Analysis of magnetic field patterns of single Josephson tunnel junctions with large idle regions

We have experimentally investigated the effect of a combined lateral and longitudinal idle region on the magnetic field dependence of the critical current of Nb/Al−AlO_x/Nb Josephson junctions. All the samples were high quality devices with a good uniformity and reproducibility of the electrical parameters. An excellent agreement has been found with calculations of the magnetic field patterns made using a simple model of junction with idle region. A comparison with the magnetic field response of the same devices after a proper trimming of the idle region has also been done.     

Approximate analysis for stationary current flow in two-dimensional Josephson tunnel junctions

The stationary current distribution in Josephson tunnel junctions is discussed within the framework of an approximate linear analysis. It is shown that such an approach can provide very useful information also on the behavior of two-dimensional junctions. Self-limiting effects and the magnetic field dependence of the critical current for various geometries are discussed in some detail. The special case of a square junction is analyzed extensively.     

Circular and Annular Two-Dimensional Josephson Junction: Statics and Dynamics

We present the results of the numerical analysis of circular and annular two-dimensional planar Josephson junctions. The static and dynamical states of a junction with or without an externally applied uniform magnetic field have been investigated. Our model is based on the assumption that junction electrodes are sufficiently long and have a uniform cross section. Such a configuration allows to find analytically and numerically the boundary conditions, which are given by the surface current distribution on the electrodes. We found static and dynamical states of a large-area two-dimensional junction and present some of the resulting patterns. Surprisingly, in contrast to a circular one-dimensional structure (ring), we found that in a circular or annular large-area junction, the fluxons circulating along the junction border do not exist. The boundary conditions play an essential role and determine the trajectory of fluxons. Finally, we analyze a junction surrounding a current-carrying wire.     

Influence of grain properties on the Abrikosov vortices interaction with Josephson junction

The problem of the displacement of an Abrikosov vortex is investigated when the magnetic field decreases to H _cl ^G in a grain of a type II superconductor. It is shown that near grain boundaries the vortex line generates an intergranular current which depends on the normalized grain size τ and the anisotropy ratio ν. These parameters strongly influence the conditions of the Josephson vortex generation as a result of the interaction of the Abrikosov vortex with the Josephson junction. We support our theory with calculations of the intergrain critical current taking into account two types of the vortex configuration, triangular and square, for different grain characteristics. The results are of interest for the charge transport in granular superconductors as well as the relaxation process in devices that contain Josephson junctions in micro- and nanoelectronics in magnetic field.     

Phase Transitions and Phase Fluctuations in the Josephson-Junction Arrays

We study the effect of quantum and classical phase fluctuations on the phase transitions in the system of Josephson-junction arrays. We employed a variational method for calculating the Gaussian type fluctuation of the phase in the Josephson-junction array lattice systems without and with an external magnetic field. We investigate the spectrum of collective excitations and the effects of collective excitations on the transport properties of Josephson-junction arrays. We showed that the Hamiltonian for the lattice system of the Josephson junction is the same as the Hamiltonian for the classical or quantum two-dimensional interacting rotators. We also showed that the dynamics of fluctuations of the phase in the lattice system of Josephson junction is very similar to the lattice dynamics of the lattice in crystals. We also showed that in the lattice system of Josephson junctions there is the collective acoustic mode similar to the acoustic mode in the crystal lattice, and this mode may lead to the dissipation of the Josephson current in the superconducting array of Josephson junctions. The speed of sound of the collective acoustic mode of the phase fluctuation depends on the Josephson coupling energy and the Coulomb charging energy. The contribution of the collective acoustic mode to the low temperature specific heat is the same as the contribution of the acoustic phonons to the specific heat of crystals. We also discuss the future development of results and their application.     

Quasiparticles and Polaronic Effects in Crystals,Fermi Gasses and Fermi Liquids

We study the effect of quantum and classical phase fluctuations on the phase transitions in the system of Josephson-junction arrays. We employed a variational method for calculating the Gaussian type fluctuation of the phase in the Josephson-junction array lattice systems without and with an external magnetic field. We investigate the spectrum of collective excitations and the effects of collective excitations on the transport properties of Josephson-junction arrays. We showed that the Hamiltonian for the lattice system of the Josephson junction is the same as the Hamiltonian for the classical or quantum two-dimensional interacting rotators. We also showed that the dynamics of fluctuations of the phase in the lattice system of Josephson junction is very similar to the lattice dynamics of the lattice in crystals. We also showed that in the lattice system of Josephson junctions there is the collective acoustic mode similar to the acoustic mode in the crystal lattice, and this mode may lead to the dissipation of the Josephson current in the superconducting array of Josephson junctions. The speed of sound of the collective acoustic mode of the phase fluctuation depends on the Josephson coupling energy and the Coulomb charging energy. The contribution of the collective acoustic mode to the low temperature specific heat is the same as the contribution of the acoustic phonons to the specific heat of crystals. We also discuss the future development of results and their application.     

