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We have investigated numerically a system consisting of a long Josephson junction embedded into a microstrip resonator. For such a configuration the Josephson vortex dynamics in the junction is driven by the oscillating currents in the resonator. We have calculated the complex impedance of the junction at the resonant frequency. The Q factor of the resonator and the change of the resonant frequency of the whole system can then be easily calculated knowing the parameters of the resonator without Josephson junction. The fluxon dynamics and the complex impedance of the junction are simulated for different values of the junction length, damping, dc bias current, and dc magnetic field. This allows us to explain the behavior of the impedance of the junction and different loss mechanisms. In particular, we predict a nonmonotonic dependence of the both real and imaginary parts of the impedance on the amplitude of the oscillating current in the resonator for some range of parameters. Our results show that such a combination of active and passive superconducting elements forms a rather interesting nonlinear physical system which might be useful for development of tunable/switchable superconducting resonators and suggests an experimental technique for detecting trapped vortices in long junctions. [ABSTRACT FROM AUTHOR]

We have investigated the dependence of the critical current I[sub C] on the value and orientation of an externally applied magnetic field H for interface-engineered YBa[sub 2]Cu[sub 3]O[sub 7-x] ramp-type Josephson junctions. The results are compared with measurements of Nb ramp-type junctions with a PdAu interlayer. The I[sub C] versus H dependences are similar to Fraunhofer patterns and their modulation period changes several orders of magnitude with the orientation of the magnetic field. For both junction types, the dependence of the modulation period on the orientation of the magnetic field can be well described by the change of the relevant projection of the junction area and the influence of flux-focusing. Therefore the features of the I[sub C](H) curves have to be attributed to the ramp geometry and not to specific properties of the superconducting material. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Discusses the numerical investigation of the radiation emission by a system of two magnetically coupled and long Josephson junctions (LJJ). Analysis of time-dependent synchronized voltage response in the flux-flow regime; Advantages of using stacked Josephson junctions as oscillators for the sub-millimeter wave band; Applications for long Josephson junctions that operated in the flux-flow mode; Description of a theoretical model which illustrates the dynamics of N-fold stacks of magnetically coupled LJJ.

Presents a study which simulated and experimentally investigated the effect of non-uniform magnetic field penetration into a one-dimensional array of over-damped Josephson junctions. Dynamics of fluxons in underdamped junction arrays; Results of the experiments; Factors attributed to the steps with extremely low differential resistance.

We study a 0-π dc superconducting quantum interference device (SQUID) with asymmetric inductances and critical currents of the two Josephson junctions (JJs). By considering such a dc SQUID as a black box with two terminals, we calculate its effective current-phase relation Is(ψ) and the Josephson energy U(ψ), where ψ is the Josephson phase across the terminals. We show that there is a domain of parameters where the black box has the properties of a φ JJ with degenerate ground state phases ψ=±φ. The φ domain is rather large, so one can easily construct a φ JJ experimentally. We derive the current phase relation and show that it can be tuned in situ by applying an external magnetic flux resulting in a continuous transition between the systems with static solutions ψ=±φ,ψ=φ0 (φ0≠0,π) and even ψ=φ0±φ. The dependence of φ0 on applied magnetic flux is not 2π (one flux quantum) periodic. [ABSTRACT FROM AUTHOR]

The 0-π transition in Superconductor-Insulator-superconductor-Feiromagnet-Superconductor (SIsFS) Josephson junctions (JJs) was investigated experimentally. As predicted by theory, an s-layer inserted into a ferromagnetic SIFS junction can enhance the critical current density up to the value of an SIS tunnel junction. We fabricated Nb' | AlOx | Nb | Ni60Cu40 | Nb JJs with wedgelike s (Nb) and F (Ni60Cu40) layers and studied the Josephson effect as a function of the s- and F-layer thickness, ds and dF, respectively. For ds = 11 nm, π-JJs with SIFS-type jc(dF) and critical current densities up to jcπ = 60 A/cm² were obtained at 4.2 K. Thicker ds led to a drastic increase of the critical current decay length, accompanied by the unexpected disappearance of the 0-π transition dip in the jc(dF) dependence. Our results are relevant for superconducting memories, rapid single flux quantum logic circuits, and solid state qubits. [ABSTRACT FROM AUTHOR]

