For insulating Si:B with dopant concentrations from 0.75 n_c to the critical concentration n_c, the conductivity ranging over five orders of magnitude collapses using a single scaling parameter T* onto a universal curve of the form σ(T)=σ0f(T∗/T) with a temperature-independent prefactor of the order of Mott's minimum metallic conductivity. The function f(T∗/T)=e−(T∗/T)β with \beta = 1/2 when T^*/T>10, corresponding to Efros-Shklovskii variable-range hopping. For T^*/T < 8 the exponent \beta = 1/3, a value expected for Mott variable-range hopping in two rather than three dimensions. The temperature-independent prefactor implies hopping that is not mediated by phonons.
@article{arxiv.cond-mat/0109222,
title = {One-Parameter Scaling of the Conductivity of Si:B: A Temperature-Independent Variable-Range Hopping Prefactor},
author = {M. P. Sarachik and P. Dai},
journal= {arXiv preprint arXiv:cond-mat/0109222},
year = {2007}
}