In-plane magnetic-field-induced metal-insulator transition in (311)A GaAs two-dimensional hole systems probed by thermopower

We report thermopower measurements in dilute (311)A GaAs two-dimensional holes subjected to an in-plane magnetic field B that drives the system through a metal-insulator transition (MIT). The diffusion thermopower S^d decreases linearly with temperature at low B for both low-mobility [011̅ ] and high-mobility [2̅ 33] directions, as expected for metallic systems. At high B, in the insulating phase, S^d changes sign along [011̅ ], while S^d drops to zero along [2̅ 33]. This behavior suggests that the system does not undergo any ground-state modification but, rather, that the apparent MIT transition is accompanied by a dramatic change in the dominant scattering mechanisms.