Unusual anisotropy of inplane field magnetoresistance in ultra-high mobility SiGe/Si/SiGe quantum wells

We find an unusual anisotropy of the inplane field magnetoresistance in ultra-high mobility SiGe/Si/SiGe quantum wells. The anisotropy depends on the orientation between the inplane field, $B_{\parallel }$, and current, $I$, relative to the crystallographic axes of the sample and is a consequence of the intrinsic ridges on the quantum well surface. For the simplest orientations between current and crystallographic axes, a method of recalculating the magnetoresistance measured at $I\perp B_{\parallel }$ into the one measured at $I\parallel B_{\parallel }$ is suggested and is shown to yield results that agree with the experiment.