Microwave ac Conductivity Spectrum of a Coulomb Glass

We report the first observation of the transition between interacting and noninteracting behavior in the ac conductivity spectrum sigma(omega) of a doped semiconductor in its Coulomb glass state near T = 0 K. The transition manifests itself as a crossover from approximately linear frequency dependence below approximately 10 GHz, to quadratic dependence above approximately 15 GHz. The sharpness of the transition and the magnitude of the crossover frequency strongly suggest that the transition is driven by photon-induced excitations across the Coulomb gap, in contrast to existing theoretical descriptions.