METAL-INSULATOR-TRANSITION IN UNCOMPENSATED SIP IN THE PRESENCE OF A MAGNETIC-FIELD

We report on detailed measurements of the electrical conductivity sigma(T) in uncompensated Si:P in a magnetic field B of 6T between 30 mK and 1K. The approximate relation sigma(T) = sigma(0) + m(B) square-root T allows a reliable extrapolation of sigma(0). The critical behaviour sigma(0) approximately (N - N(c))mu' close to the critical P concentration N(c) yields a critical exponent mu' almost-equal-to 1.3, with a tendency to smaller values at a crossover concentration which is close to the concentration where m(B) changes strongly. The implications for the previously proposed solution to the exponent puzzle for B = 0 are discussed.