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P. Wölfle

Publications and source records attributed to P. Wölfle.

At least 37 records · Page 2Linked to original sources
arXiv2006

Energy-resolved inelastic electron scattering off a magnetic impurity

We study inelastic scattering of energetic electrons off a Kondo impurity. If the energy E of the incoming electron (measured from the Fermi level) exceeds significantly the Kondo temperature T_K, then the differential inelastic cross-section σ(E,w), i.e., the cross-section characterizing scattering of an electron with a given energy transfer w, is well-defined. We show that σ(E,w) factorizes into two parts. The E-dependence of σ(E,w) is logarithmically weak and is due to the Kondo renormalization of the effective coupling. We are able to relate the w-dependence to the spin-spin correlation function of the magnetic impurity. Using this relation, we demonstrate that in the absence of magnetic field the dynamics of the impurity spin causes the electron scattering to be inelastic at any temperature. Quenching of the spin dynamics by an applied magnetic field results in a finite elastic component of the electron scattering cross-section. The differential scattering cross-section may be extracted from the measurements of relaxation of hot electrons injected in conductors containing localized spins.

cond-mat.str-el↗
arXiv2004

The Kondo Effect in Non-Equilibrium Quantum Dots: Perturbative Renormalization Group

While the properties of the Kondo model in equilibrium are very well understood, much less is known for Kondo systems out of equilibrium. We study the properties of a quantum dot in the Kondo regime, when a large bias voltage V and/or a large magnetic field B is applied. Using the perturbative renormalization group generalized to stationary nonequilibrium situations, we calculate renormalized couplings, keeping their important energy dependence. We show that in a magnetic field the spin occupation of the quantum dot is non-thermal, being controlled by V and B in a complex way to be calculated by solving a quantum Boltzmann equation. We find that the well-known suppression of the Kondo effect at finite V>>T_K (Kondo temperature) is caused by inelastic dephasing processes induced by the current through the dot. We calculate the corresponding decoherence rate, which serves to cut off the RG flow usually well inside the perturbative regime (with possible exceptions). As a consequence, the differential conductance, the local magnetization, the spin relaxation rates and the local spectral function may be calculated for large V,B >> T_K in a controlled way.

cond-mat.str-el↗
arXiv2004

Dynamical Properties of the Anderson Impurity Model within a Diagrammatic Pseudoparticle Approach

The Anderson model of a twofold spin degenerate impurity level in the limit of infinite Coulomb repulsion, $U \to \infty$, coupled to one and two degenerate conduction bands or channels, is considered in pseudo-particle representation. We extend the Conserving T-Matrix Approximation (CTMA), a general diagrammatic approximation scheme based on a fully renormalized computation of two-particle vertex functions in the spin and in the charge channel, to the calculation of thermodynamic and spectral properties. In the single-channel case, the CTMA yields in the Kondo regime a temperature independent Pauli spin susceptibility for temperatures below the Kondo temperature $T_K$ and down to the lowest temperatures considered, reproducing the exact spin screening in the Fermi liquid state. The impurity spectral density appears to remain non-singular down to the lowest temperatures, in agreement with Fermi liquid behavior. However, the unitarity sum rule, which is crucial for an impurity solver like the CTMA to be applicable within Dynamical Mean Field Theories for strongly correlated lattice models, is overestimated at the lowest temperatures. We argue that this shortcoming may be due to numerical imprecision and discuss an appropriate scheme for its correction. In the two-channel case, the spectral density calculated within CTMA exhibit qualitatively the correct non-Fermi liquid behavior at low temperatures, i.e. a powerlaw singularity.

cond-mat.str-el↗
arXiv2004

Nonequilibrium Transport through a Kondo Dot: Decoherence Effects

We investigate the effects of voltage induced spin-relaxation in a quantum dot in the Kondo regime. Using nonequilibrium perturbation theory, we determine the joint effect of self-energy and vertex corrections to the conduction electron T-matrix in the limit of transport voltage much larger than temperature. The logarithmic divergences, developing near the different chemical potentials of the leads, are found to be cut off by spin-relaxation rates, implying that the nonequilibrium Kondo-problem remains at weak coupling as long as voltage is much larger than the Kondo temperature.

cond-mat.str-el↗
arXiv2003

Nonequilibrium Transport through a Kondo Dot in a Magnetic Field: Perturbation Theory

Using nonequilibrium perturbation theory, we investigate the nonlinear transport through a quantum dot in the Kondo regime in the presence of a magnetic field. We calculate the leading logarithmic corrections to the local magnetization and the differential conductance, which are characteristic of the Kondo effect out of equilibrium. By solving a quantum Boltzmann equation, we determine the nonequilibrium magnetization on the dot and show that the application of both a finite bias voltage and a magnetic field induces a novel structure of logarithmic corrections not present in equilibrium. These corrections lead to more pronounced features in the conductance, and their form calls for a modification of the perturbative renormalization group.

