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 Sd decreases linearly with temperature at low B for both low-mobility [01 1̄] and high-mobility [2̄ 33] directions, as expected for metallic systems. At high B, in the insulating phase, Sd changes sign along [01 1̄], while Sd drops to zero along [2̄ 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.