Hall Anomaly, Quantum Oscillations and Possible Lifshitz Transitions in Kondo Insulator YbB_{12}: Evidence for Unconventional Charge Transport

Abstract

In correlated electronic systems, strong interactions and the interplay between different degrees of freedom may give rise to anomalous charge-transport properties, which can be tuned by external parameters like temperature and magnetic field. Recently, magnetic quantum oscillations and metallic low-temperature thermal conductivity have been observed in the Kondo insulator YbB_{12}, whose resistivity is a few orders of magnitude higher than those of conventional metals. As yet, these unusual observations are not fully understood. Here we present a detailed investigation of the behavior of YbB_{12} under intense magnetic fields using both transport and torque magnetometry measurements. The Hall resistivity displays a strongly nonlinear field dependence which cannot be described using a standard two-band Drude model. A low-field Hall anomaly, reminiscent of the Hall response associated with “strange-metal” physics, develops at T<1.5 K. At two characteristic magnetic fields (μ_{0}H_{1}=19.6 T and μ_{0}H_{2}∼31 T), signatures appear in the Hall coefficient, magnetic torque, and magnetoresistance; the latter characteristic field coincides with the occurrence of a change in quantum-oscillation frequency. We suggest that they are likely to be field-induced Lifshitz transitions. Moreover, above 35 T, where the most pronounced quantum oscillations are detected, the background resistivity displays an unusual, nonmetallic T^{α} behavior, with α being field dependent and varying between -1.5 and -2 By normalizing the Shubnikov–de Haas oscillation amplitude to this T^{α} dependence, the calculated cyclotron mass becomes more consistent with that deduced from de Haas–van Alphen oscillations. Our results support a novel two-fluid scenario in YbB_{12}: A Fermi-liquid-like fluid of charge-neutral quasiparticles coexists with charge carriers that remain in a nonmetallic state. The former experience successive Lifshitz transitions and develop Landau quantization in applied magnetic fields, while scattering between both fluids allows the Shubnikov–de Haas effect to be observed in the electrical transport. The verification of this two-fluid scenario by the data in the current work strongly suggests that YbB_{12} represents a new phase of matter.