A piezoelectric quantum spin Hall insulator with Rashba spin splitting in Janus monolayer SrAlGaSe4

Abstract

The realization of multifunctional two-dimensional (2D) materials is fundamentally intriguing, such as the combination of piezoelectricity with a topological insulating phase or ferromagnetism. In this work, a Janus monolayer SrAlGaSe₄ is built from the 2D MA₂Z₄ family with dynamic, mechanical and thermal stabilities, which is piezoelectric due to the lack of 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 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 the nontrivial band topology can hold until the considered 1.16 tensile strain. Moreover, Rashba spin splitting in the conduction bands can exist in a PQSHI due to the absence of a horizontal mirror symmetry and the presence of SOC. For monolayer SrAlGaSe₄, both in-plane and very weak out-of-plane piezoelectric polarizations can be induced with the application of uniaxial strain. The calculated piezoelectric strain coefficients d₁₁ and d₃₁ of monolayer SrAlGaSe₄ are −1.865 pm V⁻¹ and −0.068 pm V⁻¹ at 1.06 tensile strain as a representative TI. In fact, many PQSHIs can be realized from the 2D MA₂Z₄ family. To confirm that, similar to SrAlGaSe₄, the coexistence of piezoelectricity and topological orders can be realized by applying strain (about 1.04 tensile strain) in the CaAlGaSe₄ monolayer. This study suggests that Janus monolayer SrAlGaSe₄ is a pure 2D system for a PQSHI, enabling future studies exploring the interplay between piezoelectricity and topological order, which can lead to novel applications in electronics and spintronics.