Anomalous thermopower in the metalliclike phase of a 2D hole system

We report very low temperature (T) thermopower and resistivity (rho) measur ements on variable-density, two-dimensional hole systems confined to GaAs q uantum wells. As the hole density is lowered from 1.49 X 10(11) cm(-2) to 0 .14 X 10(11) cm(-2), the system crosses from an insulating (d rho /dT less than or similar to 0) to a metallic regime (d rho /dT > 0) and finally disp lays insulating behavior (d rho /dT < 0). Diffusion thermopower shows a str iking sign reversal in a narrow range of density in the metallic regime, su ggesting a qualitative change in the conduction or the scattering mechanism .