Sharp Metal-Insulator Transition in a Random Solid

Zero-temperature metallic conductivities have been measured above and below Mott's minimum value ${\sigma }_{min}$ in bulk crystals of P-doped Si. Conductivities below ${\sigma }_{min}$ increase by over ${10}^3$ as the density is raised by less than 1%, and do not rule out a discontinuous transition. However, over a wider density range the data can be fitted with a scaling form with a characteristic length that tends to diverge with the same exponent in the metal and insulator.