Josephson Weak Links Research Articles

Quasiparticle energy spectrum in ferromagnetic Josephson weak links

The quasiparticles energy spectrum in clean ferromagnetic weak links between conventional superconductors is calculated. Large peaks in density of states (DOS), due to a special case of the Andreev reflection at the ferromagnetic barrier, correspond to spin-split bound states. Their energies are obtained as a function of the barrier thickness, exchange field strength, and of the macroscopic phase difference φ at the link, related to the Josephson current. In the ground state, φ can be 0 or π, depending on the ferromagnetic barrier influence. Conditions for the appearance of the zero-energy bound states (ZES) and for the spin polarized ground state (SPGS) are obtained analytically. It is shown that ZES appear only outside the weak link ground state.

NON-EQUILIBRIUM RADIATION RESPONSE AND DECOUPLING OF VORTEX-ANTIVORTES PAIRS IN TWO-DIMENSIONAL HIGH-Tc SUPERCONDUCTORS

On the basis of the two-dimensional (2D) characteristics of the superconducting transportation as well as Josephson weak link behaviors in granular YBCO films,a 2D Josephson junction array is proposed as a model system for this film.Using this simplified model,we have discussed the decoupling procedure of vortex-antivortex pairs by bias current,and analytically developed temperature distribution of free vortices n(T,I) below TKT.As compared with experimental results,we find that the temperature dependency of n(T,I) is similar to the behaviors of microwave response of granular YBCO films near K-T transition temperature.This similarity implies,to a certain extent,some intrinsic relationship between vortex-antivortex decoupling and non-equilibrium radiation response dissipation in the high-Tc superconducting granular films.

Optically-induced effects in Y–ba–cu–O Josephson junctions

Abstract We report our studies on light-induced changes in electrical properties of YBa 2 Cu 3 O x (YBCO) grain-boundary Josephson weak links. We discuss the erasable process (photodoping), in which light-induced enhancement of junctions properties at low temperatures relaxes back above 250 K. Our studies are focused on the junction critical current versus time, light intensity, and magnetic field measurements. In tested few- μ m-wide step-edge junctions, photodoping manifested itself as above 50% increase of the critical current and the decrease of the junction magnetic penetration depth, leading to reduction of the effective junction width. Contrary to the photodoping effect, the high-intensity laser modification of junctions led to permanent modification of their electrical characteristics that remained unchanged after subsequent room-temperature/helium thermal cycling of the sample. The laser-modified junctions exhibited up to 25% increase of the critical current times normal-state resistance product.

Crossover effects for critical currents within Landau-Ginzburg phenomenology incorporating Josephson weak links in superconducting thin films ofYBaCu2O7−δ

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Phase-dependent energy spectrum in Josephson weak links

The quasiparticles energy spectrum in clean Josephson weak links is studied theoretically. The density of states (DOS) is calculated for various situations: s-wave or d-wave symmetry of the order parameter in the superconducting electrodes, the barrier consisting of a normal or a monodomain ferromagnetic metal. The number of peaks in DOS, corresponding to the Andreev bound states, in the anisotropic d-wave case can be greater than in the isotropic s-wave case, provided that the barrier acceptance angle is not too small. Energy levels of the bound states are explicitly obtained as a function of the barrier thickness and exchange field and in the d-wave case of the orientation of the crystalline axes of the electrodes. The spectra are strongly influenced by the macroscopic phase difference at the link.

