One-Parameter Scaling of the Conductivity of Si:B: A Temperature-Independent Variable-Range Hopping Prefactor

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For insulating Si:B with dopant concentrations from 0.75 nc to the critical concentration nc, the conductivity ranging over five orders of magnitude collapses using a single scaling parameter T* onto a universal curve of the form σ(T) =σ₀ f (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 β= 1/2 when T*/T>10, corresponding to Efros-Shklovskii variable-range hopping. For T*/T < 8 the exponent β= 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.