Kondo Impurities at a Finite Concentration of Impurities

TL;DR

This paper investigates the behavior of Kondo impurities at finite concentrations, identifying two regimes with distinct scattering mechanisms and providing analytical insights into the discrepancy between theoretical and experimental Kondo temperatures.

Contribution

It introduces a theoretical framework distinguishing single impurity and dilute impurity regimes, explaining experimental-theoretical discrepancies in Kondo temperature values.

Findings

Derived the ratio between Kondo temperatures in two regimes.

Identified the independence of the ratio from impurity concentration.

Provided analytical evidence within the Reed-Newns meanfield approximation.

Abstract

In this work we study the Kondo impurity problem - at a finite concentration of impurities. We identify two parameter regimes for the Kondo impurity problem. 1) The single impurity limit, where the concentration of Kondo impurities is so low that the background scattering mechanisms (non-magnetic impurities, Umklapp scattering, etc.) of the metal considered are the dominant conduction electron scattering mechanisms at zero temperature. 2) The dilute impurity system limit where the concentration of magnetic impurities is such that they form the dominant mechanism of conduction electron scattering at zero temperature of the metal in question (this is accompanied by a variety of easily detectable Kondo signatures (resistance minimum, specific heat measurements, magnetization as a function of external magnetic field, conduction electron dephasing rates as well as ARPES, RIXS and NMR spectroscopies)) while still being very dilute. Most theoretical efforts are currently in regime where a single isolated impurity is considered - regime 1) while most experimental efforts are in regime 2). We present analytical evidence that this explains the well known discrepancy between experiment and theory as to the value of the Kondo temperature. We find that the ratio between the two Kondo temperatures in regime 1) and regime 2) is given by: $\mathcal{R}=\exp\left[\frac{\pi^{2}\rho v_{F}}{2k_{F}^{2}Vol}\right]$ where $\rho$ is the density of states, $v_{F}$ is the fermi velocity, and $k_{F}$ is the Fermi wavevector and $Vol$ is the volume of a unit cell. We note that there is no dependence on the impurity concentration in this ratio so it is possible to define a single Kondo temperature for limit 2) for the dilute Kondo impurity system. In this work we present results within the Reed-Newns Kondo meanfield approximation and to leading order of the linked cluster expansion.