McMillan Tunneling Model Research Articles

Generalized McMillan tunneling model of the superconducting proximity effect

We have extended the McMillan tunneling model (MTM) of the superconducting proximity effect to the case of strongly coupled normal-superconducting composites. The generalization has been carried out by treating the tunneling Hamiltonian to all orders of self-consistent perturbation theory. The electronic density of states, the real and imaginary parts of the order parameter, the energy gap, and the transition temperatureT c calculated in the generalized model are significantly different from the MTM results. Generally, the peak inN N (E) is broader, shifted, and damped. The energy gap is lower and the depression inT c is softened.

Theoretical investigation of the transition temperature of dirty proximity-effect sandwich systems

The influence of impurities and of an interface barrier on the critical temperature of a superconductor-normal (SN) metal sandwich is studied in the Cooper limit. The investigations are based on new equations which allow us to deal with inhomogeneous, dirty superconducting systems near the critical temperature. In contrast with previous treatments, it is shown that at finite film thicknesses,T c is not only dependent on the N to S thickness ratio, but also on the absolute thickness values, the transmission probability of the interface barrier, and the mean free electron paths. Our results permit the determination of the stay probabilities of McMillan 's tunneling model and the effective length of de Gennes' extended boundary condition on the integral kernel which determinesT c. Finally, it is shown how the conventional Cooper limit result forT c by de Gennes is approached.

Proximity effect for superconductors containing transition metal impurities. II

The superconducting proximity effect in a superconductor-normal metal sandwich system in which the metal on the normal side contains dilute 3d transition metal impurities is investigated theoretically. The Kaiser-Zuckermann theory, based on McMillan's tunneling model, is extended by the use of the Hartree-Fock version of the Anderson model to give rise to a bound state in an effective energy gap. For the purpose of analyzing experimental results, the transition temperature and the critical concentration for gapless superconductivity are calculated. It is also demonstrated that with increasing impurity concentration, an impurity band merges into a gap, which may correspond to recent experimental findings.

ANALYSIS OF PROXIMITY EFFECTS BETWEEN SPIN SINGLET AND SPIN TRIPLET SUPERCONDUCTORS

Using McMillan's tunneling model of the superconducting proximity effect we show that the possible existence of an attractive p-wave interaction, leading to p-state superconductivity at some low temperature, cannot be tested experimentally using the proximity effect.

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 A thick) and Pb or Sn superconducting layers (∼3000 A 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.

Effect of metal-superconducting interfaces in proximity-effect sandwiches on fluctuations aboveTc

The effect of the proximity effect between magnetic and superconducting films on fluctuations of the order parameter above ${T}_{c}$ is reexamined. Using McMillan's tunneling model for the proximity effect, it is found that the finite penetration probability of the barrier between the magnetic and the superconducting films acts to reduce the relaxation frequency of the fluctuations. This reduction is in addition to that caused by magnetic impurities.

Proximity Effects in Pb-I-Bi8Te7S5-I-Pb Sandwiches as Determined by Electron Tunneling

Pb-I-Bi/sub 8/Te/sub 7/S/sub 5/-I-Pb proximity sandwiches have been produced b y evaporating Pb onto both surfaces of atomically smooth films of Bi/ sub 8/Te/sub 7/S/sup 5/. Tunnelng experiments have been performed on these sandwiches and the results are shown to be in agreement with the McMillan tunneling model of the superconducting proximity effect. (auth)

Tunneling into Weakly Coupled Copper and Lead in Proximity

Tunneling experiments from superconducting Al base electrodes into both sides of weakly coupled Pb–Cu proximity sandwiches have been performed. The temperature variation and energy of multiple-peaked structure in the normalized tunneling conductance curves compare very well with those calculated using the McMillan tunneling model of the proximity effect, although the calculated curves exhibit more sharply peaked structure than the observed ones. A simple model which takes into consideration the thickness inhomogeneities in the proximity sandwich is proposed in order to account for the sharpness differences.