A15 NIOBIUM-TIN: FABRICATION AND SUPERCONDUCTIVE TUNNELING SPECTROSCOPY

The material and superconducting properties of A15 phase niobium-tin have been studied by tunneling spectroscopy. The ability to fabricate high-quality tunnel junctions on transition-metal compounds has been made possible by the combined use of thin-film deposition techniques and an artificial tunneling barrier. The material is synthesized by co-deposition of niobium and tin in an electron-beam evaporator. Careful control of the substrate temperature during deposition has proved critical to improving the homogeneity and reproducibility of the films. The tunneling barrier is formed from a thin (2-3 nm) layer of amorphous silicon, deposited in vacuum on the freshly-made base electrode, and then oxidized in air. This barrier has proved generally applicable to a wide variety of materials.; The tunnel junctions formed on these films are of high quality, but exhibit some non-ideal behavior independent of the deposition techniques used. Most notably, the measured tunneling density of states at the superconducting gap edge is much broader than for a BCS superconductor, suggesting a distribution of gap values at the surface of the material. In addition, off-stoichiometry samples (tin poor) deposited at substrate temperatures below 800(DEGREES)C have gap values (determined by tunneling) that are too small for the inductively measured critical temperature. This produces calculated values of the coupling strength below the BCS weak-coupling limit. It has been found that increasing the deposition temperature above 850(DEGREES)C eliminates this problem.; The tunneling density of states can be used with the proximity-effect modified Eliashberg equations to calculate the electron-phonon coupling spectrum. As the tin composition is decreased from stoichiometry, there is an overall decrease in the electron-phonon coupling strength. At the same time there is an increase in the average phonon frequency due to an increase in energy of the lowest phonon branch. These two factors contribute to the decrease in critical temperature observed in off-stoichiometry material.