Transport Theory of Monolayer Transition-Metal Dichalcogenides through Symmetry

We present a theory that elucidates the major momentum and spin relaxation processes for electrons, holes, and hot excitons in monolayer transition-metal dichalcogenides. We expand on spin flips induced by flexural phonons and show that the spin relaxation is ultrafast for electrons in free-standing membranes while being mitigated in supported membranes. This behavior due to interaction with flexural phonons is universal in two-dimensional membranes that respect mirror symmetry, and it leads to a counterintuitive inverse relation between mobility and spin relaxation.