Tunneling Study of Niobium Using Aluminum-Aluminum Oxide-Niobium Juctions: Fid-Bau Portal

Tunneling Study of Niobium Using Aluminum-Aluminum Oxide-Niobium Juctions

Robinson, Bennett / Geballe, T. H. / Rowell, J. M.

Abstract Given the success of the tunneling techique in the determination of the important superconducting parameters, [Δ(ω), α2 (ω) F(ω), λ, <ω> etc.] in the s-p metals [1], it was natural to hope that similarly detailed studies could be made of the transition metals, where it would be particularly valuable to have measurements of these parameters for families of alloys traversing the well-known peaks in Tc versus electron per atom ratio. As outlined by Shen [2] in the previous Rochester conference, only in the case of tantalum has an entirely satisfactory tunneling study of a transition metal been possible [3]. This is still true today, despite considerable effort in the intervening years. In the case of niobium, investigators have used cleaner and cleaner bulk samples and employed ultrahigh vacuum systems to outgas bulk samples for long periods of time at elevated temperatures before preparation of the tunnel junctions using, as the insulating barrier, the oxide layer grown on these Nb surfaces. Extensive studies of this kind have been carried out by MacVicar and coworkers [4] and by Gartner [5]. In this paper we first present a speculation, based on existing studies of the oxidation of clean Nb, as to why limited progress has been made with tunnel junctions prepared on bulk Nb. We then describe an alternative approach which relies on evaporation of a Nb film over an A1 counterelectrode, thus using aluminum oxide as the tunneling barrier. We will show that the results obtained illustrate the importance of ultrahigh vacuum in the preparation of the Nb film and that initial tunneling results are extremely encouraging. This system allows not only the study of annealed Nb films, but also those deposited at low temperature (1.5 K) and at all stages of their anneal.