Comparing the effects of oxygen content on the superconducting properties of YBa 2Cu 3O x and RBa 2Cu 3O x with R=Nd, Eu and Gd

Previous studies have shown that a Josephson weak link transition occurs in some YBa 2Cu 3O x samples at very low fields, H< H C1. This transition is distinct from the vortex glass–fluid transition which of course occurs for H> H C1. The Josephson weak link transition for YBa 2Cu 3O x was observed only in samples annealed in oxygen. Moreover, the transition temperature T c of YBa 2Cu 3O x is also sensitive to oxygen content. T c is 93 K for air-sintered and vacuum-annealed samples but only 90 K for oxygen-annealed samples. We have extended these studies to RBa 2Cu 3O x samples with R=Nd, Eu and Gd. Like the Y-based systems, all three show the Josephson weak link transition but only for oxygen-annealed samples. Unlike the Y-based system, the values of T c for RBa 2Cu 3O x are not sensitive to oxygen content with T c being 95 K for all samples. Moreover, the Y- and R-based systems show different specific heat behavior. C/ T vs. T for both oxygen and vacuum-annealed RBa 2Cu 3O x samples show a single peak at 95 K. Vacuum-annealed YBa 2Cu 3O x samples also show a single C/ T peak at 93 K, but oxygen-annealed YBa 2Cu 3O x samples show two peaks in C/ T at 90 and 93 K.

A potential method to correlate electrical properties and microstructure of a unique high-Tc superconducting Josephson junction

A method to correlate microstructure from cross-section transmission electron microscopy (TEM) investigations and transport properties of a single well characterized high-Tc artificial grain boundary junction is reported. A YBa2Cu3O7−δ 45° twist junction exhibiting the typical phenomenology of high-Tc Josephson weak links was employed. The TEM sample preparation is based on focused ion beam etching and allows to easily localize the electron transparent area on a microbridge. The reported technique opens clear perspectives in the determination of the microstructural origin of variations in Josephson junction properties, such as the spread in Ic and IcRN values and the presence of different transport regimes in nominally identical junctions.

Adiabatic modulation of a superconducting quantum interference device (SQUID) ring by an electromagnetic field

In this paper we consider the modulation of a SQUID ring (a Josephson weak link enclosed by a thick superconducting ring) by an external electromagnetic (em) field for the case where the ring remains adiabatically in its ground state. We demonstrate that very good agreement can be found between experimental modulation data and the results predicted theoretically by solving the time-dependent Schrödinger equation for the ring-em-field system. We also show that the non-linear dynamical coupling between the ring and an external resonant circuit can influence the exact form of the modulation. Again we find consistent agreement between experiment and theory.

Supercurrent flow through a semiconductor: the transport properties of superconductor–semiconductor hybrid structures

When heavily modulation-doped InAs quantum wells are contacted by a pair of closely spaced superconducting electrodes, the electrical resistance through the structure gradually vanishes when the temperature is dropped below the critical temperature of the electrodes. The I– V characteristics eventually become qualitatively similar to those of more conventional Josephson junctions and weak links. Quantitatively, though, major differences exist, believed to be a consequence of electrode separations that are large compared to those in more conventional SNS weak links. The underlying physics is that of ballistic electron transport in the semiconductor, combined with Andreev reflections at the superconducting electrodes.

Effect of the RE 3+ ionic size on the REBa 2Cu 3O 7− δ ceramics oxygenated at 250 bar

REBa 2Cu 3O 7− δ (RE=Ho, Y, Dy, Gd, Sm, Nd) ceramics have been oxygenated at 1 bar pressure (LP) and, subsequently, at 250 bar (HP). Despite the noticed slight uptake of oxygen (up to the value of 0.07), after the HP processing, the electrical resistivity ( ρ) of all samples increased, what was attributed to the deterioration of the grain boundaries. The increase of ρ was much more pronounced and also accompanied by a change of ρ( T) characteristics into a semiconducting-like one in the case of 123 compounds based on REs which ionic size is large enough to form solid solutions of RE 1+ x Ba 2− x Cu 3O y type (i.e., RE=Gd, Sm, Nd). As shown in the literature, such 123s usually contain more structural defects. Thus, the observed effect may be attributed to the migration of the defects induced by the elevated pressure oxygenation. The defects could be trapped near the grain boundaries resulting in the deterioration of their electrical properties. The possible role of the oxygen-pressure-induced modifications of the impurity phases has been also discussed. The materials obtained in the HP process may be regarded as 3D arrays of superconducting grains coupled by the Josephson junction or weak links only, as was shown in a tunneling experiment.

