E. L. Wolf

Proximity electron tunneling spectroscopy I. Experiments on Nb

A quantitative proximity electron tunneling spectroscopy (PETS) is demonstrated for the study of strong coupling superconductors which do not form suitable insulating oxides for conventional McMillan-Rowell tunneling spectroscopy. Proximity junctions of the form C-Al2O3-Al/S are employed, with Al thickness dN ≤ 100 ». Here S is the superconductor of interest and C is any convenient counterelectrode. The physical basis for the method, experimental techniques, and data obtained from foils of Nb are presented. The results for Nb include the energy-dependent pair potential δS(E), the renormalization function Z(E), effective phonon spectrum α 2F(Ω), electron-phonon coupling constant λ, and Coulomb pseudopotential Μ*. A full discussion of the underlying theory and details of the methods of analysis employed to obtain, in addition, the pair potential of the Al proximity layer are contained in a following paper.

Proximity electron tunneling spectroscopy. II. Effects of the induced N-metal pair potential on calculated S-metal properties

The general problem of determining the N-metal pair potential from tunneling characteristics of proximity effect N-S sandwiches is assessed, and two specific methods are presented. These methods are applied to the treatment of data on PET junctions of the form In-Al2O3-Al/Nb. Inclusion of the influence of the Nb-induced Al pair potential produces important corrections to Nb properties (α2F(E), λ, Μ*) initially obtained by neglecting this influence.

Proximity electron tunneling spectroscopy

Abstract The theory and experimental techniques of proximity electron tunneling spectroscopy are reviewed, with emphasis on determination of effective phonon spectra α 2 F (ω) of strong and weak coupling superconductors. Experimental results to date are summarized.

A resolution of the controversy on tunneling in Nb

Abstract Conventionally acceptable values of the superconducting parameters λ and δ∗ can be obtained from published “anomalous” tunneling data from thermally oxidized Nb junctions by the assumption of a weak superconductor surface layer, most probably NbO, and an appropriately extended analysis.

Quantitative proximity tunneling spectroscopy

Abstract Using the new Greens function calculation of Arnold and an appropriate modification of the McMillan-Rowell procedure, it is demonstrated that tunneling characteristics of suitable proximity junctions may be inverted to determine the properties of superconductors which do not oxidize satisfactorily. The junctions employed are of the form M-Al 2 O 3 -Al/S (where S is the superconductor of interest). The Al thickness is less than 100 A and the Al/S interface is specularly transmitting. Quantitative values for Δ S (E) and Δ N (E) are obtained for S = Nb , N = Al .

McMillan-Rowell tunneling spectroscopy in the Nb-Zr system

Abstract We report the first full application of tunneling spectroscopy to a superconducting transition metal alloy: Nb1−xZrx at x = 0.25, corresponding to the maximum Tc in the Nb-Zr system. The spectral function α2 F(ω) and related parameters, when compared to those for the Nb, confirm that the increase in Tc from 9.22 K (x = 0) to 10.8 K(x = 0.25) arises largely by softening of the effective phonon spectrum.

Proximity electron tunneling spectroscopy. III. Electron-Phonon coupling in the Nb-Zr system

The technique of McMillan-Rowell tunneling spectroscopy, extended by a method of proximity junctions to poorly oxidizing materials, is applied to study electron-phonon coupling in Nb1−xZrxat x = 0.25, corresponding to the maximum Tcin the Nb-Zr alloy system. The resulting α2F(Ω) function provides λ = 1.3 for the alloy, and indicates that the primary benefit to superconductivity upon alloying arises from softening of the effective phonon spectrum.

Proximity electron tunneling spectroscopy. IV. Electron-phonon coupling and superconductivity of tantalum

An extensive superconducting electron tunneling study of Ta, using both a proximity method and an optimized conventional spectroscopy, is reported. The results for Ta confirm the electron-phonon origin of superconductivity in this element, and provide slightly improved superconducting parameters: λ =0.73 and Μph* = 0.11. The results from the PET and optimized conventional methods are in substantial agreement, although the proximity method appears to provide somewhat better resolution of structure in the Eliashberg function α2F(E).

On the observation of superconducting phonon structure in proximity tunneling

Abstract Unusually clear phonon structure from superconducting Nb is observed in tunneling into the normal side of ultrathin AlNb (NS) proximity sandwiches. Contrary to recent reports, this phonon structure is not inverted. The possible utility of such spectra in deducing the properties of the superconducting state in the underlying S electrode is pointed out.

Josephson tunneling in LiTi2O4

Abstract Josephson point contact tunneling between LiTi 2 O 4 and Nb reveals, under K-band microwave radiation. Shapiro steps of spacing approximately hv /2 e . This appears to confirm that superconductivity in this material has s-wave pairing, of interest in connection with recent suggestions of Kulik and of Ranninger.