Conductivity of Metallic Si:B Near the Metal-Insulator Transition: Comparison between Unstressed and Uniaxially Stressed Samples

The low-temperature dc conductivities of barely metallic samples of ptype Si:B are compared for a series of samples with different dopant concentrations, n, in the absence of stress (cubic symmetry), and for a single sample driven from the metallic into the insulating phase by uniaxial compression, S. For all values of temperature and stress, the conductivity of the stressed sample collapses onto a single universal scaling curve, σ(S, T ) = σ0(∆S/Sc) μG[T/T ∗(S)], with T ∗ ∝ (∆S)zν . The scaling fit indicates that the conductivity of Si:B is ∝ T 1/2 in the critical range. Our data yield a critical conductivity exponent μ = 1.6, considerably larger than the value reported in earlier experiments where the transition was crossed by varying the dopant concentration. The larger exponent is based on data in a narrow range of stress near the critical value within which scaling holds. We show explicitly that the temperature dependences of the conductivity of stressed and unstressed Si:B are different, suggesting that a direct comparison of the critical behavior and critical exponents for stress-tuned and