Tunable Metal–Insulator\nand Topological Phase\nTransitions in Piezoelectric Janus Monolayers: Theoretical Simulations\nwith Implications for Reconfigurable Strain-Biased Electronic Devices

Quantum\nspin Hall (QSH) insulators represent a quintessential\nexample\nof a topological phase of matter, characterized by a conducting edge\nmode existing within a bulk energy gap. The pursuit of a tunable QSH\nstate stands as a pivotal objective in the development of QSH-based\ntopological devices. In this study, we employ first-principles calculations\nto identify three strain-tunable quantum spin Hall (QSH) insulators\nbased on monolayer MAlGaTe<sub>4</sub> (where M represents Mg, Ca,\nor Sr). These monolayers exhibit dynamic stability, with no imaginary\nmodes detected in their phonon dispersion. While MgAlGaTe<sub>4</sub> is a normal insulator under zero strain, it transitions into the\nQSH phase when subjected to external strains. Conversely, CaAlGaTe<sub>4</sub> and SrAlGaTe<sub>4</sub> already exhibit the QSH phase at\nzero strain. Intriguingly, upon the application of biaxial strain,\nthese two compounds undergo phase transitions, encompassing metallic\n(M), normal/trivial insulator (NI), and topological insulator (TI)\nphases, thereby illustrating their strain-tunable electronic and topological\nproperties. (Ca, Sr)AlGaTe<sub>4</sub>, in particular, undergo M-TI/TI-M\ntransitions under applied strain, while MgAlGaTe<sub>4</sub> additionally\nexperiences an M-NI/NI-M transition, signifying it as a material featuring\na metal–insulator transition (MIT). Remarkably, the observation\nof metal-trivial insulator-topological insulator transitions in MgAlGaTe<sub>4</sub> introduces it as a unique material platform in which both\nMIT and topological phase transitions can be controlled through the\nsame physical parameter. Moreover, the piezoelectric properties of\nthese materials enable strain-induced tuning, allowing them to be\nstrain-biased for application in novel devices such as reconfigurable\ncircuit components for compact electronic devices and topological\nfield-effect transistors. Our study thus introduces a novel material\nplatform distinguished by highly strain-tunable electronic and topological\nproperties, offering promising prospects for the development of next-generation,\nlow-power topological devices.