Anomalous Effective Mass of Two-dimensional Holes in a Strong Parallel Magnetic Field

We report effective hole mass (m*) measurements through analyzing the temperature dependence of the Shubnikov-de Haas oscillations in dilute (density $\sim $ 5x10$^{10}$cm$^{-2})$ two-dimensional (2D) hole systems confined to a 20nm-wide, (311)A GaAs quantum well. In this system the 2D holes occupy two spin-subbands whose m* we measure to be $\sim $ 0.2 (in units of free electron mass), in good agreement with the theoretical band calculations. We then apply a sufficiently strong ($>$10T) parallel magnetic field to fully depopulate one of the spin subbands, and measure m* for the populated subband. We find that this latter m* is close in magnitude to the m* we measure in the absence of the parallel field. This is a surprising observation as it is in stark disagreement with the results of our band calculations which take into account the spin-orbit interaction and the holes' finite layer thickness, and predict a large enhancement of m* in a strong parallel magnetic field.