Modulation characteristics of DC Josephson current through Nb tunnel junction by applying external magnetic field in perpendicular direction

Perpendicular magnetic field dependences of the Josephson current through tunnel junction were first measured. Niobium/aluminum-oxide/niobium (200/5/150 nm in thickness) superconducting tunnel junctions were fabricated by DC-magnetron apparatus with a load-lock chamber. Josephson current Ic through the superconducting junction is usually changed by the external magnetic field in one direction to check the barrier uniformity. To obtain more information of the barrier uniformity, we have changed the external magnetic field in two directions (Hx and Hy: parallel field to the junction plane) parallel to the junction plane. The shape of Ic-Hx curve and Ic-Hy curve were the Fraunhofer pattern in the Ic-Hx-Hy dependence of the junction with uniform barrier. This Ic-Hx-Hy dependence has no hysteresis. In this study, we have first applied the external magnetic field Hz perpendicular to the junction plane and have obtained Ic-Hz characteristics using three pairs of Helmholtz coils. In the case that the perpendicular field Hz < 2400 A/m, the Ic-Hz characteristics have a little hysteresis. The shape of Ic-Hz characteristics was similar to the Fraunhofer pattern. In the case that the perpendicular field Hz > 2400 A/m, the Ic-Hz characteristics have strong hysteresis. The Josephson current Ic always disappeared in the case that Hz > 7000 A/m. Two recovering methods of this current Ic were (a) alternating Hz field and (b) heating the sample to the room temperature and again cooling to the liquid He temperature.     

Sidelobe suppression in arbitrarity shaped quadrangle Josephson junctions

We consider the effects of the junction geometry in view of the requirements imposed by a possible use of Josephson tunnel junctions as nuclear detectors. The magnetic field dependence of the critical-current of rhombus shaped junctions has been investigated both experimentally and theoretically.     

Fluxon Centering in Josephson Junctions with Exponentially Varying Width

Nonlinear eigenvalue problems for fluxons in long Josephson junctions with exponentially varying width are treated. Appropriate algorithms are created and realized numerically. The results obtained concern the stability of the fluxons, the centering both magnetic field and current for the magnetic flux quanta in the Josephson junction as well as the ascertaining of the impact of the geometric and physical parameters on these quantities. Each static solution of the nonlinear boundary-value problem is identified as stable or unstable in dependence on the eigenvalues of associated Sturm-Liouville problem. The above compound problem is linearized and solved by using of the reliable Continuous analogue of Newton method.     

The effect of the external magnetic field on the current-voltage characteristic of HTS Josephson junction arrays

The role of the external magnetic field in performance specialty of the high-temperature superconducting (HTS) Josephson junction array (JJA): HTS YBa₂Cu₃O_7−0.05 bicrystal JJA with 180 junctions, is considered. The junctions are created on the yttrium-stabilized zirconium (fianite) substrate with the bicrystal grain boundary. The experimental confirmation of the current density changes under the influence of the external DC magnetic field is obtained. The dependence of current density on the penetrated magnetic field is investigated. The optimal shielding factor needed to obtain high supercurrents in considered system is determined.     

On some effects in a system of two interacting Josephson junctions

We consider two long, weakly interacting Josephson junctions in an external magnetic field. A uniformly distributed direct current applied to the system excites the movement of a vortex chain within each junction. The weak interaction between the vortex chains results in monochromatical radiation from the system and some peculiarities in theI–V curve.     

Propagation of single flux quantum pulse of superconducting transmission line

Josephson tunnel junction switching by single flux quantum (SFQ) pulses generated by a two-junction interferometer and propagated on a superconducting transmission line have been investigated experimentally and by numerical simulation. High current density tunnel junctions have been used to enhance voltage SFQ pulse amplitudes and reduce junction damping parameters. The measured effective amplitudes of SFQ pulses allow an evaluation of the applicability of transmission lines to SFQ logic circuits and Josephson samplers.     

Magnetic field patterns in Josephson junctions

The critical currents of Pb/Ag/Pb Josephson junctions have been measured as a function of magnetic field and temperature in order to study the types of magnetic field distributions that occur in superconductor-normal metal-super-conductor (SNS) junctions. A study is made of the quantum interference regime near the critical temperature where the junction is filled with vortices and the crossover to Meissner-like behavior at low temperature occurs. Special emphasis is placed on the changes in effective area of the junction as the London and Josephson penetration depths change with temperature. A mapping of the crossover regime in the temperature-magnetic field plane is given.This work was supported by the office of Basic Energy Sciences of the Department of Energy.     

