Superconductivity in Artificial Nb/Cu Heterostructures

摘要

Transport and tunneling measurements of artivicially prepared Nb/Cu heterostructures have been made. Samples studied had individual layers in the range 3.6 to 5000 A with total thickness approx. 1 mu m. The dependence of T/sub c/ above 300 A layer thickness agrees well with proximity effect theory with no adjustable parameters. Below 300 A the data indicate the T/sub c/ of Nb decreases with layer thickness. The coupling strength 2 delta /kT/sub c/ exhibits a continuous decrease from the strong coupled value of approx. 3.8 (thick layers) toward the weak coupled value of approx. 3.5 (thin layers). No evidence is found for a change in the Nb LA and TA phonons energies from their bulk values down to approx. 10 A. These results, in conjunction with Brillouin scattering measurements of zone center phonons, imply large changes in the phonon dispersion relation as a function of layer thickness in these materials. (ERA citation 08:021052)