f -electron hybridised Fermi surface in magnetic field-induced metallic YbB 12

The nature of the Fermi surface observed in the recently discovered family of unconventional insulators starting with SmB 6 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 YbB 12 , 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 YbB 12 to field-induced metallic YbB 12 , points to an origin of quantum oscillations in insulating YbB 12 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 YbB 12 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 YbB 12 , in contrast to the unhybridised conduction electron Fermi surface observed in unconventional insulating SmB 6 . Our findings thus require an alternative model for YbB 12 , of neutral in-gap low energy excitations, wherein the f -electron hybridisation is retained.