Phonon Spectrum and Strong-Coupling Superconductivity in Nb: A Single-Crystal Tunneling Study.
Abstract
Superconducting tunneling junctions of single crystal Nb grown in ultrahigh vacuum by the electron beam floating zone technique have been fabricated with counterelectrodes of Au, In, and Pb. The tunneling characteristics are nearly ideal and uniformly yield a Nb energy gap value of 2 delta (0) - 3.12+-/5 mV. The first and second derivatives of the I-V curves of these junctions are measured with a precision better than one part in 0.0001 at 0.9 K using an AC bridge circuit. This tunneling data is deconvoluted to give the electronic density of states, N(E) of superconducting Nb. Further, the experimental data is used to determine the electron-coupled phonon spectrum, (alpha squared)F, the electron-phonon coupling constant, lambda, and the dimensionless Coulomb pseudo-potential, mu*, characteristic of Nb. These parameters, as determined from each of 11 different junctions, are compared and discussed in relation to crystal orientation, resistivity ratio, sample fabrication, etc. Finally, the general character of the (alpha squared)F spectrum is discussed in relation to a lattice dynamics calculation and the neutron diffraction data of the Nb phonon spectrum, F(w).
Document Details
- Document Type
- Technical Report
- Publication Date
- Feb 21, 1975
- Accession Number
- ADA006930
Entities
People
- Judith L. Bostock
- Kenneth K. Lo
- Margaret L. A. Macvicar
- Robert Marc Rose
Organizations
- Massachusetts Institute of Technology