Low Temperature Differential Conductance in Al/AlOx/Sc Tunnel Junctions

We have fabricated several Al/AlO$_{x}$/Sc tunnel junctions and measured the differential conductances $G\equiv dI/dV$ at low temperatures. Our objective is to study the effect of the coupling between tunneling electrons and localized magnetic impurities (which sat in the insulating barrier) on $G(V,T)$. We observed a crossover from the weak coupling regime to the strong coupling regime. We found that, in both regimes, the $dI/dV$ spectra could be well described by the Appelbaum's $s$-$d$ exchange interaction theory, with a Kondo temperature $T_{K}^{Appelbaum} \approx 34.8$ K. On the other hand, our measured zero-bias conductances could be well described by a scaling form predicted by the NRG calculations and a $T_{K}^{NRG} \approx$ 38 K was deduced, being in close agreement with the value of $T_{K}^ {Appelbaum}$. A magnetic field of 4 T was applied at 2.5 K, but no Zeeman splitting in the $dI/dV$ spectra was observed. This absence of Zeeman splitting resulted as a consequence of the high $T_{K}$ value found in our junctions.