormalization group we calculate the sublattice magnetization and the staggered susceptibility to determine the phase diagram in the particle-hole symmetric case. We find that two kinds of magnetically ordered states compete with the Kondo insulating state at zero temperature, which induces non-monotonic behavior in the temperature-dependent magnetization. It is furthermore clarified that a novel magnetic metallic state is stabilized at half filling by the competition between Hund's coupling and the hybridization.

cond-mat.str-el↗

arXiv2007

Quantum phase transitions in the extended periodic Anderson model

We investigate quantum phase transitions in the extended periodic Anderson model, which includes electron correlations within and between itinerant and localized bands. We calculate zero and finite temperature properties of the system using the combination of dynamical mean-field theory and the numerical renormalization group. At half filling, a phase transition between a Mott insulating state and a Kondo insulating state occurs in the strong coupling regime. We furthermore find that a metallic state is stabilized in the weak coupling regime. This state should be adiabatically connected to the orbital selective Mott state with one orbital localized and the other itinerant. The effect of hole doping is also addressed.

cond-mat.str-el↗

arXiv2007

Transport through single-level quantum dot in a magnetic field

We study the effect of an external magnetic field on the transport properties of a quantum dot using a recently developed extension of the functional renormalization group approach to non-equilibrium situations. We discuss in particular the interplay and competition of the different energy scales of the dot and the magnetic field on the stationary non-equilibrium current and conductance. As rather interesting behavior we find a switching behavior of the magnetic field for intermediate correlations and bias voltage.

cond-mat.mes-hall↗

arXiv2007

Magnetic phases in the correlated Kondo-lattice model

We study magnetic ordering of an extended Kondo-lattice model including an additional on-site Coulomb interaction between the itinerant states. The model is solved in the dynamical mean-field theory using Wilson's numerical renormalization group approach as impurity solver. For a bipartite lattice we find at half filling the expected antiferromagnetic phase. Upon doping this phase is gradually suppressed and hints towards phase separation are observed. For large doping the model exhibits ferromagnetism, the appearance of which can at first sight be explained by Rudermann-Kittel-Kasuya-Yosida interaction. However, for large values of the Kondo coupling $J$ significant differences to a simple Rudermann-Kittel-Kasuya-Yosida picture can be found. We furthermore observe signs of quantum critical points for antiferromagnetic Kondo coupling between the local spins and band states.

cond-mat.str-el↗

arXiv2007

The numerical renormalization group method for quantum impurity systems

In the beginning of the 1970's, Wilson developed the concept of a fully non-perturbative renormalization group transformation. Applied to the Kondo problem, this numerical renormalization group method (NRG) gave for the first time the full crossover from the high-temperature phase of a free spin to the low-temperature phase of a completely screened spin. The NRG has been later generalized to a variety of quantum impurity problems. The purpose of this review is to give a brief introduction to the NRG method including some guidelines of how to calculate physical quantities, and to survey the development of the NRG method and its various applications over the last 30 years. These applications include variants of the original Kondo problem such as the non-Fermi liquid behavior in the two-channel Kondo model, dissipative quantum systems such as the spin-boson model, and lattice systems in the framework of the dynamical mean field theory.

cond-mat.str-el↗

arXiv2006

Transport properties of heavy-fermion systems

We calculate the temperature dependence of the transport properties of heavy-fermion systems such as resistivity, optical conductivity, thermoelectric power, the electronic part of the thermal conductivity, and the "figure of merit." The one-particle properties of the periodic Anderson model are obtained within dynamical mean-field theory for the paramagnetic phase using Wilson's numerical renormalization group and the modified perturbation theory as impurity solvers. We discuss the dependence of the transport properties on the band filling, valence, and Coulomb correlation $U$. The typical experimental findings can be reproduced and understood, in particular the temperature dependence of the resistance and the thermoelectric power and their absolute magnitude for both metallic heavy-fermion systems and Kondo insulators. For large values of $U$, we find a negative Seebeck coefficient $S(T)$ for an intermediate-temperature regime as observed in $S(T)$ of CeCu$_2$Si$_2$. We analyze different estimates for possible characteristic low-temperature scales of the lattice. Our results indicate a one-parameter scaling of thermodynamic and some transport properties with a strongly occupancy-dependent scaling function. This is consistent with a strong-coupling local Fermi-liquid fixed point of the effective site governing all low-lying excitations for $T\to 0$ in the paramagnetic phase.

cond-mat.str-el↗

arXiv2006

A Numerical Renormalization Group approach to Green's Functions for Quantum Impurity Models

We present a novel technique for the calculation of dynamical correlation functions of quantum impurity systems in equilibrium with Wilson's numerical renormalization group. Our formulation is based on a complete basis set of the Wilson chain. In contrast to all previous methods, it does not suffer from overcounting of excitation. By construction, it always fulfills sum rules for spectral functions. Furthermore, it accurately reproduces local thermodynamic expectation values, such as occupancy and magnetization, obtained directly from the numerical renormalization group calculations.

cond-mat.str-el↗

arXiv2006

Relevance of quantum fluctuations in the Anderson-Kondo model

We study a localized spin coupled to an Anderson impurity to model the situation found in higher transition metal or rare earth compounds like e.g.\ LaMnO$_3$ or Gd monopnictides. We find that, even for large quantum numbers of the localized spin, quantum fluctuations play an essential role for the case of ferromagnetic coupling between the spin and the impurity levels. For antiferromagnetic coupling, a description in terms of a classical spin is appropriate.

cond-mat.str-el↗

arXiv2005

Can competition between crystal field and Kondo effect cause non-Fermi liquid like behavior?

