  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.
