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

We have investigated commensurability oscillations in the magnetoresistance s of two-dimensional lateral surface superlattices with square patterns and periods of 100 nm. In some of our samples the symmetry of the potential wa s broken by the presence of stress and strong piezoelectric effects. Oscill ations were weak in symmetric samples, but became much stronger for transpo rt along the [01(1) over bar] direction [on a (100) wafer] when the symmetr y was broken. For transport along the [010] and [001] directions in the asy mmetric samples, the dominant Fourier component in the potential was at an angle of 45 degrees to the transport direction, and the commensurability os cillations had an effective period of 100/root 2 nm. All of these observati ons are fully in accord with a recent semi-classical theory based on the gu iding center drift concept.