Quantum-critical conductivity scaling for a metal-insulator transition

Temperature (T)- and frequency (omega)-dependent conductivity measurements are reported here in amorphous niobium-silicon alloys with compositions (x) near the zero-temperature metal-insulator transition. There is a one-to-on e correspondence between the frequency- and temperature-dependent conductiv ity on both sides of the critical concentration, thus establishing the quan tum-critical nature of the transition. The analysis of the conductivity lea ds to a universal scaling function and establishes the critical exponents. This scaling can be described by an x-, T-, and omega-dependent characteris tic length, the form of which is derived by experiment.