Metal-Insulator Transition in Uncompensated Si:P in the Presence of a Magnetic Field

We report on detailed measurements of the electrical conductivity σ( T ) in uncompensated Si:P in a magnetic field B of 6 T between 30 mK and 1 K. The approximate relation σ( T ) = σ(0) + m B √ T allows a reliable extrapolation of σ(0). The critical behaviour σ(0) ∼ ( N - N e ) μ' close to the critical P concentration N e yields a critical exponent μ' ≈ 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.