Hexagonal warping effect in the Janus group-VIA binary monolayers with large Rashba spin splitting and piezoelectricity

Abstract

In this paper, the electronic band structure, Rashba effect, hexagonal warping, and piezoelectricity of Janus group-VIA binary monolayers STe₂, SeTe₂, and Se₂Te are investigated based on density functional theory (DFT). Due to the inversion asymmetry and spin–orbit coupling (SOC), the STe₂, SeTe₂ and Se₂Te monolayers exhibit large intrinsic Rashba spin splitting (RSS) at the Γ point with the Rashba parameters 0.19 eV Å, 0.39 eV Å, and 0.34 eV Å, respectively. Interestingly, based on the k·p model via symmetry analysis, the hexagonal warping effect and a nonzero spin projection component S_z arise at a larger constant energy surface due to nonlinear k³ terms. Then, the warping strength λ was obtained by fitting the calculated energy band data. Additionally, in-plane biaxial strain can significantly modulate the band structure and RSS. Furthermore, all these systems exhibit large in-plane and out-of-plane piezoelectricity due to inversion and mirror asymmetry. The calculated piezoelectric coefficients d₁₁ and d₃₁ are about 15–40 pm V⁻¹ and 0.2–0.4 pm V⁻¹, respectively, which are superior to those of most reported Janus monolayers. Because of the large RSS and piezoelectricity, the studied materials have great potential for spintronic and piezoelectric applications.