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Journal of the Physical Society of Japan

Repulsion between Conduction Electrons as a Relevant Perturbation against the Stability of Non-Fermi-Liquid Fixed Point of Multichannel Kondo Problem

Kazumasa Miyake, Hiroaki Kusunose, Osamu Narikiyo

Published Aug 15, 1993• 6 citations
Publisher

Abstract

It is shown that a non-Fermi-liquid fixed point of the multichannel Kondo problem is unstable for repulsive interaction at the impurity site between conduction electrons with different flavors and the same pseudospin. The anomalous fixed point is shown, by the perturbative renormalization group argument, to move into the strong coupling region with increasing repulsion. The low-temperature properties at the so-called “Toulouse limit” are calculated by the bosonization method to show that the Fermi-liquid like behavior is recovered quickly as soon as repulsion is switched on. This result poses a question as to the reality of the non-Fermi-liquid state for the multichannel Kondo model which has been examined extensively so far.

DOI
10.1143/jpsj.62.2553

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It is shown that a non-Fermi-liquid fixed point of the multichannel Kondo problem is unstable for repulsive interaction at the impurity site between conduction electrons with different flavors and the same pseudospin. The anomalous fixed point is shown, by the perturbative renormalization group argument, to move into the strong coupling region with increasing repulsion. The low-temperature properties at the so-called “Toulouse limit” are calculated by the bosonization method to show that the Fermi-liquid like behavior is recovered quickly as soon as repulsion is switched on. This result poses a question as to the reality of the non-Fermi-liquid state for the multichannel Kondo model which has been examined extensively so far.
Particle-hole symmetry breaking perturbation in the two-channel pseudospin Kondo problem is studied by the numerical renormalization-group method. It is shown that the repulsion among conduction electrons at the impurity site and the single particle potential are relevant perturbations against the conventional non-Fermi-liquid fixed point. Although the repulsion (potential) with realistic strength prevents the overscreening of the pseudospin, it induces in turn a real spin, which is also overscreened again. Thus the real spin susceptibility becomes anomalous, contrary to the conventional two-channel Kondo problem.
The effects of interchannel scattering of conduction electrons by the impurity and repulsion of conduction electrons at the impurity site on the two-channel Kondo model are simultaneously considered in this paper. It is shown that these two perturbations will substantially modify the usual local non-Fermi liquid behavior of the two-channel Kondo model. With bosonization and unitary transformations we find that the system can be transformed into a single channel Kondo model with anisotropy between longitudinal and transverse exchange couplings. Whatever for originally antiferromagnetic or ferromagnetic isotropic coupling, the system always flows to strong-coupling limit, which exhibits local Fermi liquid behavior at low temperatures.
Using the numerical renormalization group method, we study a generalized two-channel Kondo model where the impurity has a quadrupole moment (pseudospin) and a magnetic moment (flavor). This model may be of relevance to the Ce ions in CeB6{\mathrm{CeB}}_{6}CeB6​ and the Cu ions in cuprate superconductors. We find that the nontrivial fixed point of the two-channel Kondo model is unstable against flavor exchange coupling. The system flows to a non-Fermi-liquid fixed line. In the presence of a double tensor interaction, which breaks particle-hole symmetry, the system flows to a Fermi-liquid fixed line.
We study an exchange interaction in which conduction electrons with pseudospin Sc{\mathrm{S}}_{\mathrm{c}}Sc​=3/2 interact with the impurity spin SI{\mathrm{S}}_{\mathrm{I}}SI​=1/2. Due to the overscreening of the impurity spin by higher conduction electron spin, a nontrivial intermediate-coupling-strength fixed point is realized. Using the numerical renormalization group (NRG), we show that the low-energy spectra are described by a non-Fermi-liquid excitation spectrum. A conformal field theory analysis is compared with NRG results and excellent agreement is obtained. Using the double-fusion rule to generate the operator spectrum with the conformal theory, we find that the specific heat coefficient and magnetic susceptibility will diverge as ${\mathrm{T}}^{\mathrm{\ensuremath{-}}2\mathrm{/}3}$, that the scaling dimension of an applied magnetic field is 5/6, and that exchange anisotropy is always relevant. We discuss the possible relevance of our work to two-level-system Kondo materials and dilute cerium alloys, and we point out a paradox in understanding the Bethe-ansatz solutions to the multichannel Kondo model.
We present an analytical solution of a SU(N)\ifmmode\times\else\texttimes\fi{}SU(M) generalization of the multichannel single-impurity Kondo model in the limit N\ensuremath{\rightarrow}\ensuremath{\infty}, M\ensuremath{\rightarrow}\ensuremath{\infty}, with \ensuremath{\gamma}=M/N fixed. Non-Fermi-liquid behavior of the single electron Green function and of the local spin and flavor susceptibilities occurs in both regimes, N\ensuremath{\le}M and N>M, with leading critical exponents identical to those found in the conformal field theory solution for all N and M (with M\ensuremath{\ge}2). We explain this remarkable agreement and connect it to ``spin-flavor separation,'' the essential feature of the non-Fermi-liquid, fixed point.
We study triangular clusters of three spin-1/2 Kondo or Anderson impurities that are coupled to two conduction leads. In the case of Kondo impurities, the model takes the form of an antiferromagnetic Heisenberg ring with Kondo-type exchange coupling to continuum electrons. We show that this model exhibits many types of the behavior found in various simpler one- and two-impurity models, thereby enabling the study of crossovers between a number of Fermi-liquid (FL) and non-Fermi-liquid (NFL) fixed points. In particular, we explore a direct crossover between the two-impurity Kondo-model NFL fixed point and the two-channel Kondo-model NFL fixed point. We show that the concept of the two-stage Kondo effect applies even in the case when the first-stage Kondo state is of NFL type. In the case of Anderson impurities, we consider the transport properties of three coupled quantum dots. This class of models includes, as limiting cases, the familiar serial double quantum dot and triple quantum dot nanostructures. By extracting the quasiparticle scattering phase shifts, we compute the low-temperature conductance as a function of the interimpurity tunneling coupling. We point out that due to the existence of exponentially low-temperature scales, there is a parameter range where the stable ``zero-temperature'' fixed point is essentially never reached (not even in numerical renormalization group calculations). The zero-temperature conductance is then of no interest and it may only be meaningful to compute the conductance at finite temperature. This illustrates the perils of studying the conductance in the ground state and considering thermal fluctuations only as a small correction.