Thermopower in the re-entrant insulating phase of a two-dimensional hole system

We report thermopower measurements on a low-disorder, dilute two-dimensional hole system in a GaAs/AlGaAs heterostructure exhibiting a re-entrant insulating phase around filling factor ν=1/3. As the temperature is lowered, the thermopower vanishes at the integral and fractional quantum Hall states but diverges in the insulating phase, suggesting that an energy gap separates the insulating ground state from its conducting excitations. Our results are consistent with the condensation of the holes into a pinned Wigner solid and, from the onset of the divergence of the thermopower, we derive a phase diagram for the solid-liquid transition.