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The continuous development of superconducting electronics is encouraging several studies on hybrid Josephson junctions (JJs) based on superconductor-ferromagnet-superconductor (SFS) heterostructures, as either spintronic devices or switchable elements in quantum and classical circuits. Recent experimental evidence of macroscopic quantum tunneling and of an incomplete 0-$\ensuremath{\pi}$ transition in tunnel-ferromagnetic spin-filter JJs could also enhance the capabilities of SFS JJs as active elements. Here, we provide a self-consistent electrodynamic characterization of $\mathrm{Nb}\mathrm{N}$/$\mathrm{Gd}\mathrm{N}$/$\mathrm{Nb}\mathrm{N}$ spin-filter JJs as a function of the barrier thickness, disentangling the high-frequency dissipation effects due to the environment from the intrinsic low-frequency dissipation processes. The fitting of the $I$-$V$ characteristics at 4.2 K and at 300 mK by using the tunnel-junction-microscopic model allows us to determine the subgap resistance ${R}_{\mathrm{sg}}$, the quality factor $Q$, and the junction capacitance $C$. These results provide the scaling behavior of the electrodynamic parameters as a function of the barrier thickness, which represents a fundamental step for the feasibility of tunnel-ferromagnetic JJs as active elements in quantum and classical circuits, and are of general interest for tunnel junctions other than conventional SIS JJs.

Triplet supercurrents in multilayer ferromagnetic Josephson junctions with misaligned magnetization can penetrate thicker ferromagnetic barriers compared to the singlet component. Although the static properties of these junctions have been extensively studied, the dynamic characteristics remain largely unexplored. Here we report a comprehensive electrodynamic characterization of multilayer ferromagnetic Josephson junctions composed of Co and Ho. By measuring the temperature-dependent current-voltage characteristics and the switching current distributions down to 0.3 K, we show that phase dynamics of junctions with triplet supercurrents exhibits long (in terms of proximity) junction behavior and moderately damped dynamics with renormalized capacitance and resistance. This unconventional behavior possibly provides a different way to dynamically detect triplets. Our results show new theoretical models are required to fully understand the phase dynamics of triplet Josephson junctions for applications in superconducting spintronics., DM, RC, FT would like to thank NANOCOHYBRI project (Cost Action CA 16218). NB acknowledges funding from the British Council through UKIERI programme and Loughborough University. MGB acknowledges funding from EPSRC Programme Grant EP/N017242/1.

High critical temperature superconductors (HTS) based devices can have impact in the study of the phase dynamics of Josephson junctions (JJs) thanks to the wide range of junction parameters they offer and to their unconventional properties. Measurements of current-voltage characteristics and of switching current distributions constitute a direct way to classify different regimes of the phase dynamics and of the transport, also in nontrivial case of the moderately damped regime (MDR). MDR is going to be more and more common in JJs with advances in nanopatterning superconductors and synthesizing novel hybrid systems. Distinctive signatures of macroscopic quantum tunneling and of thermal activation in presence of different tunable levels of dissipation have been detected in YBCO grain boundary JJs. Experimental data are supported by Monte Carlo simulations of the phase dynamics, in a wide range of temperatures and dissipation levels. This allows us to quantify dissipation in the MDR and partially reconstruct a phase diagram as guideline for a wide range of moderately damped systems. (C) 2014 Elsevier B.V. All rights reserved.

The interfacial coupling of two materials with different ordered phases, such as a superconductor (S) and a ferromagnet (F) is driving new fundamental physics and innovative applications. For example, the creation of spin-filter Josephson junctions and the demonstration of triplet supercurrents have suggested the potential of a dissipationless version of spintronics based on unconventional superconductivity. Here we demonstrate evidence for active quantum applications of S-F-S junctions, through the observation of macroscopic quantum tunneling in Josephson junctions with GdN ferromagnetic insulator barriers. We prove a clear transition from thermal to quantum regime at a crossover temperature of about 100 mK at zero magnetic field in junctions which demonstrate a clear signature of unconventional superconductivity. Following previous demonstration of passive S-F-S phase shifters in a phase qubit, our result paves the way to the active use of spin filter Josephson systems in quantum hybrid circuits., Comment: 16 pages, 4 figures

