$f$-electron hybridised metallic Fermi surface in magnetic field-induced metallic YbB$_{12}$

TL;DR

This study reveals that YbB$_{12}$ maintains an $f$-electron hybridised Fermi surface in both its insulating and field-induced metallic states, challenging previous models and highlighting the role of neutral low-energy excitations.

Contribution

It provides direct experimental evidence of persistent $f$-electron hybridisation across insulating and metallic regimes in YbB$_{12}$, contrasting with SmB$_6$ and prompting new theoretical models.

Findings

Quantum oscillations indicate a heavy, hybridised Fermi surface in metallic YbB$_{12}$

In-gap neutral excitations contribute to quantum oscillations in the insulating state

Multiple Fermi surface sheets with heavy quasiparticles emerge in high magnetic fields

Abstract

The nature of the Fermi surface observed in the recently discovered family of unconventional insulators starting with SmB$_6$ and subsequently YbB$_{12}$ is a subject of intense inquiry. Here we shed light on this question by comparing quantum oscillations between the high magnetic field-induced metallic regime in YbB$_{12}$ and the unconventional insulating regime. In the field-induced metallic regime beyond 47 T, we find prominent quantum oscillations in the contactless resistivity characterised by multiple frequencies up to at least 3000 T and heavy effective masses up to at least 17 $m_\text{e}$, characteristic of an $f$-electron hybridised metallic Fermi surface. The growth of quantum oscillation amplitude at low temperatures in electrical transport and magnetic torque in insulating YbB$_{12}$ is closely similar to the Lifshitz-Kosevich low temperature growth of quantum oscillation amplitude in field-induced metallic YbB$_{12}$, pointing to an origin of quantum oscillations in insulating YbB$_{12}$ from in-gap neutral low energy excitations. The field-induced metallic regime of YbB$_{12}$ is characterised by more Fermi surface sheets of heavy quasiparticle effective mass that emerge in addition to the heavy Fermi surface sheets yielding multiple quantum oscillation frequencies below 1000 T observed in both insulating and metallic regimes. We thus observe a heavy multi-component Fermi surface in which $f$-electron hybridisation persists from the unconventional insulating to the field-induced metallic regime of YbB$_{12}$, which is in distinct contrast to the unhybridised conduction electron Fermi surface observed in the case of the unconventional insulator SmB$_6$. Our findings require a different theoretical model of neutral in-gap low energy excitations in which the $f$-electron hybridisation is retained in the case of the unconventional insulator YbB$_{12}$.