A comprehensive study of the energy, temperature, and magnetic field dependence of the anomalous Kondo and spin-flip conductances G(3)(V) and G(2)(V), in vacuum-cleaved metal-semiconductor tunnel junctions is reported. Localized Anderson magnetic moments are characteristic of Schottky barriers at donor concentrations a few times the Mott critical concentration Nc. These moments, coupled to the conduction electrons in the semiconductor by the s-d exchange interaction -2JS→•σ→, are lightly screened neutral donors at the inner edge of the depletion region. The background conductance in vacuum-cleaved junctions on Si: (1.6×1019 cm-3 donors) agrees satisfactorily with the parabolic barrier model, extended to the thin-barrier limit. A detailed study shows the energy dependence of the zero-bias Kondo scattering peak G(3)(V) to be in good agreement with the third-order perturbation theory of Kondo and Appelbaum. High magnetic field studies confirm that a large negative g shift Δg=2JρF∼-1. 0 and related broadening Γ=π(JρF)2gμBH of the Zeeman transition of the local moment occur via the exchange interaction. The broadening Γ additionally produces, for ΓkBT, a quenching of the Kondo scattering peak from -logT to -log(ΓkB). The directly measured parameters JρF and E0 determine a divergence temperature TK=(E0kB)×exp(1JρF)=(2±1)°K, significantly higher than an upper bound ∼0.5°K, indicated by the observed accurate logT dependence of G(3)(0) to 0.4°K. The discrepancy is removed by assuming simultaneous potential and exchange scattering in a ratio determined via Appelbaum's theory from the observed ratio G(3)G(2). In summary, the tunneling spectra and an extended Kondo-Appelbaum perturbation theory, including the implied g shift, broadening Γ, and an added potential scattering, are in good agreement.
ASJC Scopus subject areas
- Condensed Matter Physics