Ultra-high magnetic field study of the Kondo-type zero-bias conductance peak in magnetically doped metal-insulator-metal tunnel junctions

Abstract

We have measured the zero-bias conductance peak in $\mathrm{Al}\ensuremath{-}I\ensuremath{-}\mathrm{Al}$ tunnel junctions with Fe impurities incorporated into the insulator in magnetic fields up to 181 kG and at temperatures from 4.2 to 0.4 K. In zero field, the voltage and temperature dependence of the peak fits the Appelbaum theory quite well, providing a direct measure of the Kondo scattering amplitude in the perturbational limit. In high field, we have made a very careful comparison of the data to the Appelbaum theory modified to include the possibility of magnetic-field-induced lifetime broadening of the kind suggested by Wolf and Losee. We find that although it is possible at any given field to obtain a reasonable fit to the experimental line shapes, the fit at other fields becomes poor. The observed curves do exhibit an increased broadening with increasing magnetic field, but the deduced values of the broadening parameter $\ensuremath{\Gamma}$ versus field do not agree with the predicted broadening based upon the measured $g$ shift. The broadening can quantitatively account for the temperature dependence of the conductance in a given field, but fails to predict the observed dependence of the magnetoconductance at zero bias, in particular, the lack of saturation at very high magnetic field. No evidence is found for the existence of the type of triangular well observed in metal-semiconductor tunnel junctions at low temperatures.