Search Results (16 total)

We study classical resonant activation (CRA) for a Josephson junction embedded in a shell circuit consisting of one capacitive and one inductive element. The low dissipation quality factor of this device is calculated analytically at the fundamental frequency for two different models of the shell circuit. From our numerical results it emerges that CRA and energy level quantization (ELQ) are distinct processes that can be distinguished in measurements on Josephson devices at low temperatures. CRA at subharmonic frequencies is a highly nonlinear process very different from the corresponding quantum multiphoton process. These results are relevant for the interpretation of a recent experiment on ELQ in high-temperature superconductors Josephson junctions.

We have modeled flux quanta propagation in two-dimensional square arrays of small Josephson junctions when the motion is hampered by the presence of an effective barrier due to the superconducting loops. The energy barriers have been estimated simulating the effect of thermal fluctuations and evaluating the barrier via the Arrhenius factor. The results have been compared with a much simpler semianalytic method, showing that the method is able to give an acceptable estimate. The strength of the fluxon-(anti)fluxon interaction as a function of the loop inductance and the distance between the excitations has been also evaluated. It is reported that the presence of a finite inductance substantially affects the interaction potential, and the contributions due to mutual inductances are found to further change the behavior of the interaction.

By introducing a concept of thermal expansion (TE) of a Josephson junction as an elastic response to an effective stress field, we studied (both analytically and numerically) the temperature and magnetic field dependences of the TE coefficient α in a single small junction and in a square array. In particular, we found that in addition to field oscillations due to Fraunhofer-type dependence of the critical current, α of a small single junction also exhibits strong flux driven temperature oscillations near T_C. We also numerically simulated stress-induced response of a closed loop with finite self-inductance (a prototype of an array) and found that α of a 5×5 array may still exhibit temperature oscillations provided the applied magnetic field is strong enough to compensate for the screening-induced effects.

The escape rate from the zero voltage state in a superconducting Josephson junction (JJ) is determined by the temperature, but it saturates at low temperature due to macroscopic quantum tunneling (MQT). Complications due to d-wave symmetry in a high temperature superconductor, like low energy quasiparticles and an unconventional current-phase relation, may influence the escape rate. We report, for the first time to our knowledge, the observation of MQT in a YBa₂Cu₃O_7-δ grain boundary biepitaxial JJ. This proves that dissipation can be significantly reduced by a proper junction configuration, which is of significance for quantum coherence.

A novel dynamical state has been observed in the dynamics of a perturbed sine-Gordon system. This resonant state has been experimentally observed as a singularity in the dc current-voltage characteristic of an annular Josephson tunnel junction, excited in the presence of a magnetic field. In this respect it can be assimilated to self-resonances known as Fiske steps. Differently from these, however, we demonstrate, on the basis of numerical simulations, that its detailed dynamics involves rotating fluxon pairs, a mechanism associated, so far, to self-resonances known as zero-field steps. This occurs because the size of nonlinear excitations is comparable with that of the system.

By irradiating with a single ultrafast laser pulse a superconducting electrode of a Josephson junction, it is possible to drive the quasiparticles (qp’s) distribution strongly out of equilibrium. The behavior of the Josephson device can, thus, be modified on a fast time scale, shorter than the qp’s relaxation time. This could be very useful, in that it allows fast control of Josephson charge qubits and, in general, of all Josephson devices. If the energy released to the top layer contact S1 of the junction is of the order of ∼μJ, the coherence is not degradated because the perturbation is very fast. Within the framework of the quasiclassical Keldysh Green’s function theory, we find that the order parameter of S1 decreases. We study the perturbed dynamics of the junction, when the current bias is close to the critical current, by integrating numerically its classical equation of motion. The optical ultrafast pulse can produce switchings of the junction from the Josephson state to the voltage state. The switches can be controlled by tuning the laser light intensity, the pulse duration, and the bias current of the Josephson junction.

Recent experiments on high-temperature superconductors show a paramagnetic behavior localized at grain boundaries (GBs). This paramagnetism can be attributed to the presence of unconventional d-wave induced π junctions. By modeling the GBs as an array of π and conventional Josephson junctions we determine the conditions of the occurrence of the paramagnetic behavior.

