We report on an ultrahigh Hall mobility exceeding 40 000 cm2/V s and a very long traditional scattering time in a trivial layered semiconductor Bi2O2Se. Shubnikov-de Haas (SdH) oscillations were observed in both the unsaturated longitudinal linear magnetoresistance Rxx and the transverse Hall resistance Rxy. The amplitude ΔRxy of SdH oscillations was phase-shifted approximately 180° with respect to ΔRxx, indicating the strong suppression of electron backward scattering. This was further proved by the evidence of transport lifetime that is 10 times longer than the quantum lifetime. Our results show that the suppressed backward scattering in nontrivial Dirac semimetals can also occur in the trivial semiconductor Bi2O2Se.

The pressure-induced phase transition of bismuth selenide (Bi2Se3) was investigated by combining theoretical calculations with synchrotron powder X-ray diffraction measurements up to 57.4 GPa. We demonstrated that the ambient-pressure rhombohedral Bi2Se3 crystallized into a monoclinic structure with the space group C2/m at 9.1 GPa, and eventually to a body-centered tetragonal structure with the space group I4/mmm at about 27.2 GPa. This behavior was different from the transformation sequences of Bi2Te3 and Sb2Te3. The stabilities of five structures of Bi2Se3 were studied by density functional theory calculations. Furthermore, an unusual irreversible relaxation process was observed. We attempted to make clear the unusual structural behavior of Bi2Se3 occurring in the compression process and the relaxation process.

Circularly polarized light (CPL) is essential for optoelectronic and chiro-spintronic applications. Hybrid perovskites, as star optoelectronic materials, have demonstrated CPL activity, which is, however, mostly limited to chiral perovskites. Here, we develop a simple, general, and efficient strategy to stimulate CPL activity in achiral perovskites, which possess rich species, efficient luminescence, and tunable bandgaps. With the formation of van der Waals heterojunctions between chiral and achiral perovskites, a nonequilibrium spin population and thus CPL activity are realized in achiral perovskites by receiving spinpolarized electrons from chiral perovskites. The polarization degree of room-temperature CPL in achiral perovskites is at least one order of magnitude higher than in chiral ones. The CPL polarization degree and emission wavelengths of achiral perovskites can be flexibly designed by tuning chemical compositions, operating temperature, or excitation wavelengths. We anticipate that unlimited types of achiral perovskites can be endowed with CPL activity, benefiting their applications in integrated CPL sources and detectors.