Recent findings by G.Pepe et al. (Appl.Phys.Lett.89, 2770, 2001) have given new insights over the dynamics of Josephson junctions subject to electronic pulse injection. These experiments can be considered important steps toward the study of non-equilibrium effects involved in junctions under very fast (e.g. laser induced) pumping signals. In the governing equations (sine-Gordon or coupled sine-Gordon depending on the system) pulse operation enters via Josephson current distribution or via bias current distribution. By means of numerical simulations here we study the dependence of the switching to resistive state on fast local depression of Josephson current. Next, using simulated electronic pulse train we show how the flip-flop operating mode found in G.Pepe et al. is a common feature of single or stacked Josephson systems without regard for the pulse waveform or the presence of the gap structure in the I-V characteristics. The effect of the length and of the magnetic field is also discussed.

The behavior of a Josephson system under pulsed operation is of utmost importance for developing reliable digital devices working at very high clock frequencies. Information about effect of thermal noise over pulsed operation is also useful to design errors free devices. Recent experiments in a system of two stacked junctions show that pulses applied to the first junction (injector) of the stack drive the Josephson biased second junction (detector) in the resistive state. The experiment was interpreted as pulsed-assisted escape toward resistive state as a consequence of an excitation of large non-linear oscillations in the detector junction that, with the contribution of thermal noise, drives out the junction from the zero voltage state. Moreover, an unstability of the resistive state toward the zero voltage state under the same pulsed operation was also noted. By means of a systematic numerical approach to the problem, we present a study of the pulsed-assisted escape using the framework of the thermal escape theory for the direct transition from zero voltage state to resistive state. We study the single junction case and a stacked configuration as examples of different systems showing pulsed-assisted escape. (C) 2002 Elsevier Science B.V. All rights reserved.

A ferromagnetic Josephson junction (JJ) represents a special class of hybrid system where different ordered phases meet and generate novel physics. In this work we report on the transport measurements of underdamped ferromagnetic NbN/GdN/NbN JJs at low temperatures. In these junctions the ferromagnetic insulator gadolinium nitride barrier generates spin-filtering properties and a dominant second harmonic component in the current-phase relation. These features make spin filter junctions quite interesting also in terms of fundamental studies on phase dynamics and dissipation. We discuss the fingerprints of spin filter JJs, through complementary transport measurements, and their implications on the phase dynamics, through standard measurements of switching current distributions. NbN/GdN/NbN JJs, where spin filter properties can be controllably tuned along with the critical current density (J c ), turn to be a very relevant term of reference to understand phase dynamics and dissipation in an enlarged class of JJs, not necessarily falling in the standard tunnel limit characterized by low J c values.

We have observed energy level quantization in an all high critical temperature superconductor d-wave Josephson junction. From the measurements we have also extracted the quality factor Q of the junction which is of the order of 40. These results indicate that the role of dissipation mechanisms in high temperature superconductors has to be revised, and may also have consequences for the class of solid state “quiet” quantum bit with longer coherence time.

We have measured the energy level separation from the ground state to the first excited state in a high critical temperature superconductor d-wave Josephson junction. We observe that the bias current dependence of the energy level separation is strongly affected by the large shunt capacitance and inductance of the junction electrodes. In order to describe the effect of a shunt capacitance and shunt inductance on the junction dynamics we derive the equation of motion of the total system. In general such a circuit displays two resonance frequencies corresponding to two different energy scales. We discuss various limits for the values of the shunt inductor and capacitor.

We report on the latest developments in the study of transport properties of YBa2Cu3O7-delta (YBCO)-based grain boundary (GB) biepitaxial Josephson junctions. We have successfully extended the fabrication process, originally developed for (110) oriented SrTiO3 substrates, to (110) oriented LaAlO3 substrates in view of microwave applications. We will report, also, on the observation of conductance fluctuations, we believe related to the coherent diffusion of quasi-particles across the GB. These results, together with the recent observation of macroscopic quantum tunneling and energy level quantization in these junctions, could help cast some light on the mechanism involved in transport across grain boundaries and in revising the role and the effect of quasi particles in d-wave transport.

