Low-density finite-temperature apparent insulating phase in two-dimensional semiconductor systems

We propose that the observed low-density ``insulating'' phase of a two-dimensional (2D) semiconductor system, with the carrier density being just below (n<nc) the so-called critical density where the derivative of resistivity changes sign at low temperatures [i.e., resistivity ρ(T) increases with increasing T for n>nc whereas it decreases with increasing T for n<nc], is in fact a high-temperature crossover version of the same effective metallic phase seen at higher densities (n>nc). This low density (n<nc) finite-temperature crossover 2D effective insulating phase is characterized by ρ(T) with power-law temperature dependence in contrast to the truly insulating state (occurring at still lower densities) whose resistivity increases exponentially with decreasing temperature.