Toggling Valley-Spin Locking and Nonlinear Optical Properties of Single-Element Multiferroic Monolayers via Light

Ferroicities in structural ferroic materials can effectively tune many different fascinating physical properties, leading to potential applications in the future. Usually, in order to break inversion symmetry in ferroelectrics, the material needs to be composed of multiple types of element. Here, we study a single element multiferroic material with time-reversal symmetry, monolayer Bi(110), which possesses multiple types of ferroic (ferroelectric and ferroelastic) order. By performing first-principles calculations combined with a group-theory analysis, we reveal that this multiferroic material exhibits various interesting properties, such as an out-of-plane persistent spin helix, valleytronics, and promising nonlinear optical properties, which depend on its orientation variant. We show further that under illumination with linearly polarized light, different orientation variants in the Bi(110) monolayer possess different free energies. Thus, one can use this noncontacting approach to control the thermodynamic stability of different types of order. The switching of the orientation variant is ultrafast and can occur in times on the order of picoseconds. During this process, no electron-hole pairs are generated. Therefore, one can eliminate waste heat during operation, ensuring good reversibility.