Tunneling α2F( ω) on high Tc A15 and B1 compounds

Abstract

Recent advances in artificial tunneling barriers have resulted in excellent tunnel junctions on several materials whose native oxide are of poor quality. Among the high T c A15 compounds tunneling α 2 F( ω) data are now available for Nb 3Sn, Nb 3Al and Nb 3Ge. Among B1 compounds there have been no published α 2 F( ω) results. Since this is another class of high T c materials, the contrast would be interesting. This paper will give the first tunneling α 2 F( ω) spectrum for the B1 compound NbN. The spectrum show two large phonon peaks at energies of 13 meV and 47 meV. The λ value of 1.46±0.10 along with the 2 Δ/ k B T c of 4.25 show this to be a strong coupled material. In addition, we will present α 2 F( ω) as a function of composition for A15 VSi, comparing it with previously measured A15 compounds. Here in V 3Si unlike the other A15 compounds measured, there is no evidence of mode softening which had been presented as an explanation for the high T c's in the A15 compounds. Also unusual for the A15 compounds previously measured by tunneling, is that V 3Si remains weak coupled as silicon concentration increases. Both the λ value (remaining about 1.0) and the sΔ/ k B T c of about 3.5 show this.