| Doped semiconductors are an ideal playground to study the physics of quantum impurities in a disordered environment, due to the random distribution of donor atoms in a crystalline host. We will specifically discuss Si:P where the P donors give rise to an S = 1/2 magnetic impurity. Long-range Coulomb interactions lead to Altshuler-Aronov corrections to the density of states N(EF) at the Fermi level and the electrical conductivity σ(T) on the metallic side of the MI transition, and to a soft Coulomb gap at EF and Efros-Shklovskii variable-range hopping on the insulating side. On-site Coulomb interactions, on the other hand, lead to the formation of localized magnetic moments and the Kondo effect on the metallic side, and to a Hubbard splitting of the donor band on the insulating side. The MI transition in Si:P can be tuned by varying the P concentration or - for barely insulating samples - by application of uniaxial stress S. The continuous stress tuning allows the observation of dynamic scaling of σ(T,S) and hence a reliable determination of the critical exponent μ of the extrapolated zero-temperature conductivity σ (0) ∼ |S - Sc|μ, i.e. μ = 1, and of the dynamical exponent z = 3. The issue of half-filling vs. away from half-filling of the donor band (i.e., uncompensated vs. compensated semiconductors) is discussed in detail. |
|