The low-temperature dc conductivities of barely metallic samples of $p$-type 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, $S.\mathrm{}$ For all values of temperature and stress, the conductivity of the stressed sample collapses onto a single universal scaling curve, $\sigma \mathrm{}(S,T)={\sigma }_0\left(\Delta {S/S}_c{)}^{\mu }G\right[{T/T}^{*}\mathrm{}\left(S\right)\mathrm{}],$ with $T^{*}\propto (\Delta {S)}^{z\nu }.$ The scaling fit indicates that the conductivity of Si:B is $\propto T^{1/2}$ in the critical range. Our data yield a critical conductivity exponent $\mu =1.6,$ considerably larger than the value reported in earlier experiments 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 concentration-tuned transitions may not be warranted.

• Received 27 October 1998

DOI:https://doi.org/10.1103/PhysRevB.60.2292