Many experiments on high-temperature superconductors have shown paramagnetic behavior when the sample is field cooled. The paramagnetism was attributed to a d-wave order parameter creating π-junctions in the samples. However, the same effect was later discovered in traditional low-temperature superconductors and conventional Josephson-junction arrays which are s wave. By simulating both conventional and mixed π/conventional Josephson-junction arrays we determine that differences exist which may be sufficient to clearly identify the presence of π junctions. In particular, the π junctions cause a symmetry breaking providing a measurable signature of their presence.

We simulate two-dimensional Josephson-junction arrays, including full mutual-inductance effects, as they are cooled below the transition temperature in a magnetic field. We show numerical simulations of the array magnetization as a function of position, as detected by a scanning superconducting quantum interference device which is placed at a fixed height above the array. The calculated magnetization images show striking agreement with the experimental images obtained by Nielsen et al. [Phys. Rev. B 62, 14 380 (2000)]. The average array magnetization is found to be paramagnetic for many values of the applied field, confirming that paramagnetism can arise from magnetic screening in multiply connected superconductors without the presence of d-wave superconductivity.

We have measured the rate of thermally induced escape from the zero-voltage state in long Josephson junctions of both overlap and in-line geometry as a function of applied magnetic field. The statistical distribution of switching currents is used to evaluate the escape rate and derive an activation energy ΔU for the process. Because long junctions correspond to the continuum limit of multidimensional systems, ΔU is in principle the difference in energy between stationary states in an infinite-dimensional potential. We obtain good agreement between calculated and measured activation energies for junctions with lengths a few times the Josephson penetration depth λ_J.

The purpose of this work is to compare the dynamics of arrays of Josephson junctions in the presence of a magnetic field in two different frameworks: the so-called XY frustrated model with no self-inductance and an approach that takes into account the self-field generated by the screening currents (considering self-inductances only). We show that, while for a range of parameters the simpler model is sufficiently accurate, in a region of the parameter space solutions arise that are not contained in the XY model equations. © 1996 The American Physical Society.

We study the phase locking of a long Josephson junction operating in a fluxon oscillator regime to external rf signals whose frequency is an even harmonic of the oscillator frequency. The phenomenon is investigated for different values of the dc bias current in the junction, and around each value the current intervals that allow phase locking are evaluated. These intervals can be well identified even when the drive frequency is sixteen times the fluxon oscillator frequency. The explanation of the numerical data that we obtain is given in terms of analytical approximations for a long Josephson junction model. An equation containing relevant experimental parameters is derived for the current-locking ranges generated by the different harmonics. This equation establishes that the amplitude of these intervals decreases exponentially with the harmonic number and we show how this result can be related to a property of the Fourier spectrum of fluxon oscillations. In all the cases that we analyze very good agreement is found between the numerical evidence and theoretical analysis.

We investigate fluxon dynamics in underdamped one-dimensional parallel arrays of small Josephson tunnel junctions. The current-voltage characteristics of the arrays show various resonant steps depending upon the temperature of the sample and the externally applied magnetic field. Experimental results on fluxon propagation in arrays are compared with that for continuous Josephson transmission lines fabricated on the same chip. By modeling fluxon dynamic states in a one-dimensional array as a propagation of kinks in a discrete sine-Gordon lattice, we find consistency between numerical results and the experimental data. This consistency indicates that the concept of fluxons as moving relativistic particles can be still used even for strongly discrete lines. However, two important classes of phenomena are found which do not have any counterparts in the continuum case. These are concerned with the data that we obtain in the short-wavelength limit (determined by line discreteness) and with damping requirements that are necessary in order to stabilize kink propagation.

Biharmonic-driven long Josephson junctions are studied within the framework of a perturbative approach for the motion of fluxons in the junctions. A thorough study of phase-locking states is carried out giving a generalization of the phase-locking conditions valid for any type of periodic external drive. These conditions give rise to a classification of phase-locked steps on the current-voltage characteristics of the junction. Enhancement of dynamical phase-locked regions and stabilization in connection with the phenomenon of chaos suppression are two interesting characteristics of biharmonic phase-locking states. Numerical simulations show that the theoretical predictions are, in general, well reproduced.

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.

There is experimental evidence in long Josephson junctions of rf-induced dynamical states that are not observed in small junctions. Such states consist of groups of current steps, asymmetrical with respect to the McCumber curve, having amplitudes much larger than the expected rf-induced (Shapiro) steps. Numerical solutions of the model equation (the perturbed sine-Gordon equation) obtained from the multimode expansion are in good agreement (at least qualitatively) with the experimental data. The basic dynamic configuration consists of multifluxon bunches phase locked to the rf signal.