Fermi surface in the absence of a Fermi liquid in the Kondo insulator SmB$_6$

TL;DR

This paper provides experimental evidence of a Fermi surface in the Kondo insulator SmB$_6$ without the presence of a conventional Fermi liquid, indicating novel low-energy excitations near the insulator-metal transition.

Contribution

It demonstrates the existence of a Fermi surface in SmB$_6$ through various experimental signatures, revealing a new route to Fermi surface formation absent of Fermi liquid behavior.

Findings

Bulk Fermi surface signatures at very low temperatures

Observation of magnetic quantum oscillations in SmB$_6$

Enhanced thermal conductivity below the charge gap energy

Abstract

The search for a Fermi surface in the absence of a conventional Fermi liquid has thus far yielded very few potential candidates. Among promising materials are spin-frustrated Mott insulators near the insulator-metal transition, where theory predicts a Fermi surface associated with neutral low energy excitations. Here we reveal another route to experimentally realise a Fermi surface in the absence of a Fermi liquid by the experimental study of a Kondo insulator SmB$_6$ positioned close to the insulator-metal transition. We present experimental signatures down to low temperatures ($\ll 1$ K) associated with a Fermi surface in the bulk, including a sizeable linear specific heat coefficient, and on the application of a finite magnetic field, bulk magnetic quantum oscillations, finite quantum oscillatory entropy, and substantial enhancement in thermal conductivity well below the charge gap energy scale. Thus, the weight of evidence indicates that despite an extreme instance of Fermi liquid breakdown in Kondo insulating SmB$_6$, a Fermi surface arises from novel itinerant low energy excitations that couple to magnetic fields, but not weak DC electric fields.