  The manipulation of the Rashba spin-splitting is crucial for the development
of nanospintronic technology. Here, it is proposed that the Rashba
spin-splitting can be turned on and off by reversing electric-field direction.
By the first-principle calculations, our proposal is illustrated by a concrete
example of Janus monolayer RbKNaBi. The designed RbKNaBi possesses dynamical,
thermal and mechanical stability, and is a large-gap quantum spin Hall
insulator (QSHI) with Rashba spin-splitting near the Fermi level. A small
built-in electric field is predicted due to very small electronegativity
difference between the bottom and top atoms, which is very key to switch Rashba
spin-splitting through the experimentally available electric field intensity.
Due to out-of-plane structural asymmetry, the Janus monolayer has distinctive
behaviors by applying external electric field $E$ with the same magnitude but
different directions ($z$ or $-z$). Our results reveal that the Rashba energy
($E_R$) and Rashba constant ($\alpha_R$) are increased by the positive $E$,
while a negative $E$ suppresses the Rashba splitting to disappear, and then
appears again. In a certain $E$ region (0.15 $\mathrm{V/\AA}$ to 0.25
$\mathrm{V/\AA}$), switching Rashba spin-splitting can be achieved by only
reversing electric-field direction. Besides, the piezoelectric strain
coefficients $d_{11}$ and $d_{31}$ (5.52 pm/V and -0.41 pm/V) are predicted,
which are higher than or compared with those of many 2D materials. By
piezoelectric effect, the strain can also be used to tune Rashba spin-splitting
of RbKNaBi. Moreover, a possible spintronic device is proposed to realize the
function of spintronic switch.
