Piezoelectric quantum spin Hall insulator with Rashba spin splitting in Janus monolayer $\mathrm{SrAlGaSe_4}$

Abstract

The realization of multifunctional two-dimensional (2D) materials is fundamentally intriguing, such as combination of piezoelectricity with topological insulating phase or ferromagnetism. In this work, a Janus monolayer SrAlGaSe4 is built from 2D MA2Z4 family with dynamic, mechanical and thermal stabilities, which is piezoelectric due to lacking inversion symmetry. The unstrained SrAlGaSe4 monolayer is a narrow gap normal insulator (NI) with spin orbital coupling (SOC). However, the NI to topological insulator (TI) phase transition can be induced by the biaxial strain, and a piezoelectric quantum spin Hall insulator (PQSHI) can be achieved. More excitingly, the phase transformation point is only about 1.01 tensile strain, and nontrivial band topology can hold until considered 1.16 tensile strain. Moreover, a Rashba spin splitting in the conduction bands can exit in PQSHI due to the absence of a horizontal mirror symmetry and the presence of SOC. For monolayer SrAlGaSe4, both in-plane and much weak out-of-plane piezoelectric polarizations can be induced with a uniaxial strain applied. The calculated piezoelectric strain coefficients d11 and d31 of monolayer SrAlGaSe4 are -1.865 pm/V and -0.068 pm/V at 1.06 tensile strain as a representative TI. In fact, many PQSHIs can be realized from 2D MA2Z4 family. To confirm that, similar to SrAlGaSe4, the coexistence of piezoelectricity and topological orders can be realized by strain (about 1.04 tensile strain) in the CaAlGaSe4 monolayer. Our works suggest that Janus monolayer SrAlGaSe4 is a pure 2D system for PQSHI, enabling future studies exploring the interplay between piezoelectricity and topological orders, which can lead to novel applications in electronics and spintronics.

Intrinsic room-temperature piezoelectric quantum anomalous hall insulator in Janus monolayer $\mathrm{Fe_2IX}$ (X=Cl and Br)

“…The coexistence of intrinsic piezoelectricity and ferromagnetism, namely piezoelectric ferromagnetism (PFM), has been predicted in 2D vanadium dichalcogenides, VSi 2 P 4 , CrBr 1.5 I 1.5 and InCrTe 3 [16][17][18][19] . The combination of piezoelectricity with topological insulating phase has also been achieved in monolayer InXO (X=Se and Te) 20 and Janus monolayer SrAlGaSe 4 21 . A natural idea is to search for PQAHI with piezoelectricity, topological and FM orders.…”