Evidence of two-stage melting of Wigner solids

Abstract

Ultralow carrier concentrations of two-dimensional holes down to $p=1\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ are realized. Remarkable insulating states are found below a critical density of ${p}_{c}=4\ifmmode\times\else\texttimes\fi{}{10}^{9}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ or ${r}_{s}\ensuremath{\approx}40$. Sensitive dc V-I measurement as a function of temperature and electric field reveals a two-stage phase transition supporting the melting of a Wigner solid as a two-stage first-order transition.