The predominant d-wave pairing symmetry in high temperature superconductors leads to an unconventional current - phase relation in Josephson junctions. This circumstance may induce new effects in the dynamics of dc SQUIDs. In this contribution we report on the measurements of the dependence of the SQUID Josephson current on the external magnetic field taken at very low temperatures, down to 20 mK. Different grain boundaries have been fabricated by using the biepitaxial and the bicrystal technique. Some of the effects which are induced by a nonsinusoidal current - phase relation can be clearly identified in the dynamics of the SQUIDs. The experimental data are also compared with theoretical simulations taking into account the inductance of the loop. The data show that, in specific conditions, a non-negligible inductance of the loop can induce effects similar to an unconventional current - phase relation, with a pronounced second harmonic sin( 2 phi) term. This fact has to be taken into account when designing d- wave SQUIDs for quantum circuitry.

We describe the main ideas and some experimental outcomes of two experiments, both aimed to study quantum tunneling effects in high-T-C structures. In the first experiment, macroscopic quantum tunneling is demonstrated in grain boundary YBaCuO Josephson junctions revealing dissipation levels lower than expected and opening novel perspectives for quantum circuitry. In the second, the low temperature dissipation is dominated by quantum tunneling of individual Pearl vortices in ultra-thin CaBaCuO systems characterized by extremely long Pearl lengths. (c) 2006 Elsevier B.V. All rights reserved.

One of the stimulating area of superconductors investigations lies in the achieved and potential applications as radiation detectors. Results concerning annular Josephson junctions in this context are discussed. Fundamental aspects, mainly related to the fluxon dynamics in such structures, are discussed in detail. The results confirm the importance of the precious sharing of technological requests with fundamental physical implications. Peculiar results are reported dealing with new resonances occurring on these Josephson junctions of annular configuration.

The presence of spontaneous currents in unconventional Josephson junctions gives rise to the possibility of using zero magnetic field magnetization states in an unconventional loop as qubit states in a Josephson quantum computing device. However, the advantage of zero field may be insufficient to overcome the problems arising in the design and fabrication of low-noise, low-dissipation, reproducible devices. On the other hand, low-T/sub c/ "flux" devices suffer the same problems though their perspectives are more promising. The solution could be an increase in the device complexity, but with the gain of an effective advantage on the external noise rejection of the system during quantum evolution. Using advanced fabrication techniques it is now possible to build Josephson junction arrays made of superconducting unconventional loops. On this basis, a new qubit device can be designed as a ring array of unconventional loops. From a theoretical point of view this device is analogous to an annular Josephson junction, but with a "built in" natural degeneracy. For their topological and energetical properties, such arrays could be excellent devices for the qubit circuitry usable as the basic building blocks for quantum computing devices. In this work, we show how some of the principles of qubit design could be implemented in such devices.

Dissipation encodes the interaction of a quantum system with the environment and regulates the activation regimes of a Brownian particle. We have engineered grain boundary biepitaxial YBaCuO junctions to drive a direct transition from a quantum activated running state to a phase diffusion regime. The crossover to the quantum regime is tuned by the magnetic field and dissipation is described by a fully consistent set of junction parameters. To unravel phase dynamics in moderately damped systems is of general interest for advances in the comprehension of retrapping phenomena and in view of quantum hybrid technology.

We report a systematic study of the transport properties of high critical temperature superconductor (HTS) biepitaxial Josephson junctions in the submicron range. Junction performances point to more uniform and reproducible devices and to better control of d-wave intrinsic properties. Outcomes promote novel insights into the transport mechanisms across grain boundaries and encourage further developments in the control of dissipation in HTS devices. The application of nanotechnology to HTS could be an additional tool to properly engineer the junction properties to match specific circuit design also in view of the integration into hybrid quantum circuits. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3388035]

The Josephson effect is a probe of unparalleled performances in the study of a variety of macroscopic quantum phenomena. In the present article an overview of important achievements and challenging trends is given referring, in particular, to macroscopic quantum tunneling and energy level quantization. The attention is mainly addressed to high-TC superconducting structures and recent investigations concerning nanostructures.

We report on $\hbox{YBa}_{2}\hbox{Cu}_{3}\hbox{O}_{7-x} $ biepitaxial grain boundary Josephson junctions (JJs) demonstrating significant tunability of their circuital parameters. We show how the enhanced control of the junction properties, and particularly of the capacitance, leads to advances in the understanding of phase dynamics of high-critical-temperature superconductor JJs and potentially of their intrinsic dissipation mechanisms.

