Stress-tuned metal-insulator transition in Si:B: dynamical scaling

Uniaxial stress S was used to tune the critical density of barely metallic Si:B at low temperatures through the metal-insulator transition. For all values of stress and temperature, the DC conductivity exhibits an excellent fit to the scaling form σ(S,T) = AT^μ/zv f[(S - S_c)/T^1/zv] on both sides of the transition. Finite temperature scaling yields a critical conductivity exponent μ = 1.6, considerably larger than the value obtained in experiments where the transition was crossed by varying the dopant concentration. Direct comparison of the temperature dependence of stressed and unstressed Si:B suggests that the stress-driven and concentration-driven metal-insulator transitions are not equivalent. We show that the conductivity in the critical region on the metallic side follows σ_c ∝ T ^1/2 , the form that has been calculated and observed in the perturbative region for weakly disordered metals. The conductivity in the insulating phase follows the Efros-Shklovskii form, σ(S,T) ∝ exp([-(S - S_c)^zv/T] ^1/2 ), expected for variable-range hopping in the presence of Coulomb interactions, with a prefactor ∝ T ^1/2 corresponding to the temperature dependence of the critical curve.