We have conducted temperature and frequency dependent transport measurements in amorphous ${\mathrm{Nb}}_x{\mathrm{Si}}_{1-x}$ samples in the insulating regime. We find a temperature dependent dc conductivity consistent with variable range hopping in a Coulomb glass. The frequency dependent response in the millimeter-wave frequency range can be described by the expression $\sigma \left(\omega \right)\propto (-i\omega {)}^{\alpha }$ with the exponent somewhat smaller than 1. Our ac results are not consistent with extant theories for the hopping transport.

• Received 8 March 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.116602