ANHARMONIC PERIODIC MODULATION IN LATERAL SURFACE SUPERLATTICES

We have measured the longitudinal magnetoresistance of a lateral surfa ce superlattice with a period of 270 nm where the electrons are only 2 8 nm deep. The commensurability oscillations have a strong second harm onic content. This reflects a non-sinusoidal potential in the two-dime nsional electron gas, a consequence of the shallow structure. The shap e of the potential cannot be explained by a pinned GaAs surface and in dicates that the surface charge is frozen or that the electrons feel a n elastic strain field from the metal gates.