Anomalous magneto-oscillations at very low magnetic fields in ultra-high-mobility GaAs two-dimensional hole systems

GaAs two-dimensional (2D) holes are a promising platform for the study of many-body phenomena thanks to their large effective mass which implies a smaller Fermi energy and thus strong interaction between carriers. In addition, due to the strong spin-orbit interaction, 2D holes in GaAs demonstrate a tunable spin-orbit-induced subband splitting, anisotropic Fermi contours as well as nonlinear Landau levels, adding further complexity to their band structure. Here we present low-temperature magneto-transport measurements of an ultra-high-mobility 2D hole system confined to a symmetric (001) GaAs quantum well. The measurements reveal anomalous oscillations in magneto-resistance, periodic in 1/B, persisting beyond the damping of the Shubnikov-de Haas (SdH) oscillations at low fields. The frequency of these oscillations appears to be related to the difference between the SdH frequency for the two spin-orbit-split, heavy-hole subbands. However, the amplitude of the low-field oscillations does not match what is observed in the SdH oscillations. We discuss possible origins of these features.