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.

The Reverse Quantum Limit: Implications for Unconventional Quantum Oscillations in YbB$_{12}$

“…Importantly, the quantum oscillations in Fig. 2 are in good agreement with previous reports in both the insulating [20,31] and metallic states [21,22] of YbB 12 . This demonstrates quantum oscillations in high-quality YbB 12 are a robust and reproducible phenomenon.…”

“…First, we motivate the idea that the first Landau level crossing may correspond to the lowest Landau level by presenting high-field measurements on the Kondo insulator YbB 12 . We establish YbB 12 as an ideal system to investigate reverse Landau level filling because it has (1) large Zeeman and small cyclotron energies, (2) a small gap (which sets a field scale amenable to laboratory magnets), and (3) reproducible high-field, low-temperature behavior (including unconventional quantum oscillations [20][21][22]). Then, we demonstrate the angular dependence of the quantum oscillations matches that of the insulatormetal transition field.…”

The reverse quantum limit and its implications for unconventional quantum oscillations in YbB12

“…First, we motivate the idea that the first Landau level crossing may correspond to the lowest Landau level by presenting high-field measurements on the Kondo insulator YbB 12 . We establish YbB 12 as an ideal system to investigate reverse Landau level filling because it has (1) large Zeeman and small cyclotron energies, (2) a small gap (which sets a field scale amenable to laboratory magnets), and (3) reproducible high-field, low-temperature behavior (including unconventional quantum oscillations 18 – 20 ). Then, we demonstrate the angular dependence of the Landau level indices matches that of the insulator–metal transition field.…”

Quantum oscillations in an impurity-band Anderson insulator

“…In recent years, this association has been called into question, in light of remarkable experimental discoveries of quantum oscillations in insulators. Quantum oscillations have been observed in the Kondo insulators SmB 6 [2][3][4][5] and YbB 12 [6][7][8][9], and in insulating InAs/GaSb quantum well devices [10][11][12]. More recently there have been reports of oscillatory behaviour in WTe 2 [13] and for the insulating spin-liquid α-RuCl 2 [14,15], though these may be of different origin [16][17][18].…”