  We study hole spin resonance in a p-channel silicon metal-oxide-semiconductor
field-effect transistor. In the sub-threshold region, the measured source-drain
current reveals a double dot in the channel. The observed spin resonance
spectra agree with a model of strongly coupled two-spin states in the presence
of a spin-orbit-induced anti-crossing. Detailed spectroscopy at the
anti-crossing shows a suppressed spin resonance signal due to
spin-orbit-induced quantum state mixing. This suppression is also observed for
multi-photon spin resonances. Our experimental observations agree with
theoretical calculations.
