Minimum of spin-orbit coupling in two-dimensional structures

Spin-orbit coupling in symmetrically doped Si/Si1-xGex quantum wells is investigated theoretically. Unavoidable fluctuations of concentration of dopant ions lead to a finite spin-orbit coupling there even if the mean dopant concentrations are equal on both sides of the well. This effect, being a realization of the minimal possible strength of spin-orbit coupling, leads to a measurable intensity of electric-dipole spin resonance, that is, to electron spin-flip transitions caused by electric field of an incident electromagnetic wave.