  We present a unified theoretical framework for the study of spin dynamics and
relativistic transport phenomena in disordered two-dimensional Dirac systems
with pseudospin-spin coupling. The formalism is applied to the paradigmatic
case of graphene with uniform Bychkov-Rashba interaction and shown to capture
spin relaxation processes and associated charge-to-spin interconversion
phenomena in response to generic external perturbations, including spin density
fluctuations and electric fields. A controlled diagrammatic evaluation of the
generalized spin susceptibility in the diffusive regime of weak spin-orbit
interaction allows us to show that the spin and momentum lifetimes satisfy the
standard Dyakonov-Perel relation for both weak (Gaussian) and resonant
(unitary) nonmagnetic disorder. Finally, we demonstrate that the spin
relaxation rate can be derived in the zero-frequency limit by exploiting the
SU(2) covariant conservation laws for the spin observables. Our results set the
stage for a fully quantum-mechanical description of spin relaxation in both
pristine graphene samples with weak spin-orbit fields and in graphene
heterostructures with enhanced spin-orbital effects currently attracting much
attention.
