Orbital Kondo-effect from tunneling impurities

TL;DR

This paper reviews how fast tunneling centers in metals can exhibit an orbital Kondo effect, leading to non-Fermi-liquid behavior, explained through a mapping to the multichannel Kondo model.

Contribution

It provides a theoretical framework linking tunneling center models to multichannel Kondo models using renormalization group methods and discusses experimental implications.

Findings

Identification of orbital Kondo effect in tunneling centers

Mapping to multichannel Kondo model explains non-Fermi-liquid behavior

Discussion of experimental observations and M-state systems

Abstract

The article reviews recent results for the low energy physics of fast tunneling centers in metallic environments. For strong enough couplings to the environment these tunneling centers display an orbital Kondo effect and give rise to a non-Fermi-liquid behavior. This latter property is explained by establishing a mapping of the tunneling center model to the multichannel Kondo model via the renormalization group transformation combined with a $1/N_f$ expansion. The case of M-state systems, the role of the splittings and the present experimental situation are also discussed.