Dimensional Josephson Junction Research Articles

Pure odd-frequency superconductivity at the cores of proximity vortices

After more than a decade, direct observation of the odd frequency triplet pairing state in super- conducting hybrid structures remains elusive. We propose an experimentally feasible setup that can unambiguously reveal the zero energy peak due to proximity-induced equal spin superconducting triplet correlations. We theoretically investigate a two dimensional Josephson junction in the diffu- sive regime. The nanostructure consists of a normal metal sandwiched between two ferromagnetic layers with spiral magnetization patterns. By applying an external magnetic field perpendicular to the junction plane, vortices nucleate in the normal metal. The calculated energy and spatially resolved density of states, along with the pair potential, reveal that remarkably, only triplet Cooper pairs survive in the vortex cores. These isolated odd frequency triplet correlations result in well defined zero energy peaks in the local density of states that can be identified through tunneling spectroscopy experiments. Moreover, the diffusive regime considered here rules out the possibility of Andreev bound states in the vortex core as contributors to the zero energy peaks.

Curved Josephson junction

The constant curvature one and quasi-one dimensional Josephson junction is considered. On the base of Maxwell equations, the sine–Gordon equation that describes an influence of curvature on the kink motion was obtained. It is showed that the method of geometrical reduction of the sine–Gordon model from three to lower dimensional manifold leads to an identical form of the sine–Gordon equation.

Highly sensitive magnetometers for absolute magnetic field measurements based on quantum interference filters

We present an experimental study on absolute field magnetometers based on superconducting quantum interference filters (SQIFs). Two different prototype SQIF circuits have been designed and fabricated. The first type comprises a one dimensional Josephson junction array possessing unconventional grating structure. The second type is a series array of loops with different areas each loop having two Josephson junctions. The flux to voltage transfer function of both SQIFs has a unique voltage signal around zero applied flux. Our results show that SQIFs can be applied as highly sensitive magnetometers for absolute magnetic field measurements.

Vortex pairs and nonlinear inductance of high-T/sub c/ superconducting microwave resonators

Recently, an unusual power dependence of two-tone intermodulation has been observed in high-T/sub c/ superconducting microwave resonators. In order to address this problem, we study the nonlinear response of a two dimensional Josephson junction array as a coupled grain model for the high-T/sub c/ films. On a microscopic scale the calculated current distributions within the array show a strong tendency to form topological Josephson vortices which organize themselves in vortex-antivortex clusters even in the absence of an external magnetic field. These vortex clusters are stable within an external ac drive and cause a redistribution of the input current. The resulting current distributions possess pronounced transversal components. On the macroscopic scale such current distributions lead to a nonlinear inductance that varies linearly with the magnitude of the input current consistent with the observed power dependence. This result suggests the dynamics of topological Josephson vortices within the coupled grains as a possible explanation for the unusual nonlinear response.

Kinetic inductance and Coulomb blockade in one dimensional Josephson junction arrays

One dimensional Josephson junction arrays have been fabricated andcurrent-voltage characteristics (IVC) have been measured at cryogenic temperatures. The arrays were fabricated in a SQUID-geometry which allowed an in situ tuning of the Josephson energy by application of a magnetic field. The IVC of the arrays shows a clear Coulomb blockade state. In the Coulomb blockade regime the IVC are hysteretic. The array is modeled using a serial resistive-inductive-junction model which is able to qualitatively explain the IVC. In this model an inductance of the order of 0.1–10 mH per junction is needed to account for the hysteresis. Kinetic inductance, stemming from the inertia of the Cooper pairs, gives the correct order of magnitude. The problem of self-heating is also discussed as an alternative explanation of the hysteresis.

Two Dimensional Josephson Junction Arrays

Two dimensional Josephson junction arrays (JJAs) offer theopportunity to study a variety of basic physical concepts. Thepresent review focuses on recent experimental work on thedynamics of JJAs, as characterized by ac conductancemeasurements. The review starts with a discussion of basicphysics necessary to describe JJAs. Some experimentalissues, array fabrication and measurement techniques areconsidered next. In a perpendicular magnetic field, a JJA isan experimental realization of the frustrated XY model, withthe frustration parameter f, corresponding to the number offlux quanta in a unit cell of the array, adjusted by themagnetic field. It is thereby possible to investigate thenature of the ground states at arbitrary frustrations. Phasetransitions are the next topic: the vortex unbindingtransition is observed at integer f-values, while, if thejunction coupling energies are appropriately varied across thearray, at half-integer f-values the Ising transition,associated with chiral symmetry breaking, may be observed.Some aspects of vortex dynamics, a subject which is not yetcompletely understood, are then considered. Under certainconditions there is virtually no pinning in JJAs, they aretherefore ideally suited for the study of vortex dynamics. Thenext topic of this review is concerned with the influence ofdisorder on the ground states and on the phase transitions inJJAs. Site percolation in JJAs has provided some insightsinto the physics of disordered systems and allowed to verifysome theoretical predictions on percolation in two dimensions.A quick look at JJAs in the underdamped regime concludesthis review.

Phase locking of Josephson junction arrays achieved by a non-traditional bias scheme

We propose to modify the usual biasing scheme of a two dimensional Josephson junction array by connecting the wires in a special cross-type fashion. Numerical simulations show that a simple two dimensional circuit of Josephson junctions with the new biasing scheme, to the contrary of the traditional scheme, one can achieve a stable frequency locked state without the use of an external load. We also show that four of these circuits can be series connected and that the output voltage is further increased. The new scheme may dramatically improve the performance of Josephson junction microwave oscillators.

Vortex structure and resistive transitions in high-Tc superconductors.

The nature of the resistive transition for a current applied parallel to the magnetic field in high-Tc materials is investigated by numerical simulation on the three dimensional Josephson junction array model. It is shown by using finite size scaling that for samples with disorder the critical temperature Tp for the c axis resistivity corresponds to a percolation phase transition of vortex lines perpendicularly to the applied field. The value of Tp is higher than the critical temperature for j perpendicular to H, but decreases with the thickness of the sample and with anisotropy. We predict that critical behavior around Tp should reflect in experimentally accessible quantities, as the I-V curves.

Josephson-vortex Cherenkov radiation.

We predict the Josephson vortex Cherenkov radiation of an electromagnetic wave. We treat a long one--dimensional Josephson junction. We consider the wave length of the radiated electromagnetic wave to be much less than the Josephson penetration depth. We use for calculations the nonlocal Josephson electrodynamics. We find the expression for the radiated power and for the radiation friction force acting on a Josephson vortex and arising due to the Cherenkov radiation. We calculate the relation between the density of the bias current and the Josephson vortex velocity.

Modeling of the critical state in the granular high-Tcsuperconductors

Abstract Considering granular superconductors as the Josephson structures, we study the process of the magnetic flux penetration into the high-T c superconductor ceramics. We consider the models of the one-, two-, and three-dimensional Josephson lattices, which are formed of the mutually crossed Josephson junctions in a superconductor. In the simplest case of the one dimensional Josephson junction with the dense periodical inhomogeneity the modeling gives the picture of the critical state that is in accordance with the estimations obtained by accounting the pinned vortices interaction. Connections with the stochasticity in the Frenkel-Kontorova model are discussed.