cond-mat.str-el↗
arXiv2003

Spectral function of the Kondo model in high magnetic fields

Using a recently developed perturbative renormalization group (RG) scheme, we calculate analytically the spectral function of a Kondo impurity for either large frequencies w or large magnetic field B and arbitrary frequencies. For large w >> max[B,T_K] the spectral function decays as 1/ln^2[ w/T_K ] with prefactors which depend on the magnetization. The spin-resolved spectral function displays a pronounced peak at w=B with a characteristic asymmetry. In a detailed comparison with results from numerical renormalization group (NRG) and bare perturbation theory in next-to-leading logarithmic order, we show that our perturbative RG scheme is controlled by the small parameter 1/ln[ max(w,B)/T_K]. Furthermore, we assess the ability of the NRG to resolve structures at finite frequencies.

cond-mat.str-el↗
arXiv2003

Pseudogaps in the t-J model: Extended DMFT study

We investigate the highly incoherent regime of hole-doped 2d Mott-Hubbard insulators at moderately small doping and temperatures T>=0.1J, where J is the exchange coupling. Within an extended dynamical mean-field theory of the t-J model and a generalized non-crossing approximation we calculate the single-particle spectral function, the dynamical susceptibility, and thermodynamic and transport quantities. Short-ranged antiferromagnetic fluctuations lead to strongly incoherent single-particle dynamics, large entropy and large electrical resistivity. At low doping a pseudogap is found to open up both in the single-particle and the spin excitation spectra leading to a decrease in entropy and resistivity. The Hall coefficient changes sign to positive values upon lowering the doping level and increases inversely proportional to doping.

cond-mat.str-el↗
arXiv2003

Non-Equilibrium Transport through a Kondo-Dot in a Magnetic Field: Perturbation Theory and Poor Man's Scaling

We consider electron transport through a quantum dot described by the Kondo model in the regime of large transport voltage V in the presence of a magnetic field B with max(V,B) >> T_K. The electric current I and the local magnetization M are found to be universal functions of V/T_K and B/T_K, where T_K is the equilibrium Kondo temperature. We present a generalization of the perturbative renormalization group to frequency dependent coupling functions, as necessitated by the structure of bare perturbation theory. We calculate I and M within a poor man's scaling approach and find excellent agreement with experiment.

cond-mat.str-el↗
arXiv2002

Conductance distribution in disordered quantum wires: Crossover between the metallic and insulating regimes

We calculate the distribution of the conductance P(g) for a quasi-one-dimensional system in the metal to insulator crossover regime, based on a recent analytical method valid for all strengths of disorder. We show the evolution of P(g) as a function of the disorder parameter from a insulator to a metal. Our results agree with numerical studies reported on this problem, and with analytical results for the average and variance of g.

cond-mat.mes-hall↗
arXiv2002

Magnetic Field Effects on the Transport Properties of One-sided Rough Wires

We present a detailed numerical analysis of the effect of a magnetic field on the transport properties of a `small-$N$' one-sided surface disordered wire. When time reversal symmetry is broken due to a magnetic field $B$, we find a strong increase with $B$ not only of the localization length $ξ$ but also of the mean free path $\ell$ caused by boundary states. Despite this, the universal relationship between $\ell$ and $ξ$ does hold. We also analyze the conductance distribution at the metal-insulator crossover, finding a very good agreement with Random Matrix Theory with two fluctuating channels within the Circular Orthogonal(Unitary) Ensemble in absence(presence) of $B$

cond-mat.mes-hall↗
arXiv2002

Pseudogaps in an Incoherent Metal

How are the properties of a metal changed by strong inelastic scattering? We investigate this question within the two-dimensional t-J model using extended dynamical mean field theory and a generalized non-crossing approximation. Short-ranged antiferromagnetic fluctuations lead to a strongly incoherent single particle dynamics, large entropy and resistance. Close to the Mott transition at low hole doping a pseudogap opens, accompanied by a drop in resistivity and an increase in the Hall constant for both lower temperatures and doping levels. The behavior obtained bears surprising similarity to properties of the cuprates.

cond-mat.str-el↗
arXiv2001

Anderson impurity model at finite Coulomb interaction U: generalized Non-crossing Approximation

We present an extension of the non-crossing approximation (NCA), which is widely used to calculate properties of Anderson impurity models in the limit of infinite Coulomb repulsion $U\to\infty$, to the case of finite $U$. A self-consistent conserving pseudo-particle representation is derived by symmetrizing the usual NCA diagrams with respect to empty and doubly occupied local states. This requires an infinite summation of skeleton diagrams in the generating functional thus defining the ``Symmetrized finite-U NCA'' (SUNCA). We show that within SUNCA the low energy scale $T_K$ (Kondo temperature) is correctly obtained, in contrast to other simpler approximations discussed in the literature.

