Hall Anomaly, Quantum Oscillations and Possible Lifshitz Transitions in Kondo Insulator YbB12: 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 YbB12, 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 YbB12 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 (μ0H1=19.6 T and μ0H2∼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 Tα behavior, with α being field dependent and varying between -1.5 and -2 By normalizing the Shubnikov-de Haas oscillation amplitude to this Tα 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 YbB12: 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 YbB12 represents a new phase of matter.