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×{}10¹⁸ As/cm³, the conductivity approached a temperature-independent value {σ}({ω}) which exhibited a superlinear frequency dependence [{σ}({ω}){∞}ωˢ, s>1] for sufficiently low donor density. The superlinear dependence is not in agreement with a Shklovskii-Efros prediction in the regime {}{ω}{}e²/{ε}r_ω. Although in qualitative agreement with the Mott ω² 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.