Kondo insulators, a class of heavy fermion material in which the narrow band gap is opened by the interaction between localized and conduction electrons, have recently become a hot topic in condensed matter physics. In the exemplary Kondo insulator SmB6, quantum oscillations (QOs) were observed in the magnetization, but not in the electrical transport. This unusual behavior raised many speculations about the unconventional QOs in insulating systems, such as the Landau quantization of charge neutral quasiparticles. To clarify this question, we studied the high magnetic field properties of another Kondo insulator YbB12. In this material, QOs were resolved in both the magnetization and the resistivity. The temperature dependence of the oscillation amplitude obeys the Lifshitz-Kosevich formula of Fermi liquid theory. However, the QOs appear to be unconventional since (i) the quasiparticles that give rise to the QOs have very weak contribution to the electrical conductivity, (ii) the QOs in magnetization and resistivity reveal different effective masses and orbit areas. Our findings confirm that the hybridization gap in YbB12 is a pseudogap with the existence of exotic three-dimensional in-gap quasiparticle states. The nature of these in-gap states and their potential link to topological nontriviality is to be further explored in future research in YbB12 and other insulating/semimetallic Kondo systems.
Dr. Xiang completed Ph.D. in March 2016, at the Hefei National Laboratory for Physical Science at Microscale, University of Science and Technology of China (USTC). In the graduate school at USTC, Dr. Xiang learned the techniques of the growth and characterization of single crystals under the supervision of Prof. Xianhui Chen. The main projects in Prof. Chen’s group focused on iron-based superconductors and the tunable topological orders in three-dimensional materials. Since June 2016 Dr. Xiang has been working in Prof. Lu Li’s group in the Department of Physics at the University of Michigan as a postdoctoral researcher. Recently Dr. Xiang discovered unconventional quantum oscillations in Kondo insulator YbB12 under high magnetic fields. Dr. Xiang's current research interests are in the area of strongly correlated electron systems with possible topological nontriviality, including Kondo insulators, heavy fermion semimetals and quantum magnets.