We demonstrate a high-frequency generator operating at ~200 GHz based on flipping a semifluxon in a Josephson junction of moderate normalized length. The semifluxon spontaneously appears at the π discontinuity of the Josephson phase artificially created by means of two tiny current injectors. The radiation is detected by an on-chip detector (tunnel junction). The estimated radiation power (at the detector) is ~8 nW and should be compared with the dc power of ~100nW consumed by the generator. The measured radiation linewidth, as low as 1:1MHz, is typical for geometrical (Fiske) resonances, although we tried to suppress such resonances by placing well-matched microwave transformers at its both ends. Making use of a phase-locking feedback loop, we are able to reduce the radiation linewidth to less than 1Hz measured relative to the reference oscillator and defined just by the resolution of our measurement setup. [ABSTRACT FROM AUTHOR]

We consider a f Josephson junction, which has a bistable zero-voltage state with the stationary phases ...=±f. In the nonzero voltage state the phase "moves" viscously along a tilted periodic double-well potential. When the tilting is reduced quasistatically, the phase is retrapped in one of the potential wells. We study the viscous phase dynamics to determine in which well (-f or +f) the phase is retrapped for a given damping, when the junction returns from the finite-voltage state back to the zero-voltage state. In the limit of low damping, the f Josephson junction exhibits a butterfly effect-extreme sensitivity of the destination well on damping. This leads to an impossibility to predict the destination well. [ABSTRACT FROM AUTHOR]

The φ Josephson junction has a doubly degenerate ground state with the Josephson phases ±φ. We demonstrate the use of such a φ Josephson junction as a memory cell (classical bit), where writing is done by applying a magnetic field and reading by applying a bias current. In the 'store' state, the junction does not require any bias or magnetic field, but just needs to stay cooled for permanent storage of the logical bit. Straightforward integration with rapid single flux quantum logic is possible. [ABSTRACT FROM AUTHOR]

We consider an asymmetric 0-π Josephson junction consisting of 0 and π regions of different lengths L0 and Lπ. As predicted earlier this system can be described by an effective sine-Gordon equation for the spatially averaged phase ψ so that the effective current-phase relation of this system includes a negative second harmonic ∝ sin(2ψ). If its amplitude is large enough, the ground state of the junction is doubly degenerate ψ = ± φ, where φ depends on the amplitudes of the first and second harmonics. We study the behavior of such a junction in an applied magnetic field H and demonstrate that H induces an additional term ∝ H cos ψ, in the effective current-phase relation. This results in a nontrivial ground state tunable by magnetic field. The dependence of the critical current on H allows for revealing the ground state experimentally. [ABSTRACT FROM AUTHOR]

We extend the theory of self-resonances in short Josephson junctions to the case of a piecewise constant critical current density and a κ discontinuity in the Josephson phase. We calculate the amplitude of the self-resonances as a function of applied magnetic field by using an extension of the approach introduced by Kulik for conventional Josephson junctions (I. O. Kulik, JETP Lett. 2, 84 (1965)). The theory given here agrees with existing experiments on superconducting-insulator-ferromagnet-superconductor 0-π Josephson junctions. The results are relevant to the characterization of all modern 0-π junctions as well as 0-κ junctions with artificially created phase discontinuities: high-temperature grain boundary junctions, junctions with a ferromagnetic barrier, and junctions with current injectors. [ABSTRACT FROM AUTHOR]

