Majoranas in mixed-valence insulators

TL;DR

This paper presents a supersymmetric model for mixed-valence insulators that predicts Majorana fermions, explains observed magneto-oscillations, and suggests extensive ground state entropy as a key feature.

Contribution

It introduces a novel supersymmetric framework for mixed-valence insulators, revealing Majorana excitations and explaining experimental oscillations without fine-tuning.

Findings

Prediction of a protected gapless Majorana band in lattice models.

Explanation of magneto-oscillations in insulators.

Proposal of extensive ground state entropy in these compounds.

Abstract

A physical model for a mixed-valence impurity in a metal must satisfy the Friedel screening theorem for both valences. Such a model is shown, following earlier work which showed low energy singularities in it, to be supersymmetric, leading to a free Majorana and a phase-shifted Majorana excitation. The theory extended approximately to a lattice of mixed-valence ions at appropriate filling gives, without fine-tuning the parameters, a protected gapless Majorana fermion band across the chemical potential, besides the mixed-valence particle and hole bands separated by gaps. In this situation the system is electrically neutral in linear response but has de Haas-van Alphen oscillations. This is used to explain the recently observed magneto-oscillations in mixed-valence insulators as well as their accompanying low energy thermodynamic and relaxation rate anomalies. Some predictions to test the validity of the theoretical results are provided, the most striking of which is that there should be extensive ground state entropy in such compounds.