Anomalous Thermopower in the Metalliclike Phase of a 2D Hole System

We report very low temperature ( $T$) thermopower and resistivity ( $\rho$) measurements on variable-density, two-dimensional hole systems confined to GaAs quantum wells. As the hole density is lowered from $1.49\times {10}^{11}{\mathrm{cm}}^{-2\mathrm{}}$ to $0.14\times {10}^{11}{\mathrm{cm}}^{-2\mathrm{}}$, the system crosses from an insulating ( $\frac{d\rho }{\mathrm{dT}}\lesssim 0$) to a metallic regime ( $\frac{d\rho }{\mathrm{dT}}>0\mathrm{}$) and finally displays insulating behavior ( $\frac{d\rho }{\mathrm{dT}}<0\mathrm{}$). Diffusion thermopower shows a striking sign reversal in a narrow range of density in the metallic regime, suggesting a qualitative change in the conduction or the scattering mechanism.