Electrical conductivity and Wigner crystallization

We have measured the temperature dependence of both the dc resistance and the rf conductivity for several high quality two-dimensional GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As electron samples in the high-magnetic-field Wigner-crystallization regime. We have found an increase in the activation energy of the dc resistance at the temperature where the dynamical conductivity becomes frequency dependent. We interpret this temperature as the melting temperature of the Wigner crystal and discuss the conduction mechanisms on both sides of the transition.