Importance of symmetry breaking in two-dimensional lateral-surface superlattices

We have investigated commensurability oscillations in the magnetoresistances of two-dimensional lateral surface superlattices with square patterns and periods of 100 nm. In some of our samples the symmetry of the potential was broken by the presence of stress and strong piezoelectric effects. Oscillations were weak in symmetric samples, but became much stronger for transport along the $[011\ifmmode\bar\else\textasciimacron\fi{}]$ direction [on a (100) wafer] when the symmetry was broken. For transport along the $[010]$ and $[001]$ directions in the asymmetric samples, the dominant Fourier component in the potential was at an angle of $45\ifmmode^\circ\else\textdegree\fi{}$ to the transport direction, and the commensurability oscillations had an effective period of $100/\sqrt{2}$ nm. All of these observations are fully in accord with a recent semi-classical theory based on the guiding center drift concept.