Gerald B. Arnold

Superconducting tunneling without the tunneling Hamiltonian. II. Subgap harmonic structure

The theory of superconducting tunneling without the tunneling Hamiltonian is extended to treat superconductor/insulator/superconductor junctions in which the transmission coefficient of the insulating barrier approaches unity. The solution for the current in such junctions is obtained by solving the problem of a particle hopping in a one-dimensional lattice of sites, with forward and reverse transfer integrals that depend on the site. The results are applied to the problem of subgap harmonic structure in superconducting tunneling. The time-dependent current at finite voltage through a junction exhibiting subgap structure is found to have terms that oscillate at all integer multiples of the Josephson frequency,n(2eV/ħ). The amplitudes of these new, and as yet unmeasured, ac current contributions as a function of voltage are predicted.

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.

Phonon structure in the tunneling characteristics of thin proximity effect sandwiches

The tunneling spectra of Al-oxide-N-S proximity effect junctions show phonon structure not only from the superconducting layerS, but also from the normal metal layerN. We report experimental results for junctios with Ag, Cu, and Al normal metal layers (100–400 Å thick) and Pb or Sn superconducting layers (∼3000 Å thick). We compare these experimental results with the McMillan geometrical resonance model of the proximity effect and with an extended McMillan tunneling model. The extension includes the effect of the frequency-dependent electron-phonon interaction in the normal metal. Finally, we estimate the electron-phonon coupling constant λ for the normal metals from the size of their phonon structure in the tunneling spectra.

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 .

Theory of coherent c-axis Josephson tunneling between layered superconductors

We calculate exactly the Josephson current for c-axis coherent tunneling between two layered superconductors, each with internal coherent tight-binding intra- and interlayer quasiparticle dispersions. Our results also apply when one or both of the superconductors is a bulk material, and include the usually neglected effects of surface states. For weak tunneling, our results reduce to our previous results derived using the tunneling Hamiltonian. Our results are also correct for strong tunneling. However, the c-axis tunneling expressions of Tanaka and Kashiwaya are shown to be incorrect in any limit. In addition, we consider the c-axis coherent critical current between two identical layered superconductors twisted an angle {phi}{sub 0} about the c axis with respect to each other. Regardless of the order-parameter symmetry, our coherent tunneling results using a tight-binding intralayer quasiparticle dispersion are inconsistent with the recent c-axis twist bicrystal Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} twist junction experiments of Li et al. [Li et al., Phys. Rev. Lett. 83, 4160 (1999)]. (c) 2000 The American Physical Society.

Proximity Effect and Josephson Coupling in the SO (5) Theory of High- Tc Superconductivity

We consider proximity effect coupling in Superconducting/Antiferromagnetic/Superconducting (S-A-S) sandwiches using the recently developed SO(5) effective theory of high temperature superconductivity. We find that, for narrow junctions, the A region acts like a strong superconductor, and that there is a critical junction thickness which depends on the effective SO(5) coupling constants and on the phase difference across the junction, at which the A region undergoes a Freedericksz-like transition to a state which is intermediate between superconductor and antiferromagnet. For thick junctions, the current-phase relation is sinusoidal, as in standard S-N-S and S-I-S junctions, but for thin junctions it shows a sharp break in slope at the Freedericksz point.

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.