  The authors investigate the magnetic field dependence of the energy splitting
between low-lying valley states for electrons in a Si/SiGe quantum well tilted
with respect to the crystallographic axis. The presence of atomic steps at the
quantum well interface may explain the unexpected, strong suppression of the
valley splitting observed in recent experiments. The authors find that the
suppression is caused by an interference effect associated with multiple steps,
and that the magnetic field dependence arises from the lateral confinement of
the electronic wave function. Using numerical simulations, the authors clarify
the role of step disorder, obtaining quantitative agreement with the
experiments.
