  Electrons in graphene are theoretically expected to retain spin states much
longer than most materials, making graphene a promising platform for
spintronics and quantum information technologies. Here, we use first-principles
density-matrix (FPDM) dynamics simulations to show that interaction with
electric fields and substrates strongly enhance spin relaxation through
scattering with phonons. Consequently, the relaxation time at room temperature
reduces from microseconds in free-standing graphene to nanoseconds in graphene
on hexagonal boron nitride (hBN) substrate, the order of magnitude typically
measured in experiments. Further, inversion symmetry breaking by hBN introduces
a stronger asymmetry in electron and hole spin lifetimes, than predicted by the
conventional D'yakonov-Perel' (DP) model for spin relaxation. Deviations from
the conventional DP model are stronger for in-plane spin relaxation, resulting
in out-of-plane to in-plane lifetime ratios much greater than 1/2 with a
maximum close to the Dirac point. These FPDM results, independent of
symmetry-specific assumptions or material-dependent parameters, also validate
recent modifications of the DP model to explain such deviations. Overall, our
results indicate that spin-phonon relaxation in the presence of substrates may
be more important in graphene than typically assumed, requiring consideration
for graphene-based spin technologies at room temperature.
