Tunneling Studies of Compound Geometrical Resonances and Proximity Phenomena in Superconductive Zinc-Lead and Silver-Lead Sandwiches.

Wong and Tomasch have found that thick, clean Zn films, when backed by clean Pb films of sufficient thickness, produce first derivative tunneling spectra (T = 0.3K) that differ markedly from the predictions of Wolfram's theory in the central virtual-state region. Gallagher has recently extended Wolfram's theory to allow for quasiparticle geometrical resonances in both metal layers. By utilizing Gallagher's theory, the present analysis is able to explain both the well-behaved and the anomalous portions of these spectra. For the c-axis orientation of Zn and the (111) orientation of Pb, one obtains Fermi velocities of (1.31 (+OR-) 0.02) Mm/s and (0.78 (+OR-) 0.04) Mm/s, respectively, in good agreement with values obtained by other methods. At the Fermi surface (E = 0), the present estimate of the Zn electron -phonon coupling parameter, (lamda) = Z(,N)(0)-1 = 0.42, agrees with theoretical estimates by Tomlinson and Swihart, but away from the Fermi surface (E 0), present estimates of the complex renormalization function generally exceed theirs. Experiments with thin Ag layers (d(,N) (TURN) 100-700 (ANGSTROM)) backed by thicker Pb (d(,S) (TURN) 3,500 (ANGSTROM)) have lead Khim and Tomasch to infer large effective renormalizations for Ag that increase with decreasing d(,N), even though the electron-phonon interaction presumably retains its bulk character. Arnold has recently shown that dirty proximity superconductors violate Anderson's theorem and that, in this regime, the usual treatment of elastic scattering is inappropriate for Andreev interference phenomena. He finds that both the pairing potential (DELTA)(,N)(E), and the effective layer renormalization, Z(,N)(E), are influenced strongly by scattering. The present reanalysis, based on Arnold's s-wave scattering theory and on Gallagher's compound resonance theory, assumes bulk behavior for the Ag electron-phonon interaction. The best estimates obtained for the dressed Fermi velocities are v(,FS)('*)(Pb) = (0.72 (+OR-) 0.06) Mm/s and v(,FN)*(Ag) = (1.07 (+OR-) 0.06) Mm/s.