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We have investigated the static configurations of the phase inside an annular Josephson tunnel junction in the presence of an externally applied magnetic field. For rings having a circumference less than the Josephson penetration depth analytical results are derived which extend the previous theory to (i) rings having finite width and (ii) rings with trapped fluxons. For longer junctions an analysis based on a perturbative approach is carried out in order to find the different phase profiles existing for a given applied field. This perturbative analysis reproduces the threshold curves found numerically. \textcopyright{} 1996 The American Physical Society.

We present experimental observations at low temperature of subharmonic gap structures in high-quality Nb/AlO[sub [ital x]]/Nb tunnel junctions, with relatively low critical current density. Samples having small, though appreciable, differences in the energy gaps show split structures which permit an interpretation in terms of the multiparticle tunneling (MPT). A detailed analysis and discussion of the experimental data in terms of the MPT theories is presented. In particular, the amplitudes and the temperature dependence of the subgap structures and the voltage dependence of the multiparticle current are investigated. Finally, a model that considers the tunnel barrier as consisting of randomly distributed thicknesses is proposed to overcome the failure of the theoretically predicted structure amplitudes for a uniform barrier. Within this framework, the experimental data of this work and those recently obtained by other groups are discussed.

The behavior of a long Josephson tunnel junction drastically depends on the distribution of the dc bias current. We investigate the case in which the bias current is fed in the central point of a one-dimensional junction. Such junction configuration has been recently used to detect the persistent currents circulating in a superconducting loop. Analytical and numerical results indicate that the presence of fractional vortices leads to remarkable differences from the conventional case of uniformly distributed dc bias current. The theoretical findings are supported by detailed measurements on a number of delta-biased samples having different electrical and geometrical parameters.

In highly hysteretic long Josephson junctions irradiated with microwaves the rf-induced current steps can cross the zero-current axis. The quantized voltage states arise from the phase locking of fluxon oscillations, which in rf-driven junctions can occur even with negative current bias. This phenomenon is interesting both for nonlinear dynamics and for application to the Josephson voltage standard.

It has been argued by Zurek and Kibble that the likelihood of producing defects in a continuous phase transition depends in a characteristic way on the quench rate. In this paper we discuss our experiment for measuring the Zurek-Kibble (ZK) scaling exponent $\ensuremath{\sigma}$ for the production of fluxons in annular symmetric Josephson tunnel junctions. The predicted exponent is $\ensuremath{\sigma}=0.25,$ and we find $\ensuremath{\sigma}=0.27\ifmmode\pm\else\textpm\fi{}0.05.$ Further, there is agreement with the ZK prediction for the overall normalization.

Experimental measurements of current-voltage structure and emitted $X$-band radiation in applied magnetic field from overlap-geometry Josephson tunnel junctions of normalized length about 2 are compared with numerical simulations obtained with the use of a perturbed sine-Gordon model. The simulations furnish the current and field dependence of the oscillation configuration, from which can be calculated average voltages, frequencies, and power spectra. Simulation and experimental results are in good agreement with regard to the lobe structure of the height of the first zero-field step and/or second Fiske step in magnetic field and the field dependence of the radiation frequency within the various lobes, including details such as hysteresis between lobes. The simulations predict an alternation of the dominant frequency component with increasing field that accounts well for the experimental observations. The usefulness and limitations of cavity-mode analyses, both singlemode and multimode, are evidenced by comparison with the simulation results.