Majorana Fermi Sea in Insulating SmB6: A proposal and a Theory of Quantum Oscillations in Kondo Insulators

TL;DR

This paper proposes that SmB6 is a bulk scalar Majorana Fermi Liquid that exhibits quantum oscillations despite being an insulator, explaining recent experimental observations and predicting unique magnetic responses.

Contribution

It generalizes Coleman, Miranda, and Tsvelik's theory to non-zero magnetic fields, revealing a novel insulating phase with quantum oscillations and magnetic response.

Findings

SmB6 exhibits quantum oscillations despite being insulating.

The scalar Majorana Fermi Liquid responds to magnetic fields with diamagnetism.

Deviations from Lifshitz-Kosevich behavior are explained by Majorana fermion fluctuations.

Abstract

In an exciting development, SmB6, a Kondo insulator has been shown to exhibit bulk quantum oscillations. We propose that SmB6 is a bulk scalar Majorana Fermi Liquid} (MFL) with a finite gap for charge and spin excitations. In their study of Kondo insulators in 1993, Coleman, Miranda and Tsvelik (CMT) envisaged such a remarkable possibility, using a mean field ansatz. We generalize CMT theory to non-zero magnetic fields and show a counter intuitive result that the scalar Majorana fermi liquid, while remaining electrically insulating, responds to external magnetic field and exhibits Landau diamagnetism and quantum oscillations. Physics of an emergent compactified 2-channel Kondo lattice physics that is behind formation of the novel scalar Majorana fermi liquid phase is discussed. It is also suggested that a known resistivity saturation in SmB6 as well as a new strong deviation of quantum oscillation amplitude from Lifshitz-Kosevich formula in SmB6 at low temperatures are due to a coherent fluctuation of charge of a neutral scalar Majorana fermion. Possible presence of 2-dimensional Majorana fermion excitations in surfaces of SmB6, and other Topological Kondo Insulators (TKI) is pointed out.