Zero-Bias Conductance Peak in Al/AlOx/Sc Tunnel Junctions

We have fabricated a series of Al/AlO$_{x}$/Sc tunnel junctions and measured the differential conductances at low temperatures. 25-nm thick Al (99.999{\%}) stripes were first thermally evaporated onto a glass substrate, followed by glow discharge under an O$_{2}$ atmosphere, to form a thin insulating AlO$_{x}$ layer. Subsequently, a 60-nm thick Sc (99.99{\%}) film was thermally evaporated across the oxidized Al stripes to form tunnel junctions of 1 mm $\times $ 1 mm. Lock-in techniques were used to measure the differential conductances dI/dV(G) of the junctions. Zero-bias conductance peaks were found in all the tunnel junctions. In particular, the magnitudes of the zero-bias conductance peaks reveal a -ln$T$ dependence below about 30 K, which could be attributed to the electron-magnetic impurities interactions according to the theory of Appelbaum. However, the magnetic field has only a small effect on the conductance peaks. An asymmetric term in G(V) was observed, which is strongly temperature dependent and magnetic-field insensitive. Possible explanations will be discussed.