Conductivity of metallic Si : B near the metal-insulator transition: Comparison between unstressed and uniaxially stressed samples

The low-temperature de conductivities of barely metallic samples of p-type Si:B are compared for a series of samples with different dopant concentrati ons, 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 stres sed sample collapses onto a single universal scaling curve, sigma(S,T)=sigm a(0)(Delta S/S-c)(mu)G[T/T*(S)], with T*proportional to(Delta S)(ZV). The s caling fit indicates that the conductivity of Si:B is proportional to T-1/2 in the critical range. Our data yield a critical conductivity exponent mu= 1.6, considerably larger than the value reported in earlier experiments whe re the transition was crossed by varying the dopant concentration. The larg er 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 d ependences of the conductivity of stressed and unstressed Si:B are differen t, suggesting that a direct comparison of the critical behavior and critica l exponents for stress-tuned and concentration-tuned transitions may not be warranted.