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

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${}_{12}$.

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• Topological surface states in the Kondo insulator YbB${}_{12}$ revealed via planar tunneling spectroscopy
• Temperature-driven change in band structure reflecting spin-charge separation of Mott and Kondo insulators
• Disordered 2D ferromagnetism at the surface of FeSi
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