Bifurcations and chaos in a coupled superconducting-quantum-interference-device ring–resonator system

In this paper we consider from a theoretical viewpoint the dynamical behavior of a superconducting-quantum-interference-device ring (a single Josephson weak link enclosed by a thick superconducting ring) in its ground state when coupled inductively to a radio frequency (rf) tank circuit resonator. We show that this simple system, when strongly driven at rf, displays a rich nonlinear dynamical structure, including unusual fold bifurcations and chaos.

Hysteretic Phenomena in the Critical Current of High-Tc Superconductors

Models describing the appearance of an abnormal hysteresis in the critical current of granular superconductors are analyzed. It has been revealed that the most suitable for High T c Superconductors ( H T c S ) is a model based on an irreversibility of the grain surface current that determines the critical current of the Josephson weak links or depairing current of narrow superconducting part in ( H T c S ) oxides. This leads to the irreversibility phenomena in ( H T c S ) at the high magnetic fields H≫ H c1 .

Influence of the growth mechanism on the grain boundary structure in YBa 2Cu 3O 7 thin film bicrystals

The growth control of thin YBa 2Cu 3O 7 (YBCO) film bicrystals determines the quality of Josephson junctions and weak links. Previous TEM experiments on various thin film bicrystals showed grain boundary oscillations that were not clearly explained. We combined AFM, SEM, conventional TEM and HRTEM in order to perform a complete characterization of both substrate and YBa 2Cu 3O 7 thin film bicrystals. Our results clearly show that the grain boundary oscillations are due to the YBCO growth mechanisms leading to a conflictual crystallographic situation at the grain boundary. The resulting crystallographic structure of the grain boundary may affect the interpretation of several recent experiments purporting to determine the nature of the HTC superconductivity.

Characteristics of current flow in composites made from a high-temperature superconductor and the low-temperature superconducting metal oxide Ba(PbBi)O3

Composites are prepared from a high-temperature superconductor with the 1-2-3 structure and the low-temperature oxide BaPb3/4Bi1/4O3 with various volume concentrations of the high-temperature and low-temperature superconductors. The temperature dependences of the electrical resistivities and critical current densities of the composites are measured. It is shown experimentally that these composites comprise a synthetically created network of Josephson weak links (with a specified degree of coupling).

Oxygen dependence of the Josephson weak link effect, specific heat, and the transition temperature of YBa2Cu3Ox

We have measured χ(T) for oxygen-annealed YBa2Cu3Ox samples and observed a glass-like behavior at low magnetic fields which can be explained in terms of a Josephson weak link effect. The glass transition temperature Tg depends on the oxygen annealing temperature and sample pellet pressure. Tg disappears when a sample was powdered after receiving an oxygen anneal, indicating that the Josephson weak link effect occurs between intergranular boundaries. In addition, Tc shifts from 93 K for the air-sintered samples to 90 K for the oxygen-annealed materials. Under vacuum annealing, Tg disappears and Tc first shifts back to 93 K. Prolonged vacuum annealing or vacuum annealing at higher temperatures depresses Tc to below 90 K. The temperature dependence of the specific heat shows a single anomaly for samples sintered in air or annealed under high vacuum. But a double-peak specific heat anomaly occurs for the oxygen-annealed samples.

