Evidence of two-stage melting of Wigner solids

Ultralow carrier concentrations of two-dimensional holes down to p=1×10⁹4pt0excm^-2 are realized. Remarkable insulating states are found below a critical density of pc=4×10⁹4pt0excm^-2 or rₛ≈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.