▎ 摘　　要

Electron spin relaxation due to the D'yakonov-Perel' mechanism is investigated in bilayer graphene with only the lowest conduction band being relevant. The spin-orbit coupling is constructed from the symmetry group analysis with the parameters obtained by fitting to the numerical calculation according to the latest report by Konschuh et al. [Phys. Rev. B 85, 115423 (2012)] from first principles. In contrast to single-layer graphene, the leading term of the out-of-plane component of the spin-orbit coupling in bilayer graphene shows a Zeeman-like term with opposite effective magnetic fields in the two valleys. This Zeeman-like term opens a spin relaxation channel in the presence of intervalley scattering. It is shown that the intervalley electron-phonon scattering, which has not been reported in the previous literature, strongly suppresses the in-plane spin relaxation time at high temperature whereas the intervalley short-range scattering plays an important role in the in-plane spin relaxation especially at low temperature. A marked nonmonotonic dependence of the in-plane spin relaxation time on temperature with a minimum of several hundred picoseconds is predicted in the absence of the short-range scatterers. This minimum is comparable to the experimental data. Moreover, a peak in the electron density dependence of the in-plane spin relaxation time at low temperature, which is very different from the one in semiconductors, is predicted. We also find a rapid decrease in the in-plane spin relaxation time with increasing initial spin polarization at low temperature, which is opposite to the situation in both semiconductors and single-layer graphene. A strong anisotropy between the out-of- and in-plane spin relaxations at high temperature is also revealed with the out-of-plane spin relaxation time being about two orders of magnitude larger than the in-plane one. Detailed comparisons of the temperature and electron density dependencies of the spin relaxation with the experiments of Yang et al. [Phys. Rev. Lett. 107, 047206 (2011)], Han and Kawakami [Phys. Rev. Lett. 107, 047207 (2011)], and Avsar et al. [Nano Lett. 11, 2363 (2011)] are reported.