  Compared with direct-gap semiconductors, the valley degeneracy of silicon and
germanium opens up new channels for spin relaxation that counteract the spin
degeneracy of the inversion-symmetric system. Here the symmetries of the
electron-phonon interaction for silicon and germanium are identified and the
resulting spin lifetimes are calculated. Room-temperature spin lifetimes of
electrons in silicon are found to be comparable to those in gallium arsenide,
however, the spin lifetimes in silicon or germanium can be tuned by reducing
the valley degeneracy through strain or quantum confinement. The tunable range
is limited to slightly over an order of magnitude by intravalley processes.
