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The recently discovered anisotropic superconductor MgB2 is the first of its kind showing the intriguing properties of two-band superconductivity. By tunneling experiments using thin film tunnel junctions, electron-coupled phonon spectra were determined showing that superconductivity in MgB2 is phonon mediated. In a further analysis, which involves first principles calculations, the strongest feature in these spectra could be traced back to the key quantity of two-band superconductivity, the interband pairing interaction. For the phonons, this interaction turns out quite selective. It involves mainly low-energy optical phonon modes, where the boron atoms move perpendicular to the boron planes.

We reexamine the tunneling data on A15 superconductors by performing a generalized McMillan-Rowell tunneling inversion that incorporates a nonconstant electronic density of states obtained from band-structure calculations. For Nb 3 Sn, we find that the fit to the experimental data can be slightly improved by taking into account the sharp structure in the density of states, but it is likely that such an analysis alone is not enough to explain completely the superconducting tunneling characteristics of this material. Nevertheless, the extracted Eliashberg function displays a number of features expected to be present for the highest-quality Nb 3 Sn samples.

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We report that the normal-state. tunneling behavior of. Nb -oxide -Ag junctions is radically changed when very thin Al is deposited over the Nb before oxidation. From measurements of the oxidation rate, the conductance at high bias, the zero-bias anomaly, and observations of molecular excitations, we show that behavior generally associated with tunneling through niobium oxide disappears when the Al thickness is about two monolayers.The tunneling characteristics of metal -oxide -metal junctions in the normal state are largely dominated by properties of the tunneling barrier (almost always an oxide), whereas it is the superconducting properties of the electrodes that determine the changes in characteristic that occur below the superconducting transition tempertature.Thus the normal-state tunneling behavior of a junction can be used to infer the properties of the barrier oxide, at least qualitatively.For example, aluminum oxide junctions are marked by a slow but asymmetrical change in conductancewith voltage (say up to 0.5 V), ' and molecular excitations' are easily observed at certain voltages. Lead oxide exhibits strong and distinct coupling of the tunneling electrons to phonons of the oxide, ' whereas oxides of Nb and Ta show zero-bias anomalies (0 -10 mV) and a rapid increase of conductance at higher voltages (say 10 -100 mV). 4 We have made measurements of the normal-state tunneling behavior of Nb-Al -oxide -Ag (or Pb) junctions where the Al is a very thin layer, ranging in thickness from less than one monolayer up to 40 A, which is deposited before oxidation of the composite Nb-Al film. We find that the typical characteristics associated with tunneling through niobium oxide change markedly and show some of the features expected for aluminum oxide when the Al thickness exceeds about two monolayers. Although surprising to us, this result (in hindsight) is not inconsistent with previous studies of Nb covered by very thin metallic overlayers, for example Cu (Ref. 5) and Pd, 6 of Ta (Ref. 7) and V3Si (Ref. 8) covered by Al, and of Pt over Au and Au over Pt. As difficulties encountered with Nb in tunneling spectroscopy, Josephson tunneling, and accelerator cavities have been blamed on its oxidation, such surface modification by thin overlayers is of obvious interest.The studies reported here resulted from work in which tunnel junctions were prepared on metallic Nb-Al multilayers. ' These multilayer samples were made by rotating substrates 8 cm from two magnetron sputtering heads with 4.5-cm-diam Nb and Al targets 120'C apart on a 13-cm-diam circle. Films -3000 A thick had the individual layer thicknesses from 20 to 100 A. , and, during the same deposition, different samples had Nb or Al as the final layer of the deposit. These samples were then oxidized thermally. It was clear that the normal-state tunneling behavior was determined by the final layer of the multilayer film, suggesting that the oxide layer was primarily Nb oxide or Al oxide even down to layer thicknesses of 20 A. We therefore decided to determine ho...