Energy relaxation due to magnetic impurities in mesoscopic wires: Logarithmic approach

TL;DR

This paper investigates how magnetic impurities cause energy relaxation in mesoscopic wires under high bias, using a logarithmic approximation to explain experimental observations of anomalous relaxation rates.

Contribution

It provides a systematic study of Kondo impurity effects on energy relaxation in mesoscopic wires within the logarithmic approximation above the Kondo temperature, explaining experimental data with enhanced coupling.

Findings

Kondo resonances form at the distribution steps and narrow with increased bias.

Korringa broadening smears Kondo resonances, leading to an effective enhanced coupling.

Distribution functions exhibit scaling behavior depending on the ratio of energy to bias.

Abstract

The transport in mesoscopic wires with large applied bias voltage has recently attracted great interest by measuring the energy distribution of the electrons at a given point of the wire, in Saclay. In the diffusive limit with negligible energy relaxation that shows two sharp steps at the Fermi energies of the two contacts, which are broadened due to the energy relaxation. In some of the experiments the broadening is reflecting an anomalous energy relaxation rate proportional to $E^{-2}$ instead of $E^{-3/2}$ valid for Coulomb electron-electron interaction, where $E$ is the energy transfer. Later it has been suggested that such relaxation rate can be due to electron-electron interaction mediated by Kondo impurities. In the present paper the latter is systematically studied in the logarithmic approximation valid above the Kondo temperature. In the case of large applied bias voltage Kondo resonances are formed at the steps of the distribution function and they are narrowed by increasing the bias. An additional Korringa energy broadening occurs for the spins which smears the Kondo resonances, and the renormalized coupling can be replaced by a smooth but essentially enhanced average coupling (factor of 8-10). Thus the experimental data can be described by formulas without logarithmic Kondo corrections, but with enhanced coupling. In certain regions of large bias, that averaged coupling depends weakly on the bias. In those cases the distribution function depends only on the ratio of the electron energy and the bias, showing scaling behavior. The impurity concentrations estimated from those experiments and other dephasing experiments can be very different, and a possible explanation considering the surface spin anisotropy due to strong spin-orbit interaction is the subject of our earlier paper.