Aspects of macroscopic quantum tunnelling in small Josephson junctions

Summary We study the effect of temperature on the lifetime of the resonant macroscopic quantum tunnelling voltage state. In view of experiments we consider a merit factor in order to balance between measuring extremely small signals of longer duration or larger signals of shorter duration.     

BCS quasi-particle tunnelling current in Josephson tunnel junctions

Summary The recent improvements in the fabrication technology have succeeded in obtaining high-quality Josephson tunnel junctions. TheI–V characteristics of these junctions exhibit now lesser and lesser differences from the theoretical predictions. The various figures of merit, so far introduced to qualify the junctions, appear now focused on too limited aspects and a direct comparison with the detailed prediction of the theory is in order. For this awaken interest in the detailed predictions of the BCS theory, we have revisited the relevant formulae for the quasi-particle tunnelling current. A detailed discussion of the analytical transformation necessary for a simple numerical evaluation of this current is reported. SeveralI–V curves are obtained using the parameter values corresponding to the case of Nb-based junctions, which is the actual running technology. Peculiar interesting aspects are outlined.     

Experiments on macroscopic quantum tunnelling in Josephson structures: Influence of the uncertainty of junction parameters

Summary Some of the most interesting experiments on MQT, based on the Josephson effect, are considered in order to understand whether a definite experimental proof of the phenomenon exists. We have investigated the effects of some spurious situations related to the uncertainty on the determination of the relevant junction parameters, which can simulate the same effects of MQT. As a result we have seen that probably all the experimental data cannot be completely interpreted within the classical framework. However some ambiguities remain particularly for what concerns the implications of dissipation in the phenomenon.     

Aspects of dissipation effects in experiments on macroscopic quantum tunnelling in Josephson junctions

Summary The effect of dissipation in experiments on macroscopic quantum tunnelling in Josephson tunnel junctions are discussed. The frequency-dependent, nonlinear dynamic resistance of the junction makes it difficult to understand which effective linear resistance has to be chosen to compare the experimental results with the theory developed in the framework of the RSJ model. Various experiments reported in the literature lead to different conclusions and are presently discussed. Recently obtained experimental results on the thermal regime allow us to propose a new experimental strategy which ultimately should remove any ambiguity on the possibility to observe MQT process in the presence of dissipation in Josephson structures. To speed up publication, the authors have agreed not to receive proofs which have been supervised by the Scientific Committee.     

Comments on an experiment of resonant tunnelling between macroscopically distinct quantum levels

Summary In an important experiment performed on a SQUID system, Rouse, Han and Lukens report the first observation of resonant tunnelling between macroscopically distinct quantum levels. We support the experiment with the first theoretical approach for the phenomenon.     

Comments on experiments about the supercurrent decay of josephson junctions

Summary The subtle problems lying in the interpretation of data on the supercurrent decay of underdamped Josephson junction are approached. An analysis of the unavoidable correlation among the free fitting parameters is presented. As a result we find that even the most appropriate approach to this problem leads to a certain unavoidable ambiguity in the interpretation of the data.     

High-T c Josephson junctions for electronic applications

Summary The current status of the electronic applications of high-T _c Josephson junctions is briefly reviewed. Recent results obtained by the authors on devices employing step-edge junctions are reported. In particular the design of a microwave oscillator based on a parallel array of junctions is discussed and preliminary experimental results are presented. Paper presented at the ?VII Congresso SATT? Torino, 4–7 October 1994.     

The role of the surface depression of the pair potential on the Josephson and quasi-particle tunnelling in high-T c superconductors

Summary We discuss the influence of the intrinsic surface depression of the order parameter on the temperature dependence of the Josephson critical currentI _c(T) and, tentatively, on the quasi-particle tunnelling conductance in a superconductor with a very short coherence length. The comparison with the experiments shows that the presence of a surface-depressed pair potential can explain the large deviations of theI _c(T) from the ideal BCS behaviour that we recently observed in Bi₂Sr₂CaCu₂O_8+x Josephson break junctions and could mimic the presence of a broadening in the Superconductor-Insulator-Normal tunnelling conductance of the same high-T _c superconductor. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

Anomalies and proximity effect in high-T C tunnel junctions

Summary Some of the main anomalies in conductance characteristics of high-T _C tunnel junctions are considered. Mainly, the extreme, depression of gap structures, the presence of conductance peaks in corrispondence to the counterelectrode gap, and ?zero-bias anomalies? will be examined. The possibility of application of the proximity McMillan model to describe the behaviour in conductance of high-T _C junctions is considered. Discussion and preliminary comparison with experimental data on YBCO junctions are also reported.     

Comments about the escape temperature representation of experiments on macroscopic quantum tunnelling

Summary The technique of analysing the data on macroscopic quantum tunnelling effects in Josephson junctions in terms of the escape temperature is discussed in connection with some important experiments. In particular we analyse the effect of an uncertainty on the determination of the Josephson critical current in the current dependence of the escape temperature. As a result we point out that the experimental errors are of the same order of the effect to be observed.     

Crossover between the extremely low damping and moderate underdamped regime in the thermal activation theory: Preliminary experimental results

Summary The problem of the decay of metastable state due to thermal activation is studied. Experimental data referring to the supercurrent decay of an underdamped Josephson junction are compared with Kramers theory and its recent improvements. Significative aspects of the crossover region between the extremely underdamped regime and the moderate underdamped one are discussed.     

