Correlation between strain tunable piezoelectricity and Rashba effect in flexible Janus Ga2Ge2XY (X, Y = S, Se, and Te) monolayers with high carrier mobility

Broken inversion symmetry in Janus structures leads to superior physical properties due to the emergence of a non-zero dipole moment. Ga2Ge2XY (X, Y = S, Se, Te) monolayers have been explored for their vibrational, thermal, elastic, piezoelectric and electronic properties and additionally, Rashba effect, via first-principles methods. All the phonon modes in Janus monolayers are Raman active, contrary to their pristine counterparts. The variation of isochoric heat capacity at low temperatures is quadratic, signalling the dominant contribution from the longitudinal acoustic phonon modes, analogous to the MoS2 monolayer. Young's modulus lower than that for transition metal dichalcogenides suggests greater flexibility. Janus monolayers exhibit strain tunable piezoelectricity, Rashba-spin splitting and high electron mobility. The trend in piezoelectricity and Rashba parameter follow that of vertical dipole moment (μz), electronegativity difference ratio (rΔEN) and are linearly correlated with the out-of-plane electrostatic potential difference (ΔV) between the outer layer atoms. The latter can serve as a potential descriptor for piezoelectricity and Rashba parameter. The small strain tunability of band gap, piezoelectric coefficient and Rashba parameter broadens their spectrum of applications. Our work showcases the potential of Ga2Ge2XY monolayers in the domain of piezotronics, spintronics, flexible electronics, flexible piezo-spintronics and may trigger experimental works.