Mixed-valence insulators with neutral Fermi surfaces

TL;DR

This paper proposes that mixed-valence insulators like SmB6 can host a neutral Fermi surface of composite excitons, explaining experimental observations of gapless neutral excitations in the insulating phase.

Contribution

It introduces the concept of a composite exciton Fermi liquid as a new ground state in mixed valence insulators, with a neutral Fermi surface mechanism.

Findings

Identification of a neutral Fermi surface in SmB6

Proposal of composite exciton Fermi liquid as ground state

Comparison with experimental data on SmB6

Abstract

Samarium hexaboride is a classic three-dimensional mixed valence system with a high-temperature metallic phase that evolves into a paramagnetic charge insulator below 40 kelvin. A number of recent experiments have suggested the possibility that the low-temperature insulating bulk hosts electrically neutral gapless fermionic excitations. Here we show that a possible ground state of strongly correlated mixed valence insulators - composite exciton Fermi liquid - hosts a three dimensional Fermi surface of a neutral fermion, that we name the "composite exciton". We describe the mechanism responsible for the formation of such excitons, discuss the phenomenology of the composite exciton Fermi liquids and make comparison to experiments in SmB$_6$.