We have identified anomalous behavior of the escape rate out of the zero-voltage state in Josephson junctions with a high critical current density Jc. For this study we have employed YBa2Cu3O7-x grain boundary junctions, which span a wide range of Jc and have appropriate electrodynamical parameters. Such high Jc junctions, when hysteretic, do not switch from the superconducting to the normal state following the expected stochastic Josephson distribution, despite having standard Josephson properties such as a Fraunhofer magnetic field pattern. The switching current distributions (SCDs) are consistent with nonequilibrium dynamics taking place on a local rather than a global scale. This means that macroscopic quantum phenomena seem to be practically unattainable for high Jc junctions. We argue that SCDs are an accurate means to measure nonequilibrium effects. This transition from global to local dynamics is of relevance for all kinds of weak links, including the emergent family of nanohybrid Josephson junctions. Therefore caution should be applied in the use of such junctions in, for instance, the search for Majorana fermions., Comment: 8 pages, 4 figures

The magnetic behaviour of multiply connected superconductors (MCS) can be described by analysing the simplest loop structures containing Josephson junctions: conventional loops with all conventional Josephson junctions and π-loops with an odd subset of π-junctions. The latter are unconventional Josephson junctions in which the coupling has the reversed sign and appears in the ceramic materials as a consequence of d-pairing. Among MCS, the magnetic behaviour of large β two-dimensional Josephson junction arrays (JJA) is based on the single loop behaviour. Solving full mutual inductance Josephson junction square array equations with and without π-loops shows that the mutual inductance coupling influences the distribution of π/conventional loops without substantially altering their single loop magnetization. The JJA mean magnetic behaviour in a low field can be recovered using a simple energy approach based on the single loop solutions avoiding the solution of the array equations. Also, we draw some consequences on the behaviour of more complex MCS as high-Tc ceramics and their observed paramagnetic susceptibilities (paramagnetic Meissner effect).

LTC Josephson junction arrays (JJA) have recently gained interest as a model for the study of magnetization in superconductor materials. The presence of paramagnetic Meissner effect in such devices when field-cooled makes them an excellent candidate to discriminate between the different explanations given for such phenomenon. On the other hand, the understanding of the JJA physics is not simple due to the interplay between the non-linear behaviour of Josephson devices and the mutual induction effects in large JJA. The use of a SQUID microscope (SSM) to catch the magnetic image of the array is a distinctive feature of the experiment. So a study of SQUID read-out based on the actual JJA magnetic behaviour is needed in order to have a correct picture of what is going on. In this paper, using a simple way to reconstruct the complex array far-field viewed by the SQUID, we study how the flux read-out is dependent on array resolution, distance, tilting and noise for the typical situations in which arrays are found. This paper was presented at the 8th International Superconductive Electronics Conference, Osaka, Japan, 19–22 June 2001

We demonstrate that a large area Josephson junction oscillating in the fluxon oscillator mode can be synchronized to other large junctions and simultaneously pump, by emitted radiation, a small area junction. We study the synchronization of the oscillations of the long junctions as a function of relevant experimental parameters such as bias current and stripline coupling characteristics. Experimental results obtained on coupled series arrays of Josephson junctions designed on the basis of our calculations are presented. We have fabricated coupled arrays containing each up to 1500 junctions in order to estimate the usefulness of our calculations for voltage standard devices.

Two-dimensional (2D) Josephson Junctions arrays have been studied largely in the overdamped case. Only recently some experimental, numerical and theoretical results have been obtained in the underdamped case. The nature of dynamical states of an array when placed in a magnetic field is again not fully explained and only approximate (linear) models have been proposed. On the other hand, the comprehension of such dynamics is of great importance for understanding the behavior of such arrays. In this work, we study numerically some of the dynamical states in 2D arrays using a full inductance matrix in the array equations. The results show that the response of the array to the magnetic field is governed at large by the parameter /spl beta//sub L/. Moreover we study the underlying dynamics which is dominated by so-called checkerboard solutions. The properties of such arrays are investigated in view of applications.

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.

A locally phase-shifted Sine–Gordon model well accounts for the phenomenology of unconventional Josephson junctions. The phase dynamics shows resonant modes similar to Fiske modes that appear both in the presence and in the absence of the external magnetic field in standard junctions. In the latter case, they are also in competition with zero field propagation of Sine–Gordon solitons, i.e., fluxons, which give rise to the so-called zero field steps in the current–voltage (I–V) of the junction. We numerically study the I–V characteristics and the resonances magnetic field patterns for some different faceting configurations, in various dissipative regimes, as a function of temperature. The simulated dynamics of the phase is analyzed for lower-order resonances. We give evidence of a nontrivial dynamics due to the interaction of propagating fluxons with localized semifluxons. Numerical results are compared with experimental outcomes obtained on high-quality high-Tc grain boundary YBCO junctions.