Generation of subharmonic shapiro steps in ladder arrays of overdamped superconducting junctions

Summary Subharmonic Shapiro Steps (SSS) are usually observed in 2D arrays of superconducting junctions when their dynamical states are perturbed by factors like disorder, frustration, current inhomogeneities and so on. In order to clarify the conditions under which SSS can be generated we decided to investigate arrays of very simple shape like the ladder ones. In particular we have studied the influence of the frustration (i.e. of an external magnetic field) on the time evolution of the vortex configuration and the commensurability of the latter with the geometry and the dimension of the array. Ladders, parallel or perpendicular to the bias current, turn out to be systems where the properties of the square arrays can be suitably tested. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

Investigations on phase locking in a two-dimensional Josephson junction array within the Werthamer and the RCSJ model

The synchronization properties of a simple two-dimensional Josephson array consisting of two coupled SQUID cells are studied within the Werthamer as well as the RCSJ model. Special emphasis is placed on the role of inductances arranged perpendicular and parallel to the current bias direction for the phase locking behavior. The general behavior within the Werthamer model is found to be similar to that within the RCSJ model. However, there are quantitative differences, e.g. an enhanced phase shift between the voltage oscillations within one cell and a shift of the parameter range for the in-phase regime between different cells towards lower values of the McCumber parameter in the Werthamer model. Received: 23 March 1998 / Revised: 3 June 1998 / Accepted: 9 June 1998     

2D superconducting junction networks as detectors of radiation

Summary We present a careful study of the behaviour of the below-gap photoresponse in 2D arrays of superconducting junctions. We have included the effects of the temperature, not only through the white-noise current term, but also through theT-dependence of the junction coupling energy. In order to render our model more realistic we also consider the correction due to the self-field generated by the screening currents. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

Retrapping mechanism in the switching dynamics of a Josephson junction

Summary The switching dynamics of a Josephson junction is discussed in connection with the problem of a Brownian particle in a one-dimensional washboard potential. The particle is supposed to escape from a potential well and results concerning the probability of retrapping in another well before the transition into the running state are obtained for the first time. The analysis is restricted to the very-low-damping case and the ?first-passage time? technique is used.

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Riassunto Si discute la dinamica del decadimento dello stato a tensione nulla di una giunzione Josephson nell'ambito del problema del moto browniano. Il potenziale è unidimensionale periodico ed inclinato e sono permessi molti stati metastabili. Si considera la probabilità d'intrappolamento in una buca di potenziale di una particella proveniente da uno stato metastabile a potenziale piú alto.

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Резюме В связи с проблемой броуновской частицы в одномерном потенциале обсуждается динамика переключения перехода Джозефсона. Предполагается, что частица освобождается из потенциальной ямы, и впервые определяется вероятность повторного захвата в другую яму перед переходом в движущееся состояние. Анализ ограничивается случаем очень малого затухания и используется техника ?двремени первого хода?.

     

Lower critical field and vortices configuration in 2D superconducting junction arrays

Summary In order to describe properly the magnetic status of a 2D superconducting junction array, one has to consider not only the effect of the screening currents but also that of the particular experimental protocol followed to measure the physical quantities of interest like, for example, the magnetization. We show that the value of the lower critical field,f _c1, of the junction array depends strongly on the intensity of the screening currents,i.e. on the strength of the junction coupling,E _j, and that reliable results can be obtained only by considering the full-inductance matrix. We also show that the magnetic configuration of the vortices may depend on the particular experimental approach followed (static or dynamic) and, even, in some cases, on the initial configuration of the phases. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

YBCO/Nb bulk junctions: Observation of the Josephson effect

Summary The fabrication procedure of reliable YBCO-based bulk junctions with a Nb film as a counterelectrode is described. Typical currentvoltage characteristics are presented, clearly showing the presence of the Josephson effect.     

Detecting quantum signatures of optical fields by ultrasmall Josephson junctions

We prove that a mesoscopic Josephson junction, irradiated with a quantum superposition of two -out of phase optical coherent states, exhibits an experimentally observable sensitivity to the quantum coherences of the field state. Received 1 June 1999 and Received in final form 30 July 1999     

High-quality YBCO d.c. SQUIDs based on bicrystal substrates

Summary An analysis of superconducting transport properties and magnetic behaviour of d.c. SQUIDs employing YBCO bicrystal grain boundary junctions (GBJs) has been performed. GBJs have been obtained by deposition of ac-axis-oriented YBCO film on a SrTiO₃ bicrystal substrate by ICM sputtering technique. Experimental measurements on a YBCO d.c. SQUID with a misorientation angle θ=20° are reported. The SQUID shows a critical temperatureT _c∼89 K and a high critical current densityJ _c∼3·10⁶ A/cm² atT=4.2 K. Current-voltage characteristics are close to the behaviour predicted by the resistively shunted junction (RSJ) model and the temperature dependenceJ _c(T) shows a linear behaviour at small reduced temperatures and a depressedJ _c value close toT _c. High-quality flux-voltage curves have been found upT=87 K over a large range of magnetic field. The high reproducibility and the good control of transport properties by the variation of θ make YBCO bicrystal GBJs very useful for applications in electronics. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.     

HTSC grain boundary weak links employing high-quality ICM sputtered YBCO films

Summary Bicrystal grain boundary junctions of YBCO have been fabricated by using the Inverted Cylindrical Magnetron d.c. sputtering technique. The main characteristics of the junction behaviour are reported. Some aspects of the experimental results are discussed in the framework of existing models developed for describing weak-link structures. Paper presented at the ?VII Congresso SATT?, Torino, 4–7 October 1994.