This review provides a theoretical basis for understanding the current-phase relation (CPhiR) for the stationary (dc) Josephson effect in various types of superconducting junctions The authors summarize recent theoretical developments with an emphasis on the fundamental physical mechanisms of the deviations of the CPhiR from the standard sinusoidal form A new experimental tool for measuring the CPhiR is described and its practical applications are discussed The method allows one to measure the electrical currents in Josephson junctions with a small coupling energy as compared to the thermal energy A number of examples illustrate the importance of the CPhiR measurements for both fundamental physics and applications

/pdf/the-current-phase-relation-in-josephson-junctions-40zrgv4v2s.pdf

The current-phase relation in Josephson junctions

The paper reviews the main aspects of nonequilibrium hot-electron phenomena in superconductors and various theoretical models developed to describe the hot-electron effect. We discuss implementation of the hot-electron avalanche mechanism in superconducting radiation sensors and present the most successful practical devices, such as terahertz mixers and direct intensity detectors, for far-infrared radiation. Our presentation also includes the novel approach to hot-electron quantum detection implemented in superconducting x-ray to optical photon counters.

/pdf/hot-electron-effect-in-superconductors-and-its-applications-1lkia76469.pdf

Hot-electron effect in superconductors and its applications for radiation sensors

We have developed a theory of quasiparticle and phonon energy downconversion in nonequilibrium superconductors following the absorption of an energetic photon. This stage of energy downconversion cascade is important for the production of quasiparticles and is shown to split into two phases. The first is controlled by the evolution of the phonon distribution while the second is dominated by quasiparticle downconversion. The relative durations of the two phases and hence the rates of quasiparticle generation depend on material parameters, and most common superconductors could be classified into three different groups. For typical superconductors used for x-ray detection the downconversion cascade was shown to be fast compared to various time scales in the tunneling regime.

/pdf/quasiparticle-phonon-downconversion-in-nonequilibrium-3azfjsn7us.pdf

Quasiparticle-phonon downconversion in nonequilibrium superconductors

An overview of the current status of different types of non-hysteretic Josephson junctions is given with emphasis on double-barrier structures. The results of theoretical work on double-barrier SIS′IS Josephson junctions (I is a tunnel barrier, S′ is a thin film with TC′<TC) are presented. The microscopic model for the supercurrent is developed for two cases: the S′ interlayer in the clean and in the dirty limit. The model describes the cross-over from direct Josephson coupling of the external S electrodes to the regime of two serially connected SIS′ junctions. We calculate the ICRN product as a function of the TC′/TC ratio, the interlayer thickness and the barrier strengths and compare the theory with experimental data for Nb/AlOx/Al/AlOx/Nb junctions. We argue that these junctions are very promising in rapid single flux quantum (RSFQ) and programmable voltage standard applications, since they are intrinsically shunted and have controllable interfaces. We formulate the requirements for materials and interface barriers in order to increase critical current densities and ICRN products in double-barrier junctions.

Double-barrier Josephson structures as the novel elements for superconducting large-scale integrated circuits

This article presents a general model for calculating the density of states and the Cooper pair potential in proximity-coupled superconducting bi- and trilayer films. It is valid for any kind of bilayer S1-S2, whatever the quality of the materials S1 and S2, the quality of the S1-S2 interface, and the layer thicknesses. The trilayer model is valid for a thin S3 layer, whereas the other two layers have arbitrary thicknesses. Although the equations of the dirty limit are used, it is argued that the model stays valid in clean bi-and trilayer films. The typical example of superconducting tunnel junctions is used to show that existing models, which apply to very thin or very thick layers or to perfectly transparent S1-S2 interfaces, are too restrictive to apply to an arbitrary bilayer. The new model is applied to practical junctions, with layer thicknesses intermediate between the “thick” and the “thin” approximation.

Generalized proximity effect model in superconducting bi- and trilayer films

A specially designed device based on superconducting tunnel junctions was used to measure the diffusion of excess electronic quasiparticles generated in a superconducting film of tantalum by the absorption of x-ray photons. The device was made out of a thin film of epitaxial tantalum. At both ends, a microfabricated $\mathrm{Al}\ensuremath{-}{\mathrm{AlO}}_{x}\ensuremath{-}\mathrm{Al}\ensuremath{-}\mathrm{Nb}$ tunneling junction was placed onto the film. Both tunneling junctions were operated in a current-biased mode in a small magnetic field and were used to monitor the diffusion of excess quasiparticles generated in the Ta. For the data analysis, the Ta absorber has been modeled as a chain of cells, and the time evolution and diffusion of the distribution of energy carried by quasiparticles has been calculated using the Chang-Scalapino equations. In this way, we determined the values of the quasiparticle diffusion length $\ensuremath{\Lambda}=\sqrt{{D}_{\mathrm{abs}}{\ensuremath{\tau}}_{\mathrm{eff}}},$ the diffusion rate ${D}_{\mathrm{abs}},$ and the effective lifetime ${\ensuremath{\tau}}_{\mathrm{eff}}.$

Quasiparticle diffusion in tantalum using superconducting tunnel junctions

Results from a comparison between current-voltage data measured on Nb/Al/AlO x /Al/Nb tunneling junctions to a proximity effect model proposed by Golubov et al. [J. Low Temp. Phys. 70 (1988) 83] are used to get an estimation of the quasiparticle collection as a function of the Al-film thickness (4 and 120 nm) in X-ray detectors.

The influence of proximity effect on the quasiparticle collection

A whole wafer fabrication procedure using selective etching of refractory materials to produce Nb- or Ta/Al/AlO x Al/Nb tunneling X-ray detectors is discussed. Fabrication aspects and preliminary results are presented.

Fabrication of Nb and Ta/Al/AlOx/Al/Nb tunneling junction X-ray detectors: a list of problems

Experimental investigations of the proximity effects in symmetric Nb/Al-AlOx-Al/Nb superconducting tunneling junctions with a fixed Nb thickiness of 200 nm, and Al layer thicknesses ranging from 4 to 120 nm, are reported. Both de Josephson-tunneling and quasiparticle-tunneling current voltage characteristics were measured, and analyzed using the proximity effect model proposed by Golubov and Kupriyanov. The numerical computations were performed without the approximation used by Golubov et nl. In particular, the order parameter was allowed to spatially vary in Nb and Al and the Usadel functions were not linearized. The overall shape of the experimental I-V curves, as well as the temperature dependence of the de Josephson current and of the gap energies are satisfactorily reproduced by the model calculations provided the parameter gamma(BN) describing the resistance at the Al-Nb, interface is taken into account. From ys, we extract a value for the quasiparticle transmission at the Nb/Al interface which is smaller than unity. [S0163-1829(99)01013-9].

Proximity effects in Nb/Al-AlOx-Al/Nb superconducting tunneling junctions

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Papers

Quasiparticle diffusion in tantalum using superconducting tunnel junctions

The influence of proximity effect on the quasiparticle collection

Fabrication of Nb and Ta/Al/AlOx/Al/Nb tunneling junction X-ray detectors: a list of problems

Proximity effects in Nb/Al-AlOx-Al/Nb superconducting tunneling junctions