We investigate the phase dynamics of moderately damped NbN/AlN/NbN Josephson junctions and we present experimental results on detailed aspects of phase diffusion processes. We measure both single escape and multiple escape and retrapping events obtaining a crossover temperature T* from Kramers to phase diffusion switching. We observe a clear dependence of the crossover temperature T* by the bias current ramp rate, while the damping factor Q remains the same. The measured effect is in strong agreement with theoretical predictions reported by Fenton and Warburton. © 2014 AIP Publishing LLC.

We have measured the rate of escape out of the zero-voltage state in Josephson tunnel junctions as a function of the applied magnetic field. A marked difference is found in the behaviour of long and small junctions. In all cases, the statistical distribution of the switching currents can be described using a Kramers model for the escape process, where the barrier to be overcome is the Josephson barrier and the activation energy is due to an effective temperature T/sub e/. For small junctions T/sub e/ coincides, as expected, with the thermodynamic temperature, regardless of the applied magnetic field. For long junctions instead it is found that the escape temperature depends markedly on the magnetic field and on the junction geometry (inline or overlap), suggesting a close relationship with the magnetic field distribution inside the junction.

We report on the study of the phase dynamics of high-critical-temperature superconductor Josephson junctions. We realized YBa${}_{2}$Cu${}_{3}$O${}_{7\ensuremath{-}x}$ grain boundary biepitaxial junctions in the submicron scale using low-loss substrates and analyzed their dissipation by comparing the transport measurements with Monte Carlo simulations. The behavior of the junctions can be fitted using a model based on two quality factors, which results in a frequency-dependent damping. Moreover, our devices can be designed to have Josephson energy of the order of the Coulomb energy. In this unusual energy range, phase delocalization strongly influences the device's dynamics, promoting the transition to a quantum phase diffusion regime. We study the signatures of such a transition by combining the outcomes of Monte Carlo simulations with the analysis of the device's parameters, the critical current, and the temperature behavior of the low-voltage resistance ${R}_{0}$.

The moderately damped regime in a Josephson junction (JJ) is quite common in devices characterized by low critical currents and therefore by low Josephson energies. Measurements of switching current distribution (SCD) are a direct way of discriminating the phase dynamics also in the nontrivial case of moderate damping, which is going to be more and more common with advances in nanopatterning superconductors and in materials science finalized to build hybrid systems. We report on measurements of SCDs, both in thermal and quantum regime, on moderately damped YBaCuO grain boundary biepitaxial JJs. A direct transition from phase diffusion regime to macroscopic quantum tunnelling occurs at about 130 mK. The crossover to the quantum regime is tuned by the magnetic field and phase dynamics is described by a fully consistent set of junction parameters derived through numerical simulations.

Coherent passage of Cooper pairs in a Josephson junction (JJ) above the liquid nitrogen temperature has been the first impressive revolutionary effect induced by high critical temperature superconductors (HTS) in the domain of the study of Josephson effect (JE). But this has been only the start. A d-wave order parameter has lead to significant novel insights in the physics of the JE turning into a device the notion of a ?-junction. Spontaneous currents in a frustrated geometry, Andreev bound states, long-range proximity effect have rapidly become standard terms in the study of the JE, standing as a reference bench for conventional systems based on low critical temperature superconductors (LTS) and inspiring analogies for junctions based on novel superconductors discovered in the meantime. The extreme richness of the physics of HTS JJs has not been adequately supported by the expected impact in the applications, the main reason lying in the complexity of these materials and in the consequent unsatisfactory yield and reproducibility of the performances of the JJs within the required limits. The continuous progress in material science, and specifically in the realization of oxide multi-layers, and in nanotechnologies applied to superconductors, accompanied by the advances in a better understanding of the properties of HTS and of HTS devices, has as a matter of fact opened possible novel scenarios and interest in the field. We intend to give a brief overview on interesting new problems concerning HTS JJs of inspiration also for other systems. We also review some ideas and experimental techniques on macroscopic quantum decay phenomena occurring in Josephson structures. The attention is mainly addressed to intermediate levels of dissipation, which characterize a large majority of low critical current Josephson devices and are therefore an unavoidable consequence of nanotechnology applied more and more to Josephson devices.