Simulation of the intergranular magnetization of (Bi,Pb) 2Sr 2Ca 2Cu 3O 10+ x superconductors by using Josephson junction arrays

In this paper we present a numerical simulation study of the intergranular magnetization of (Bi,Pb) 2Sr 2Ca 2Cu 3O 10+ x superconductors. The intergranular-current system was modelled as a two-dimensional Josephson junction array, which consists of superconducting grains connected via overdamped, narrow Josephson weak links. The simulations cover magnetic-flux distribution and magnetic-hysteresis studies, including relaxation effects due to thermal fluctuations in the junctions. Special attention was paid to the influence of intergranular defects on the array magnetization and intergranular flux pinning, with the motivation to find model parameters which fit to the experimental behavior of (Bi,Pb) 2Sr 2Ca 2Cu 3O 10+ x . A correspondence with the experimental results [Paasi et al., Physica C 259 (1996) 1) (previous paper)] required that there was an intergranular defect of the μm order located in the middle of the four grains of an array mesh. Such a defect could act as a strong pinning center for intergranular flux.

Study of an array of superfluid3He weak links

An array of 1126 identical microapertures is used as the hydrodynamic inductance in a superfluid3He Helmholtz resonator. Each aperture is 0.27 micron square, fabricated by e-beam lithography in a 0.1 micron thick SiN membrane. These dimensions are not too much larger than the superfluid coherence length and therefore, particularly at higher temperatures, an aperture might be though of as a Josephson weak link (in the sense of a Dayem bridge). The superfluid3He critical mass current, Ic, through this array has been measured over a wide range of temperatures. The results indicate that the critical current has a Ginzburg-Landau form of Ic=Ico(1-T/Tc)3/2. This temperature dependence is consistent with phase slippage (not necessarily involving vortices), at the pair breaking velocity, as the critical mechanism.

Anisotropy of the critical current in silver sheathed (Bi,Pb)2Sr2Ca2Cu3O10 tapes

The transport critical current of Bi:2223 tapes was measured in magnetic fields up to 15 T and at temperatures from 4.2 to 84 K. At high temperatures, the critical current density Jc is strongly anisotropic and the anisotropy increases rapidly with magnetic field, whereas at low temperatures the critical current is less anisotropic and the anisotropy is field independent above 1 T. In the former case we believe we are in a regime, where pinning limits Jc, at least within some parts of the tape, whereas in the latter case the limitation of Jc by Josephson weak links seems to be the dominant mechanism. In addition, a critical current hysteresis induced by flux trapping in a weak link network is observed, which is more pronounced at low temperatures. From transmission electron microscopy observations of the microstructure we find that the ‘‘brick’’ in the ‘‘brick wall’’ model turns out to be the colony instead of the grain inside the colony. Additionally it is found that colony boundaries parallel the ab plane and intersected boundaries occur much more frequently than boundaries parallel to the c axis, due to the misalignment of the colonies inside the tape. In a small region near the silver sheath colony misalignment is much smaller and boundaries parallel to the c axis may act as strong links at high temperatures as their interfaces are very clean and well matched.

Displacement damage in supported YBa2Cu3O7-x thin films and finite-element simulations

The writing of Josephson weak links, in an electron microscope, onto films of YBa2Cu3O7-x is an example of the high sensitivity of high-Tc superconductors to structural disorder and oxygen concentration. Radiation damage occurs from electron beams with energy much less than the threshold normally required in compounds for knock-on of oxygen ions. Finite-element methods are used to simulate thermal and charge effects in the superconducting film and substrate. The calculations show that interstitials, under electric and thermal field gradients, drift away from the beam during damage and that diffusion-induced equilibrium is inhibited by the relatively rapid temperature fall after translating the beam.

Current-voltage characteristics of shunted Josephson weak links with excess current

Many superconducting Josephson weak link junctions are not adequately or accurately modeled by the resistively shunted junction model. For point contact junctions, step edge junctions and grain boundary junctions, a large excess current in the I-V characteristic is often observed. A new model has been proposed, which provides a more accurate technique of describing the behavior. Using this theory of shunted weak link junctions with excess current, calculations are presented here which accurately describe the magnitude and position of microwave induced current steps in the dc current versus voltage characteristics of such devices.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>