Carrier dynamics and spin-valley-layer effects in bilayer transition metal dichalcogenides

Transition metal dichalcogenides are an interesting class of low dimensional materials in mono- and few-layer form with diverse applications in valleytronic, optoelectronic and quantum devices. Therefore, the general nature of the band-edges and the interplay with valley dynamics is important from a fundamental and technological standpoint. Bilayers introduce interlayer coupling effects which can have a significant impact on the valley polarization. The combined effect of spin-orbit and interlayer coupling can strongly modify the band structure, phonon interactions and overall carrier dynamics in the material. Here we use first-principles calculations of electron-electron and electron-phonon interactions to investigate bilayer MoS₂ and WSe₂ in both the AA' and AB stacking configurations. We find that in addition to spin-orbit coupling, interlayer interactions present in the two configurations significantly alter the near-band-edge dynamics. Scattering lifetimes and dynamic behavior are highly material-dependent, despite the similarities and typical trends in TMDCs. Additionally, we capture significant differences in dynamics for the AA' and AB stacking configurations, with lifetime values differing by up to an order of magnitude between them for MoS₂. Further, we evaluate the valley polarization times and find that maximum lifetimes at room temperature are of the scale of 1 picosecond for WSe₂ in the AB orientation. These results present a pathway to understanding complex heterostructure configurations and 'magic angle' physics in TMDCs.