The moderately damped regime (MDR) in a Josephson junction (JJ) is quite distinct from the well-known case of underdamped systems and quite common in devices characterized by low critical currents. Measurements of switching current distribution (SCD) are a direct way of discriminating the phase dynamics and the transport also in nontrivial cases of moderate damping. MDR is going to be more and more common with advances in nanopatterning superconductors and in materials science finalized to build hybrid devices. We report on measurements of SCDs, both in the thermal and quantum regime, on moderately damped NbN/MgO/NbN JJs. A transition from the Thermal Activation (TA) regime to Macroscopic Quantum Tunnelling (MQT) occurs at about 90 mK, while above 1.6 K a phase diffusive dynamics is clearly observed. The combined analysis of these regimes through numerical simulations allowed us to extract the fundamental parameters, which completely characterize the phase dynamics of such moderately damped JJs.

International audience; Experiments on very small bubbles showed that classical Gibbs-Laplace theory on surface tension cannot be applied when the dimension of the interface becomes comparable with bubble radi[1]. On the other hand the pressure jump across the interface is from a experimental point of view the most suitable quantity to be measured in order to know surface tension and determine its dependence on curvature. Therefore Laplace formula needs to be generalized to the case of very small bubbles. The theory of second gradient fluids (Germain cf.[2]) accounts the effects of high density gradients (occurring in fluid interface) on fluid pressure. In [3] it is shown that second gradient theory allows the determination of the simplest consistent generalization of Laplace formula and an equivalent bubble theory in which surface tension and radius of a bubble are determined in terms of the density spatial field. This paper complements the quoted results analyzing them numerically. We find that our model: i) predicts the existence of a (minimal) nucleation radius, i.e. a radius which is a minimum possible for the equilibrium of a small bubble; ii) permits, through the new Laplace formula, to evaluate the dependence of surface tension on curvature with results very similar to actual experiments; iii) allows the determination of the range of validity of classical Laplace formula and a theoretical prediction on the departure of experimental data from it.

The Josephson effect is a privileged access to the macroscopic quantum nature of superconductors. We review some ideas and experimental techniques on macroscopic quantum decay phenomena occurring in Josephson structures. The attention is mainly addressed to intermediate levels of dissipation which characterize a large majority of low critical current Josephson devices and are therefore an avoidable consequence of nanotechnology applied more and more to Josephson devices. Phase diffusion phenomena take over thermal activation in some temperature ranges also affecting the transition to macroscopic quantum tunneling, enriching the phase diagram mostly defined by the Josephson energy, the temperature and the level of dissipation.

We report on the occurrence of multiple hopping and retrapping of a Brownian particle in a tilted washboard potential. The escape dynamic has been studied experimentally by measuring the switching current distributions as a function of temperature in a moderately damped NbN/MgO/NbN Josephson junction. At low temperatures the second moment of the distribution increases in agreement with calculations based on Kramers thermal activation regime. After a turn-over temperature T*, the shape of the distributions starts changing and width decreases with temperature. We analyze the data through fit of the switching probability and Monte Carlo simulations and we find a good agreement with a model based on a multiple retrapping process.

High quality epitaxial NbN/MgO/NbN Josephson junctions have been realized with MgO barriers up to a thickness of d = 1 nm. The junction properties coherently scale with the size of barrier, and low critical current densities down to 3 A/cm(2) have been achieved for larger barriers. In this limit, junctions exhibit macroscopic quantum phenomena for temperatures lower than 90 mK. Measurements and junction parameters support the notion of a possible use of these devices for multiphoton quantum experiments, taking advantage of the fast non equilibrium electron-phonon relaxation times of NbN. (C) 2011 American Institute of Physics. [doi:10.1063/1.3624471]

We have investigated the static and dynamic properties of long YBa2Cu3O7-delta 0-pi Josephson junctions and compared them with those of conventional 0 junctions. Scanning SQUID microscope imaging has revealed the presence of a semifluxon at the phase discontinuity point in 0-pi Josephson junctions. Zero field steps have been detected in the current-voltage characteristics of all junctions. Comparison with simulation allows us to attribute these steps to fluxons traveling in the junction for conventional 0 junctions and to fluxon-semifluxon interactions in the case of 0-pi Josephson junctions.