Within the framework of the Usadel equations, the Josephson effect in a superconductor-insulator-ferromagnet-superconductor (SIFS) structure with a spatially heterogeneous transparency of the SF interface has been studied. It is shown that, at a certain thickness of the F layer, a stepwise variation of the transparency leads to the formation of a region of size ∼ξ F (coherence length in a ferromagnet), where the Josephson supercurrents of different signs may flow. This may lead to the dependence of the junction critical current on the external magnetic field qualitatively different from the Fraunhofer pattern typically observed in usual Josephson junctions. [ABSTRACT FROM AUTHOR]

The authors fabricated high quality Nb/Al2O3/Ni0.6Cu0.4/Nb superconductor-insulatorferromagnet-superconductor Josephson tunnel junctions. Depending on the thickness of the ferromagnetic Ni0.6Cu0.4 layer and on the ambient temperature, the junctions were in the 0 or π ground state. All junctions have homogeneous interfaces showing almost perfect Fraunhofer patterns. The Al2O3 tunnel barrier allows one to achieve rather low damping, which is desired for many experiments especially in the quantum domain. The McCumber parameter βc increases exponentially with decreasing temperature and reaches βc≈700 at T=2.11 K. The critical current density in the π state was up to 5 A/cm2 at T=2.11 K, resulting in a Josephson penetration depth λJ as low as 160 μm. Experimentally determined junction parameters are well described by theory taking into account spin-flip scattering in the Ni0.6Cu0.4 layer and different transparencies of the interfaces. [ABSTRACT FROM AUTHOR]

Investigates the tunable phase locking oscillator of stacked Josephson junctions. Accounts on several kinds of mutually synchronized flux-flow states; Introduction of magnetic flux on the same polarity of both junctions; Effect of in-phase locking modes on radiation linewidth.

The possibility of a fast and precise Abrikosov vortex manipulation by a focused laser beam opens the way to create laser-driven Josephson junctions. We theoretically demonstrate that a vortex pinned in the vicinity of the Josephson junction generates an arbitrary ground state phase which can be equal not only to 0 or π but to any desired φ0 value in between. Such φ0 junctions have many peculiar properties and may be effectively controlled by the optically driven Abrikosov vortex. Also we theoretically show that the Josephson junction with the embedded vortex can serve as an ultrafast memory cell operating at sub THz frequencies. [ABSTRACT FROM AUTHOR]

We consider a short Josephson junction with a phase discontinuity κ created, e.g., by a pair of tiny current injectors, at some point x0 along the width of the junction. We derive the effective current-phase relation (CPR) for the system as a whole, i.e., reduce it to an effective pointlike junction. From the effective CPR we obtain the ground state of the system and predict the dependence of its critical current on κ. We show that in a large range of κ values the effective junction behaves as a φ0 Josephson junction, i.e., has a unique ground state phase φ0 within each 2π interval. For κ≈π and x0 near the middle of the junction one obtains a φ0±φ junction, i.e., a Josephson junction with degenerate ground state phase φ0±φ within each 2π interval. Further, in view of possible escape experiments especially in the quantum domain, we investigate the scaling of the energy barrier and eigenfrequency close to the critical currents and predict the behavior of the escape histogram width σ(κ) in the regime of the macroscopic quantum tunneling. [ABSTRACT FROM AUTHOR]

The proximity effect and the Josephson current in a superconductor-insulator-ferromagnet-superconductor junction are investigated within the framework of the quasiclassical Eilenberger equations. This investigation allows us to compare the dirty and the clean limits, to investigate an arbitrary impurity scattering, and to determine the applicability limits of the Usadel equations for such structures. The role of different types of the FS interlace is analyzed. It is shown that the decay length &xi1 and the spatial oscillation period 2πξ2 of the Eilenberger function may exhibit a nonmonotonic dependence on the properties of the ferromagnetic layer such as exchange field or electron mean-free path. The results of our calculations are applied to the interpretation of experimentally observed dependencies of the critical current density on the ferromagnet thickness in Josephson junctions containing a Ni layer with an arbitrary scattering. [ABSTRACT FROM AUTHOR]

