Evidence for a magnetic hard gap in the density of states of a magnetic semiconductor near the metal insulator transition (abstract)

Abstract

A family of resistivity curves with carrier concentrations approaching the metal insulator transition is obtained in a single sample of the dilute magnetic persistent photoconductor Cd0.91Mn0.09Te:In. In zero applied magnetic field, these curves exhibit a cross-over from an exp[(T0/T)1/2] for variable range hopping with interactions to an exp(EH/T) form upon reducing temperature. The energy EH is associated with a hard gap in the density of states which is magnetic in origin. All of the data for different carrier concentrations are shown to scale on to a single curve.1 The localization length is found to have the same critical dependence as the electronic part of the dielectric constant which has been previously measured.2 In a magnetic field, the hard gap is suppressed, and only variable range hopping is observed. The orientation of the Mn spins by the carriers contributes both to the localization (bound magnetic polaron) and is the relaxation responsible for the hard gap. A large peak in the magnetoresistance is also understood within this model. The localization length ξ and hence T0=2.8e2/κξk and the resistivity are field dependent. The magnetic field first aids the BMP formation and then destroys it. The result is first a peak and then in some cases negative magnetoresistance.