In most cases, a single element material is not ferroic. Here we unravel a new 2D single elemental material, Bi(110) monolayer, which has only four Bi atoms in a unit cell, is a multiferroic material with both ferroelectric and ferroelastic orders. According to a insightful model calculation (and verified by the state-of-the-art density functional theory), we reveal that such system exhibits a persistent spin helix, with uni-directional spin polarization. That is to say, in most area of the Brillouin zone, the electron/hole spin only points along the z-direction, symmetrically protected (by glide symmetry and time-reversal symmetry). Thus, the unwanted Dyakonov-Perel spin relaxation due to strong SOC effects can be totally avoided. We also predicted a polarization induced spin and valley interchange, thus proposing a polar-valley-spin locking in this system. In addition, the monolayer Bi(110) could be a good platform to measure nonlinear optical effects (nonlinear Hall effect and photovoltaic shift current), which is also ferroicity dependent. The ferroic phase transition can be realized by a noncontacting optical irradiation scheme, according to our linear response theory results.

See our paper appeared on Phys. Rev. Appl. 14, 014024 (2020), at https://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.14.014024