Interplay of symmetry breaking and bilayer coupling in Janus MSe_0.5Te_0.5 thermoelectrics

The interplay of symmetry breaking and interlayer coupling in Janus bilayers offers a powerful pathway for optimizing thermoelectric performance in two-dimensional materials. In this work, we systematically investigate Janus MSe_0.5Te_0.5 (M = Ga, In, Tl) homo-bilayers using first-principles density functional theory combined with Boltzmann transport formalism. Structural stability is confirmed through phonon dispersion and binding energy analyses, demonstrating both dynamical and energetic robustness. We find that the coupling between layers, together with broken out-of-plane mirror symmetry, induces significant modifications in lattice dynamics, leading to pronounced phonon softening, phonon branching, rattling motions, the emergence of low-frequency optical modes, and enhanced anharmonic scattering. These effects collectively suppress lattice thermal conductivity to ultra-low values of 0.99, 0.39, and 0.42 W m^-1 K^-1 for Ga-, In-, and Tl-based systems at 300 K, respectively. Simultaneously, the synergy between symmetry breaking and bilayer interaction coupled with spin-orbit coupling (SOC) effects reshapes the electronic structure by preserving the Mexican-hat dispersion near the valence band edge while introducing favourable parabolic band features. This dual modulation enhances carrier transport, yielding comparable electron and hole mobilities on the order of 10³-10⁴ cm² V^-1 s^-1. As a result of this coupled phononic and electronic optimization, the systems exhibit outstanding thermoelectric performance, with maximum figure of merit (ZT) values reaching 1.55 (n-type) for GaSe_0.5Te_0.5 and 2.24 (n-type) for InSe_0.5Te_0.5 at 800 K. Our findings underscore the critical role of the interplay between symmetry breaking and bilayer coupling in simultaneously engineering thermal and electronic transport, providing a viable design strategy for next-generation high-efficiency thermoelectric materials.