A topological insulator with large bulk-insulating behavior and high electron mobility of the surface state is needed urgently, not only because it would be a good platform for studying topological surface states but also because it is a prerequisite for potential future applications. In this work, we demonstrated that tin (Sn) or indium (In) dopants could be introduced into a BiSbTeSe₂ single crystal. The impacts of the dopants on the bulk-insulating property and electron mobility of the surface state were systematically investigated by electrical transport measurements. The doped single crystals had the same crystal structure as the pristine BiSbTeSe₂, no impure phase was observed, and all elements were distributed homogeneously. The electrical transport measurements illustrated that slight Sn doping could improve the performance of BiSbTeSe₂ a lot, as the longitudinal resistivity (ρ_xx), bulk carrier density (n_b), and electron mobility of the surface state (μ_s) reached about 11 Ωcm, 7.40 × 10¹⁴ cm⁻³, and 6930 cm²/(Vs), respectively. By comparison, indium doping could also improve the performance of BiSbTeSe₂ with ρ_xx, n_b, and μ_s up to about 13 Ωcm, 1.29 × 10¹⁵ cm⁻³, and 4500 cm²/(Vs), respectively. Our findings suggest that Sn- or indium-doped BiSbTeSe₂ crystals should be good platforms for studying novel topological properties, as well as promising candidates for low-dissipation electron transport, spin electronics, and quantum computing.

In 2H stacking bilayer MoS₂, the exciton with an interlayer nature has been evidenced due to the hybridization of hole states among both layers. The transition energy of this interlayer exciton is located between the A and B excitons. In this work, we investigate the evolution of optical properties in stacking MoS₂ bilayers with the twisted angles ranging from 0° to 60°, especially focusing on the interlayer exciton. The clear modulations of the exciton responses are observed by the room-temperature reflectance. The interlayer exciton transition is observed in the artificial stacking bilayer MoS₂ with the twisted angle around 60°. We found that the interlayer exciton is very sensitive to the twisted angle. Once the stacking angle deviates the 2H stacking by a few degrees, the interlayer transition is quenched. This is due to the bilayer symmetry and interlayer coupling of this unique system.

Understanding the physical and chemical processes at the interfaces of metals and topological insulators is crucial for the development of the next generation of topological quantum devices. Here, we report the discovery of robust superconductivity in Pd/Bi₂Se₃ bilayers fabricated by sputtering Pd on the surface of Bi₂Se₃. Through transmission electron microscopy measurements, we identify that the observed interfacial superconductivity originates from the diffusion of Pd into Bi₂Se₃. In the diffusion region, Pd chemically reacts with Bi₂Se₃ and forms a layer of PdBiSe, a known superconductor with a bulk transition temperature of 1.5 K. Our work provides a method for the introduction of superconductivity into Bi₂Se₃, laying the foundation for the development of sophisticated Bi₂Se₃-based topological devices.