Tunneling studies of compound geometrical resonances and proximity phenomena in superconductive Zn-Pb and Ag-Pb sandwiches
It has been 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 predictons of Wolfram's theory in the central virtual-state region, and Wolfram's theory has recently been extended to allow for quasiparticle geometrical resonances in both metal layers utilizing the extended 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 +- 0.02) Mm/s and (0.78 +- 0.04) Mm/s, respectively, in good agreement with values obtained by other methods. At the Fermi surface (E = O), the present estimate of the Zn electron-phonon coupling parameters, lambda = Z/sub n/(0)-1 = 0.42, agrees with theoretical estimates, but away from the Fermi surface (E O), present estimates of the complex renormalization function generally exceed theory. Experiments with thin Ag layers (d/sub N/ approx. 100 to 700 A) backed by thicker Pb (d/sub s/ aa 3500 A) have lead some workers to infer large effective renormalizations for Ag that increase with decreasing d/sub N/, even though the electron-phonon interaction presumably retains its bulk character. It has recently been 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. It has been found that both the pairing potential ..delta../sub N/(E), and the effective layer renormalization, Z/sub N/(E), are influenced strongly by scattering. The present reanalysis, based on Arnold's s-wave scattering theory and on a compound resonance theory, assumes bulk behavior for the Ag electron-phonon interaction. The best estimates obtained for the dressed Fermi velocities are v/sub FS/(Pb) = (0.72 +- 0.06) Mm/s and V/sub FN/ (Ag) = (1.07 +- 0.06) Mm/s.
- Research Organization:
- Notre Dame Univ., IN (USA)
- OSTI ID:
- 6268331
- Resource Relation:
- Other Information: Thesis (Ph. D.)
- Country of Publication:
- United States
- Language:
- English
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