Direct measurement of a remnant Fermi surface in SmB6

TL;DR

This study provides direct evidence of a remnant Fermi surface in SmB6 using Compton scattering, challenging the traditional view of it as a bulk insulator and shedding light on its complex electronic state.

Contribution

It is the first direct measurement of a remnant Fermi surface in SmB6, revealing unexpected metallic-like features in its bulk electronic structure.

Findings

Evidence of a three-dimensional remnant Fermi surface

Full occupancy of a conventional metal is not observed

Challenges the paradigm of SmB6 as a pure bulk insulator

Abstract

The quest to understand the nature of the electronic state in SmB6 has been challenging, perplexing and surprising researchers for over half a century. In the theoretically predicted topological Kondo insulator SmB6, the nature of the bulk electronic structure is not characterised unambiguously by quantum oscillations due to contrary interpretations. One simple definition of an electrical insulator is a material that lacks a Fermi surface and here we report the results of our investigation into its existence in SmB6 by Compton scattering. Compton scattering measures occupied electron momentum states, is bulk sensitive due to the high energy of the incoming photons and is also an ultra-fast probe of the correlated many-body electron wavefunction. Remarkably, direct evidence for a three-dimensional remnant Fermi surface is observed. However, a further dichotomy is raised in that the full occupancy expected of a conventional metal is not reproduced. Our observation of a remnant Fermi surface using a momentum-resolved probe provides significant new evidence that the paradigm of a bulk insulator is not robust for SmB6.