Stress Tuning of the Metal-Insulator Transition at Millikelvin Temperatures

A high-resolution scan of the metal-insulator transition in Si: P at millikelvin temperatures has been obtained by applying uniaxial stress. A sharp, but continuous, metal-insulator transition is resolved, with conductivities below Mott's "minimum" value ${\sigma }_{\mathrm{M}}$. These measurements join smoothly with previous low-resolution experiments, ruling out any discontinuity at ${\sigma }_{\mathrm{M}}$. The reproducible critical behavior disagrees with predictions of existing scaling theories of localization.