We used one-dimensional coupled sine-Gordon equations combined with heat diffusion equations to numerically investigate the thermal and electromagnetic properties of a 300 pm long intrinsic Josephson junction stack consisting of N = 700 junctions. The junctions in the stack are combined with M segments where we assume that inside a segment all junctions behave identically. Most simulations are for M = 20. For not too high bath temperatures there is the appearance of a hot spot at high-bias currents. In terms of electromagnetic properties, robust standing-wave patterns appear in the current density and electric field distributions. These patterns come together with vortex/antivortex lines across the stack that correspond to jr-kink states, discussed before in the literature for a homogeneous temperature distribution in the stack. We also discuss scaling of the thermal and electromagnetic properties with M, on the basis of simulations with M between 10 and 350. [ABSTRACT FROM AUTHOR]

We consider 0-π Josephson junctions consisting of 0 and π regions of lengths L0 and Lπ with critical current densities j c0 and jcπ, respectively. The dependence of the Josephson current on the phase-shift averaged along the junction is derived. We show that these systems exhibit the main features of φ Josephson junctions--the ground state is doubly degenerate and the current-phase relation can be tuned in situ by applying magnetic field. In the limit of short and long 0 and π regions, the current phase relation is derived analytically. In the case of intermediate lengths of 0 and π regions, the current-phase relation is calculated numerically. [ABSTRACT FROM AUTHOR]

We demonstrate experimentally the existence of Josephson junctions having a doubly degenerate ground state with an average Josephson phase &psgr; = ±y. The value of

In long Josephson junctions with multiple discontinuities of the Josephson phase, fractional vortex molecules are spontaneously formed. At each discontinuity point a fractional Josephson vortex carrying a magnetic flux \PΦ\ < |PΦ|0, where PΦ0 &ap; 2.07 × 10-15 Wb is the magnetic flux quantum, is pinned. Each vortex has an oscillatory eigenmode with a frequency that depends on PΦ/PΦ0 and lies inside the plasma gap. We experimentally investigate the dependence of the eigenfrequencies of a two-vortex molecule on the distance between the vortices, on their topological charge p = 2πPΦ/PΦ0, and on the bias current &ggr; applied to the Josephson junction. We find that, with decreasing distance between vortices, a splitting of the eigenfrequencies occurs that corresponds to the emergence of collective oscillatory modes of both vortices. We use a resonant microwave spectroscopy technique and find good agreement between experimental results and theoretical predictions. [ABSTRACT FROM AUTHOR]

We demonstrate experimentally the operation of a deterministic Josephson ratchet with tunable asymmetry. The ratchet is based on a φ Josephson junction with a ferromagnetic barrier operating in the underdamped regime. The system is probed also under the action of an additional dc current, which acts as a counterforce trying to stop the ratchet. Under these conditions the ratchet works against the counterforce, thus producing a nonzero output power. Finally, we estimate the efficiency of the φ Josephson junction ratchet. [ABSTRACT FROM AUTHOR]

We investigate experimentally the retrapping of the phase in a φ Josephson junction upon return of the junction to the zero-voltage state. Since the Josephson energy profile U0(ψ) in φ JJ is a 2π periodic double-well potential with minima at ψ=±φmod2π, the question is at which of the two minima -φ or +φ the phase will be trapped upon return from a finite voltage state during quasistatic decrease of the bias current (tilt of the potential). By measuring the relative population of two peaks in escape histograms, we determine the probability of phase trapping in the ±φ wells for different temperatures. Our experimental results agree qualitatively with theoretical predictions. In particular, we observe an onset of the butterfly effect with an oscillating probability of trapping. Unexpectedly, this probability saturates at a value different from 50% at low temperatures. [ABSTRACT FROM AUTHOR]