HOPPING CONDUCTION NEAR THE METAL-INSULATOR-TRANSITION IN CDMNTE - INAND EVIDENCE FOR A MAGNETIC HARD GAP IN THE DENSITY-OF-STATES

A series of resistivity curves is obtained as a function carrier conce ntration in just one sample of the dilute magnetic persistent photocon ductor Cd0.91Mn0.09Te:In. The measurements, made at carrier concentrat ions approaching the metal-insulator transition, reveal a cross-over f rom an exp(T(o)/T)1/2 dependence to an exp(E(H)/T) form when lowering temperature. The exp(T(o)/T)1/2 dependence is characteristic of variab le range hopping in the presence of Coulomb interactions, while the en ergy E(H) in the activated form is associated with a hard gap in the d ensity of states that is magnetic in origin. All the data are shown to scale onto a single curve. The localization length, xi, is found to h ave the same critical dependence on carrier concentration as that of t he measured dielectric constant, kappa, when approaching the metal-ins ulator transition. The temperature dependence of the resistivity is in terpreted in terms of the orientation of Mn spins by carriers due to t he s-d exchange interaction (the formation of magnetic polarons). This model can also account for the large positive and negative magnetores istance observed in Cd0.91Mn0.09Te:ln at low temperatures.