Transport anisotropy in Ge quantum wells in the absence of quantum oscillations

A recent study of a high-mobility two-dimensional hole gas in a strained Ge quantum well revealed strong transport anisotropy in the quantum Hall regime when the magnetic field was tilted away from the sample normal [Q. Shi, M. A. Zudov, C. Morrison, and M. Myronov, Phys. Rev. B 91, 201301(R) (2015)PRBMDO1098-012110.1103/PhysRevB.91.201301]. In the present study we demonstrate that the anisotropy persists to such high temperatures and filling factors that quantum oscillations are no longer observed. This finding rules out the formation of a stripe phase as a possible origin for the observed anisotropy. However, we also show that the observed anisotropy is not consistent with other known anisotropies, such as those arising from finite-thickness effects or surface roughness.