cond-mat.str-el↗
arXiv2001

Fermi and Non-Fermi Liquid Behavior of Quantum Impurity Models: A Diagrammatic Pseudo-particle Approach

We review a systematic many-body method capable of describing Fermi liquid and Non-Fermi liquid behavior of quantum impurity models at low temperatures on the same footing. The crossover to the high temperature local moment regime is covered as well. The approach makes use of a pseudo-particle representation of the impurity Hilbert space in the limit of infinite Coulomb repulsion $U$ as well as for finite $U$. Approximations are derived from a generating Luttinger-Ward functional, in terms of renormalized perturbation theory in the hybridization $V$. Within a ``conserving T-matrix approximation'' (CTMA), including all two-particle vertex functions, an infinite series of leading infrared singular skeleton diagrams is included. The local constraint is strictly enforced. Applied to the SU(N) $\times$ SU(M) multichannel Anderson model the method allows to recover the Fermi liquid behavior of the single channel case, as well as the non-Fermi liquid behavior in the multi-channel case. The results are compared with the ``non-crossing approximation'' (NCA) and with data obtained by the numerical renormalization group and the Bethe ansatz. The generalization of the method to the case of finite on-site repulsion $U$ is presented in a systematical way and solved on the level of a generalized NCA, fully symmetric with respect to all virtual excitations of the model.

cond-mat.str-el↗
arXiv2001

The Kondo effect in quantum dots at high voltage: Universality and scaling

We examine the properties of a dc-biased quantum dot in the Coulomb blockade regime. For voltages V large compared to the Kondo temperature T_K, the physics is governed by the scales V and gamma, where gamma ~ V/ln^2(V/T_K) is the non-equilibrium decoherence rate induced by the voltage-driven current. Based on scaling arguments, self-consistent perturbation theory and perturbative renormalization group, we argue that due to the large gamma, the system can be described by renormalized perturbation theory for ln(V/T_K) >> 1. However, in certain variants of the Kondo problem, two-channel Kondo physics is induced by a large voltage V.

cond-mat.mes-hall↗
arXiv2000

Diagrammatic theory of the Anderson impurity model with finite Coulomb interaction

We have developed a self-consistent conserving pseudo particle approximation for the Anderson impurity model with finite Coulomb interaction, derivable from a Luttinger Ward functional. It contains an infinite series of skeleton diagrams built out of fully renormalized Green's functions. The choice of diagrams is motivated by the Schrieffer Wolff transformation which shows that singly and doubly occupied states should appear in all bare diagrams symmetrically. Our numerical results for $T_K$ are in excellent agreement with the exact values known from the Bethe ansatz solution. The low energy physics of non-Fermi liquid Anderson impurity systems is correctly described while the present approximation fails to describe Fermi liquid systems, since some important coherent spin flip and charge transfer processes are not yet included. It is believed that CTMA (Conserving T-matrix approximation) diagrams will recover also Fermi liquid behavior for Anderson models with finite Coulomb interaction as they do for infinite Coulomb interaction.

cond-mat.str-el↗
arXiv2000

Fermi and non-Fermi Liquid Behavior in Quantum Impurity Systems: Conserving Auxiliary Particle Theory

We review a recently developed method, based on an exact auxiliary boson representation, to describe both Fermi liquid and non-Fermi liquid behavior in quantum impurity systems. Coherent spin and charge fluctuation processes are taken into account in a self-consistent way and are shown to include {\it all} leading and subleading infrared singularities at any given order of the self-consistent loop expansion of the free energy. As a consequence, for the SU(N)$\times$ SU(M) Anderson impurity models the correct temperature dependence of the susceptibility is recovered over the entire temperature range, including Fermi liquid or non-Fermi liquid behavior below the Kondo temperature $T_K$. As a standard diagram technique the presented method has the potential to be generalized to correlated electron systems on a lattice.

cond-mat.str-el↗
arXiv1999

`One-sided' log-normal distribution of conductances of a disordered quantum wire

We develop a simple systematic method, valid for all strengths of disorder, to obtain analytically for the first time the full distribution of conductance P(g) for a quasi one dimensional wire in the absence of electron-electron interaction. We show that in the crossover region between the metallic and insulating regimes, P(g) is highly asymmetric, given by an essentially `one sided' log-normal distribution. For larger disorder, the tail of the log-normal distribution for g > 1 is cut off by a Gaussian.

cond-mat.dis-nn↗