  The existence of a spin-orbit coupling (SOC) induced by the gradient of the
effective mass in low-dimensional heterostructures is revealed. In structurally
asymmetric quasi-two-dimensional semiconductor heterostructures the presence of
a mass gradient across the interfaces results in a SOC which competes with the
SOC created by the electric field in the valence band. However, in graded
quantum wells subjected to an external electric field, the mass-gradient
induced SOC can be finite even when the electric field in the valence band
vanishes.
