Tunable Metal-Insulator and Topological Phase Transitions in Piezoelectric Janus Monolayers: Theoretical Simulations with Implications for Reconfigurable Strain-Biased Electronic Devices

Quantum spin Hall (QSH) insulators represent a quintessential example of a topological phase of matter, characterized by a conducting edge mode existing within a bulk energy gap. The pursuit of a tunable QSH state stands as a pivotal objective in the development of QSH-based topological devices. In this study, we employ first-principles calculations to identify three strain-tunable quantum spin Hall (QSH) insulators based on monolayer MAlGaTe₄ (where M represents Mg, Ca, or Sr). These monolayers exhibit dynamic stability, with no imaginary modes detected in their phonon dispersion. While MgAlGaTe₄ is a normal insulator under zero strain, it transitions into the QSH phase when subjected to external strains. Conversely, CaAlGaTe₄ and SrAlGaTe₄ already exhibit the QSH phase at zero strain. Intriguingly, upon the application of biaxial strain, these two compounds undergo phase transitions, encompassing metallic (M), normal/trivial insulator (NI), and topological insulator (TI) phases, thereby illustrating their strain-tunable electronic and topological properties. (Ca, Sr)AlGaTe₄, in particular, undergo M-TI/TI-M transitions under applied strain, while MgAlGaTe₄ additionally experiences an M-NI/NI-M transition, signifying it as a material featuring a metal-insulator transition (MIT). Remarkably, the observation of metal-trivial insulator-topological insulator transitions in MgAlGaTe₄ introduces it as a unique material platform in which both MIT and topological phase transitions can be controlled through the same physical parameter. Moreover, the piezoelectric properties of these materials enable strain-induced tuning, allowing them to be strain-biased for application in novel devices such as reconfigurable circuit components for compact electronic devices and topological field-effect transistors. Our study thus introduces a novel material platform distinguished by highly strain-tunable electronic and topological properties, offering promising prospects for the development of next-generation, low-power topological devices.