Critical transport properties of random metals in large magnetic fields.

The threshold behavior of the transport properties of a random metal in the critical region near a metal-insulator transition is strongly affected by the measuring electromagnetic fields. In spite of the randomness, the electrical conductivity exhibits striking phase-coherent effects due to broken symmetry, which greatly sharpen the transition compared with the predictions of effective medium theories, as previously explained for electrical conductivities. Here broken symmetry explains the sign reversal of the T --> 0 magnetoconductance of the metal-insulator transition in Si(B,P), also previously not understood by effective medium theories. Finally, the symmetry-breaking features of quantum percolation theory explain the unexpectedly very small electrical conductivity temperature exponent alpha = 0.22(2) recently observed in Ni(S,Se)2 alloys at the antiferromagnetic metal-insulator transition below T = 0.8 K.