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

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 approximate to 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.