Residual resistance in two-dimensional, microwave driven Hall systems

We address the origin of the residual resistance observed in microwave irradiated two-dimensional electron gases in a weak magnetic field. We study charge modulations arising from negative photo-conductivity and show that dissipative currents are exponentially suppressed. Relating the exponent to the temperature dependence of microscopic parameters taken from experiment, we find pseudo activated behaviour. In order to obtain these results it is essential to take into account the finite range of the Coulomb interaction.