The recently reported unusual behavior of the static and dynamical magnetic susceptibility as well as the specific heat in Ce$_{1-x}$La$_{x}$Ni$_{9}$Ge$_4$ has raised the question of a possible non-Fermi liquid ground state in this material. We argue that for a consistent physical picture the crystal field splitting of two low lying magnetic doublets of the Ce $4f$-shell must be taken into account. Furthermore, we show that for a splitting of the order of the low temperature scale $T^*$ of the system a crossover behavior between an SU(4) and an SU(2) Kondo effect is found. The screening of the two doublets occurs on different temperature scales leading to a different behavior of the magnetic susceptibility and the specific heat at low temperatures. The experimentally accessible temperature regime down to 50mK still lies in the extended crossover regime into a strong coupling Fermi-liquid fixed point.

cond-mat.str-el↗

arXiv2005

Spectral Density of the Two-Impurity Anderson Model

We investigate static and dynamical ground-state properties of the two-impurity Anderson model at half filling in the limit of vanishing impurity separation using the dynamical density-matrix renormalization group method. In the weak-coupling regime, we find a quantum phase transition as function of inter-impurity hopping driven by the charge degrees of freedom. For large values of the local Coulomb repulsion, the transition is driven instead by a competition between local and non-local magnetic correlations. We find evidence that, in contrast to the usual phenomenological picture, it seems to be the bare effective exchange interactions which trigger the observed transition.

cond-mat.str-el↗

arXiv2005

Hund's coupling and the metal-insulator transition in the two-band Hubbard model

The Mott-Hubbard metal-insulator transition is investigated in a two-band Hubbard model within dynamical mean-field theory. To this end, we use a suitable extension of Wilson's numerical renormalization group for the solution of the effective two-band single-impurity Anderson model. This method is non-perturbative and, in particular, allows to take into account the full exchange part of the Hund's rule coupling between the two orbitals. We discuss in detail the influence of the various Coulomb interactions on thermodynamic and dynamic properties, for both the impurity and the lattice model. The exchange part of the Hund's rule coupling turns out to play an important role for the physics of the two-band Hubbard model and for the nature of the Mott-transition.

cond-mat.str-el↗

arXiv2004

Quantum phase transition of Ising-coupled Kondo impurities

We investigate a model of two Kondo impurities coupled via an Ising interaction. Exploiting the mapping to a generalized single-impurity Anderson model, we establish that the model has a singlet and a (pseudospin) doublet phase separated by a Kosterlitz-Thouless quantum phase transition. Based on a strong-coupling analysis and renormalization group arguments, we show that at this transition the conductance G through the system either displays a zero-bias anomaly, G ~ |V|^{-2(\sqrt{2}-1)}, or takes a universal value, G = e^2/(π\hbar) cos^2[π/(2\sqrt{2})], depending on the experimental setup. Close to the Toulouse point of the individual Kondo impurities, the strong-coupling analysis allows to obtain the location of the phase boundary analytically. For general model parameters, we determine the phase diagram and investigate the thermodynamics using numerical renormalization group calculations. In the singlet phase close to the quantum phase transtion, the entropy is quenched in two steps: first the two Ising-coupled spins form a magnetic mini-domain which is, in a second step, screened by a Kondoesque collective resonance in an effective solitonic Fermi sea. In addition, we present a flow equation analysis which provides a different mapping of the two-impurity model to a generalized single-impurity Anderson model in terms of fully renormalized couplings, which is applicable for the whole range of model parameters.

cond-mat.str-el↗

arXiv2003

From Slater to Mott-Heisenberg physics: The antiferromagnetic phase of the Hubbard model

We study the optical conductivity of the one-band Hubbard model in the Néel state at half filling at T=0 using the dynamical mean-field theory. For small values of the Coulomb parameter clear signatures of a Slater insulator expected from a weak-coupling theory are found, while the strongly correlated system can be well described in terms of a Mott-Heisenberg picture. However, in contrast to the paramagnet, we do not find any evidence for a transition between these two limiting cases but rather a smooth crossover as a function of the Coulomb interaction.

cond-mat↗

arXiv2002

Kondo screening in d-wave superconductors in a Zeeman field and implications for STM spectra of Zn-doped cuprates

We consider the screening of an impurity moment in a d-wave superconductor under the influence of a Zeeman magnetic field. Using the Numerical Renormalization Group technique, we investigate the resulting pseudogap Kondo problem, in particular the field-induced crossover behavior in the vicinity of the zero-field boundary quantum phase transition. The impurity spectral function and the resulting changes in the local host density of states are calculated, giving specific predictions for high-field STM measurements on impurity-doped cuprates.

cond-mat.supr-con↗