Broken symmetry and critical transport properties of random metals

Recent experimental data on the conductivity σ(+)(T), T → 0, on the metallic side of the metal–insulator transition in ideally random (neutron transmutation-doped) (70)Ge:Ga have shown that σ(+)(0) ∝ (N − N(c))(μ) with μ = ½, confirming earlier ultra-low-temperature results for Si:P. This value is inconsistent with theoretical predictions based on diffusive classical scaling models, but it can be understood by a quantum-directed percolative filamentary amplitude model in which electronic basis states exist which have a well-defined momentum parallel but not normal to the applied electric field. The model, which is based on a new kind of broken symmetry, also explains the anomalous sign reversal of the derivative of the temperature dependence in the critical regime.