Series-biased arrays of long Josephson junction fluxon oscillators can be phase locked by mutual coupling to a high-Q, linear distributed resonator. A simplified model of such a device, consisting of junctions described by the particle-map perturbation theory approach which are capacitively coupled to a lumped, linear tank circuit, reproduce the essential experimental observations at a very low computational cost. A more sophisticated model, consisting of partial differential equation descriptions of the junctions, again mutually coupled to a linear tank, substantially confirm the predictions of the simplified model. In the particle-map model, the locking range in junction bias current increases linearly with the coupling capacitance; in the partial differential equation (p.d.e.) model, this holds up to a certain maximum value of the capacitance, after which a saturation of the locking range is observed. In both models, for a given spread of junction lengths, the existence of a minimum value of the capacitance for locking to a tank with a given resonant frequency is evidenced. Journal of Applied Physics is copyrighted by The American Institute of Physics.

By means of numerical simulations we study the dynamics of the phase difference in faceted long Josephson junctions. We compare our simulations with experiments on a sample of high quality high-Tc biepitaxial junctions. Adjusting junction length and dissipation to reproduce the observed data we found a significant agreement, which permits to identify the observed resonances mostly as Eck steps. Some considerations are also made on the requirements to observe flux-flow or Fiske resonances in these junctions.

The influence of surface-resistance dissipation on phase locking of fluxon oscillations in long Josephson junctions of in-line geometry subjected to microwave fields which interact with the fluxon at the junction boundaries is studied by the perturbation- theory map approach and by full integration of the pde model of the junction.

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.

We study the vortex-vortex and vortex-antivortex interaction in arrays of Josephson junction particularly addressing the effect of mutual induction in the intermediate screening ranges (betaa = 0.5). The modifications of the barrier energy for the displacement of a vortex in presence of a second vortex (antivortex), or more vortices (antivortices), are derived in an "adiabatic" approach. To this end, we have developed a fast explicit algorithm, which permits to retrieve the phases and the magnetization of the array in a reasonable time and with little memory occupation for relatively large arrays [e.g. o(100 times 100)]. This new model is able to catch the essential characters of the more complex "fully mutual coupling" model. We provide also some indications on the relation between energy barriers and the observation of vortex quantum tunneling phenomena in Josephson arrays and related systems as high-TC films.

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 $\ensuremath{\alpha}$ 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, $\ensuremath{\alpha}$ 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 $\ensuremath{\alpha}$ of a $5\ifmmode\times\else\texttimes\fi{}5$ array may still exhibit temperature oscillations provided the applied magnetic field is strong enough to compensate for the screening-induced effects.

Here we present the direct observation of macroscopic quantum properties in an all high-critical-temperature superconductor d-wave Josephson junction. Although dissipation caused by low-energy excitations is expected to strongly suppress macroscopic quantum effects, we demonstrate energy level quantization in our d-wave Josephson junction. The result indicates that the role of dissipation mechanisms in high-temperature superconductors has to be revised, and it may also have consequences for the class of solid-state “quiet” quantum bits with superior coherence time.

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 ${\mathrm{Y}\mathrm{B}\mathrm{a}}_{2}{\mathrm{C}\mathrm{u}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ grain boundary biepitaxial JJ. This proves that dissipation can be significantly reduced by a proper junction configuration, which is of significance for quantum coherence.

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 $\sim \mu J$, 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 and the pulse duration of the Josephson junction., Comment: 17 pages, 5 figures

The effect of a perpendicular magnetic field on a disordered two‐dimensional Josephson junction array (JJA) is analyzed using numerical simulation of full mutual inductance array equations. Disordered JJA can represent either itself or a model for granular high‐Tc disordered thin films. In usual zero field cooling conditions and absence of disorder the field penetrate the sample following a Bean critical state model. Using a simple technique it is also possible to obtain the whole 3D field map around the array. It can be shown that disorder induces a scattering of flux lines giving rise to some flux penetration into the sample. The 3D field can be used to produce 2D images of the field intensity to be compared with existing magnetic imaging techniques. We briefly discuss the comparison of simulated images with existing imaging experiments, as SQUID Scanning Microscopy (SSM) and Magneto‐Optical techniques (MO).