No abstract

Weak topological insulators, constructed by stacking quantum spin Hall insulators with weak interlayer coupling, offer promising quantum electronic applications through topologically non-trivial edge channels. However, the currently available weak topological insulators are stacks of the same quantum spin Hall layer with translational symmetry in the out-of-plane direction—leading to the absence of the channel degree of freedom for edge states. Here, we study a candidate weak topological insulator, Bi4Br2I2, which is alternately stacked by three different quantum spin Hall insulators, each with tunable topologically non-trivial edge states. Our angle-resolved photoemission spectroscopy and first-principles calculations show that an energy gap opens at the crossing points of different Dirac cones correlated with different layers due to the interlayer interaction. This is essential to achieve the tunability of topological edge states as controlled by varying the chemical potential. Our work offers a perspective for the construction of tunable quantized conductance devices for future spintronic applications.

The planar Hall effect (PHE), which originates from anisotropic magnetoresistance, presents a qualitative and simple approach to characterize electronic structures of quantum materials by applying an in-plane rotating magnetic field to induce identical oscillations in both longitudinal and transverse resistance. In this review, we focus on the recent research on the PHE in various quantum materials, including ferromagnetic materials, topological insulators, Weyl semimetals, and orbital anisotropic matters. Firstly, we briefly introduce the family of Hall effect and give a basic deduction of PHE formula with the second-order resistance tensor, showing the mechanism of the characteristic π-period oscillation in trigonometric function form with a $\frac{\pi }{4}$ phase delay between longitudinal and transverse resistance. Then, we will introduce the four main mechanisms to realize PHE in quantum materials. After that, the origin of the anomalous planar Hall effect (APHE) results, of which the curve shapes deviate from that of PHE, will be reviewed and discussed. Finally, the challenges and prospects for this field of study are discussed.

Exploration of low-dimensional semiconductors with tunable band structures is of particular interest in the applications of nano-electronics and optoelectronics. In this work, Bi(I1−xBrx)3 single crystals have been synthesized by a flux-improved physical vapor transport method, where the electronic bandgaps of these single crystals are effectively modulated by the concentration of the halide elements ratios. The first-principle calculations confirm the modulation of bandgap and reveal the orbit contributions for the conduction band minimum and valence band maximum. The properties of Bi(I1-xBrx)3-based photodetectors are measured, where a competition mechanism is identified, leading to the realization of best performance sample with a Br content of 0.18. Our results provide a route to improve the performance of BiI3-based photodetectors and to achieve controllable response spectra.