First-Principles Calculations of Coexisting Quantum-Spin Hall Effect and Intrinsic Piezoelectricity in PbGeH and PbSnH Monolayers: Implications for Spintronic Devices

Abstract In this work, PbXH (X=Ge, Sn) monolayers are predicted to be ideal piezoelectric topological insulators (TIs) that combine the quantum spin Hall effect (QSHE) and intrinsic piezoelectric (PE). Owing to their P6m2 point group symmetry, PbXH (X=Ge, Sn) monolayers exhibit in-plane piezoelectricity with d11 values of 6.676 and 5.370 pm/V, respectively. These values are significantly larger than those of recently reported piezoelectric TIs, such as SrAlGaSe4 (d11=1.865 pm/V) and GaTeS (d11=3.988 pm/V). The spin-orbit coupling (SOC) induced topological bandgaps in PbXH (X=Ge, Sn) monolayers reach up to 0.314 eV, far exceeding thermal broadening at room temperature. Furthermore, we designed a Janus Pb2SnGeH2monolayer to introduce out-of-plane piezoelectricity together with a significant Rashba SOC effect, with the Rashba parameter reaching up to 3.11 eVÅ. The Rashba parameter is the largest among the currently known piezoelectric TI materials. By applying biaxial strain from −6% to 6% to the PbXH (X=Ge, Sn) and Pb2SnGeH2monolayers, their topological bandgaps exhibit tunable properties, while maintaining the one-dimensional topological edge states, showcasing the robustness of the topological phase. This work proposes a class of two-dimensional (2D) piezoelectric TIs that combine the QSHE with intrinsic PE, providing potential applications in low-power piezoelectric spintronic devices.