Intermediate-frequency conduction in Si:As below the metal-insulator transition

The electrical conductivity of Si:As has been measured from 0.1 to 2 GHz in the temperature range 1 to 4 K. At donor densities ≲6.4×${10}^{18}$ As/${\mathrm{cm}}^3$, the conductivity approached a temperature-independent value σ(ω) which exhibited a superlinear frequency dependence [σ(ω)∞${\mathrm{\omega }}^{\mathrm{s}}$, s>1] for sufficiently low donor density. The superlinear dependence is not in agreement with a Shklovskii-Efros prediction in the regime ħω≪${\mathrm{e}}^2$/ε${\mathrm{r}}_{\mathrm{\omega }}$. Although in qualitative agreement with the Mott ${\mathrm{\omega }}^2$ law, the density variation of s(n) is much more rapid than theoretical predictions. Photon-assisted correlated multiple-electron absorption of large clusters is considered as a possible explanation of the results.