Static Magnetic Flux Dynamics in One-Dimensional (1D) Josephson Lattices

We investigated static magnetic flux dynamical properties of one-dimensional lattices of Josephson junctions. The discretized wave equations of the Josephson junction lattice were solved using a generalized relaxation iteration algorithm. Numerical simulations indicated that transitions between periodic state and chaotic state will occur as the physical parameters and geometric parameters such as external current y _n, magnetic field h ₀, h, and the length of Josephson junction _n and d _n , varied. A shot length of the Josephson junction favors stable periodic states.     

Properties of niobium-based Josephson tunneling elements in junction microstructures

We describe the fabrication and electrical characteristics of niobium oxide-barrier tunnel junctions with counterelectrodes of lead/lead alloy. Primary attention is directed to the experimental conditions necessary to obtain high-quality tunnel barriers as well as studies on characterizing the atomic structure of the barrier region. In order to study the tunnel barrier homogeneity in the tunneling region the magnetic field dependence of the critical Josephson current is investigated. TheI–V characteristics and dependence of the critical Josephson current on temperature are analyzed quantitatively by using a proximity effect model. Finally, we discuss experimental results on the improvement of junction quality by including traces of carbon in the rf argon plasma during the sputter cleaning of niobium base electrodes.     

The Effect of the External Magnetic Field on the Current-Voltage Characteristic of HTS Josephson Junction Arrays

No Heading The role of the external magnetic field in performance specialty of the high-temperature superconducting (HTS) Josephson junction array (JJA): HTS Y Ba₂Cu₃O_7–0.05 bicrystal JJA with 180 junctions, is considered. The junctions are created on the yttrium-stabilized zirconium (fianite) substrate with the bicrystal grain boundary. The experimental confirmation of the current density changes under the influence of the external DC magnetic field is obtained. The dependence of current density on the penetrated magnetic field is investigated. The optimal shielding factor needed to obtain high supercurrents in considered system is determined.PACS numbers: 73.21.Ac, 73.30.+y, 74.72.Bk, 74.76.Bz, 74.80.Dm     

Josephson Vortex Flow and Plasma Amplification in High-T c Superconductors

We investigate Josephson vortex flow states in two device arrangements considered to amplify the excited Josephson plasma. First, we study an intrinsic Josephson junction embedded inside a wave guide. The simulation result in the system reveals that the amplitude of the excited electric field almost linearly grows along the junction plane at the resonant voltage with the Josephson plasma. Second, we simulate a system composed of two separated Josephson junctions embedded inside a long wave guide and confirm that the excited electric field is amplified by the synchronization between two junction sites.     

Josephson parameters of bicrystal junctions of a new type based on metal oxide semiconductors

We have studied symmetric bicrystal Josephson junctions of a new type based on metal oxide superconductors in which the base planes are misoriented relative to the [100] direction by an angle within 22°–28°. These junctions exhibit record high values of the critical parameters at T = 77 K: the critical current density reaches I _C = (2−5) × 10⁵ A/cm² and the characteristic voltage is V _C = I _C R _N = 0.6−0.9 mV. The properties of the new junctions have been determined for the first time under the influence of monochromatic microwave radiation in the millimeter wavelength range and have been studied as functions of the magnetic field and temperature. The observed dependences of the critical current and the Shapiro step height on the electromagnetic field amplitude agree well with the theoretical predictions according to the resistive shunted junction model of the Josephson junction. The new junctions can be used in real superconducting devices operating at liquid nitrogen temperatures.     

Carbon nanotube superconducting quantum interference device

A superconducting quantum interference device (SQUID) with single-walled carbon nanotube (CNT) Josephson junctions is presented. Quantum confinement in each junction induces a discrete quantum dot (QD) energy level structure, which can be controlled with two lateral electrostatic gates. In addition, a backgate electrode can vary the transparency of the QD barriers, thus permitting change in the hybridization of the QD states with the superconducting contacts. The gates are also used to directly tune the quantum phase interference of the Cooper pairs circulating in the SQUID ring. Optimal modulation of the switching current with magnetic flux is achieved when both QD junctions are in the 'on' or 'off' state. In particular, the SQUID design establishes that these CNT Josephson junctions can be used as gate-controlled pi-junctions; that is, the sign of the current-phase relation across the CNT junctions can be tuned with a gate voltage. The CNT-SQUIDs are sensitive local magnetometers, which are very promising for the study of magnetization reversal of an individual magnetic particle or molecule placed on one of the two CNT Josephson junctions.