Variation of the Metallic Onset with Magnetic Field in Doped Germanium

We demonstrate substantial magnetic-field tuning of the metal-insulator transition in Ge:Sb for H < 20 kOe. From a fit to the conductivity in a series of samples at millikelvin temperatures in different fields to the critical form σ ∼ ( n/n c ( H ) - 1) μ , we find that μ ≈ 1 independent of H , but that n c ( H ) increases more than 25% by H ∼ 20 kOe. Moreover, n c ( H ) - n c (0) ∝ H 1/2 , in agreement with a weak-disorder perturbative scaling approach, but in contrast to recent experimental results in Si:As.