f-electron hybridised Fermi surface in magnetic field-induced metallic YbB12

Abstract

The nature of the Fermi surface observed in the recently discovered family of unconventional insulators starting with SmB₆ is a subject of intense inquiry. Here we shed light on this question by accessing quantum oscillations in the high magnetic field-induced metallic regime above ≈47 T in YbB₁₂, which we compare with the unconventional insulating regime. In the field-induced metallic regime, we find prominent quantum oscillations in the electrical resistivity characterised by multiple frequencies and heavy effective masses. The close similarity in Lifshitz-Kosevich low-temperature growth of quantum oscillation amplitude in insulating YbB₁₂ to field-induced metallic YbB₁₂, points to an origin of quantum oscillations in insulating YbB₁₂ from in-gap neutral low energy excitations. Higher frequency Fermi surface sheets of heavy quasiparticle effective mass emerge in the field-induced metallic regime of YbB₁₂ in addition to multiple heavy Fermi surface sheets observed in both insulating and metallic regimes. f-electron hybridisation is thus observed to persist from the unconventional insulating to the field-induced metallic regime of YbB₁₂, in contrast to the unhybridised conduction electron Fermi surface observed in unconventional insulating SmB₆. Our findings thus require an alternative model for YbB₁₂, of neutral in-gap low energy excitations, wherein the f-electron hybridisation is retained.