Unusual high-field metal in a Kondo insulator

TL;DR

This study reveals a high-field metallic state in a Kondo insulator with unusual quasiparticles, showing strong field-dependent quantum oscillations and a coexistence of different fermion types, indicating exotic electronic behavior.

Contribution

It demonstrates the evolution of quantum oscillations and quasiparticle properties in a Kondo insulator under high magnetic fields, revealing a complex coexistence of fermion types.

Findings

Quantum oscillation frequency and cyclotron mass are strongly field-dependent.

The same quasiparticle band causes oscillations in both insulating and metallic states.

The metallic state exhibits heavy-fermion behavior with high resistivity and large Kadowaki-Woods ratio.

Abstract

Within condensed-matter systems, strong electronic interactions often lead to exotic quantum phases. A recent manifestation of this is the unexpected observation of magnetic quantum oscillations and metallic thermal transport, both properties of systems with Fermi surfaces of itinerant quasiparticles, in the Kondo insulators SmB6 and YbB$_{12}$. To understand these phenomena, it is informative to study their evolution as the energy gap of the Kondo-Insulator state is closed by a large magnetic field. We show here that both the quantum-oscillation frequency and the cyclotron mass display a strong field dependence in the resulting high-field metallic state in $_{12}$. By tracking the Fermi-surface area, we conclude that the same quasiparticle band gives rise to the quantum oscillations in both insulating and metallic states. These data are understood most simply using a two-fluid picture where unusual quasiparticles, contributing little or nothing to charge transport, coexist with conventional fermions. In the metallic state this leads to a heavy-fermion bad metal with negligible magnetoresistance, relatively high resistivity and a very large Kadowaki-Woods ratio, underlining the exotic nature of the fermion ensemble inhabiting $_{12}$.