Josephson Network Research Articles

Microwave absorption by the Josephson network in a low field: Application to ceramic high temperature superconductors

We discuss the applied magnetic field dependence of the microwave absorption by a three-dimensional array of up to 30×30×30 Josephson junctions with a random variation of its parameters including the resistivity and the capacity of each junction. The numerical simulation results for the networks show characteristic microwave absorption anomalies observed in the ceramic samples of high temperature superconductor YBa 2Cu 3O 7− x . A brief discussion in analytical terms is also provided.

System-size dependences and correlation effects of Josephson current through one-dimensional Josephson networks

Josephson effects are studied in Josephson junction arrays with finite number of junctions. We study how the Coulomb interaction affects the maximum Josephson current. The tendency of the charge order is also discussed by studying the phase-current relations.

Non-Abelian holonomies, charge pumping, and quantum computation with Josephson junctions.

Non-Abelian holonomies can be generated and detected in certain superconducting nanocircuits. Here we consider an example where the non-Abelian operations are related to the adiabatic charge dynamics of the Josephson network. We demonstrate that such a device can be applied both for adiabatic charge pumping and as an implementation of a quantum computer.

Non Abelian Phases, Charge Pumping and Holonomic Computation with Josephson Junctions

In 1984 Berry pointed out that abelian gauge structures exist naturally in slowly varying quantum system. Since then, a great deal of interest both theoretically and experimentally has been devoted to the interpretation, generalization and detection of geometric phases. In particular, relevant for the present work, it has been shown that a non-Abelian gauge structure emerges if a set of quantum states remains degenerate as the Hamiltonian varies. Non-Abelian phases have been originally investigated in Nuclear Quadrupole Resonance. More recently, they have found a renewed interest within the field of Quantum Computation, since it has been proved that quantum gates can be also implemented by geometrical means using both Abelian and non-Abelian phases. In this novel geometric approach, known as Holonomic Quantum Computation (HQC), information is encoded in a degenerate eigenspace of a parametric family of Hamiltonians and the computational network of unitary quantum gates is realized by driving adiabatically the Hamiltonian parameters along loops in a control manifold. By properly designing such loops, the non trivial curvature of the geometry of the control manifold gives rise to unitary transformations, i.e. holonomies, that implement desired quantum gates. Until now only one scheme has been proposed to realize HQC in trapped ions by means of pulsed coherent excitations. On the other hand, it would be of great interest to have an implementation for solid-state systems, in view of their scalability and integrability. In particular, the impressing development of nanotechnologies with the consequent control of artificial two-level system in superconducting devices stimulates a more detailed investigation of geometric interference in mesoscopic systems. Here we show that non-Abelian phases can appear in the quantum charge dynamics of Josephson networks. Besides their possible detection, we shall see how they may be employed both in adiabatic charge pumping and as an implementation of HQC with Josephson junctions. 2. Josephson network for non abelian phases

Pressure and grain-size dependences of the chiral-glass transition in superconductive ceramics of YBa2Cu3O7-y

Ceramic high-T c superconductors may be viewed as a weakly coupled random Josephson network containing π-junctions, and a chiral-glass transition is predicted to be occurred. Grain-size and pressure dependences of the chiral-glass transition have been investigated on YBa 2 Cu 3 O 7-y ceramics. With decreasing temperature, a superconducting order occurs at first inside each grain at T c1 and furthermore among the grains at T c2 (< T c1 ). The increase in T c2 with increase of synthesis temperature is considered as the change of strength of inter-grain Josephson coupling accompanying the growth of grain size. The T c2 also increases with increase of pressure, however, the sharpness of the transition at T c2 does not change. In spite of difference in T c2 's, the increasing rates of t c2 by pressure are nearly the same.

Holonomic Quantum Computation with Josephson Networks

It is well known that it is possible to build quantum gates based entirely on the action of geometric phases. This approach is called holonomic quantum computation. Here we show that it is possible to generate and detect non-Abelian geometric phases in the charge dynamics of a network of mesoscopic Josephson junctions. The scheme can be used to realize a universal set of quantum gates.

Pressure dependence of the chiral-glass transition in Y–Ba–Cu–O superconductors

Ceramic high- T c superconductors may be viewed as a weakly coupled random Josephson network containing so-called π-junctions. Kawamura and Li proposed that a novel glassy phase, a chiral-glass phase, might occur in d-wave ceramics. This prediction was supported by experimental results on YBa 2Cu 4O 8 ceramics. In the present work, the pressure dependence of the chiral-glass transition has been investigated by magnetic measurements on the ceramic YBa 2Cu 4O 8. The transition temperature increases linearly with increase in pressure, and the singular behaviors of the susceptibilities are scaled by the transition temperature. The results suggested that the transition occurs in frustrated Josephson network and depends only on the strength of Josephson junctions between grains.

Quantum algorithms for Josephson networks.

We analyze possible implementations of quantum algorithms in a system of (macroscopic) Josephson charge qubits. System layout and parameters to realize the Deutsch algorithm with up to three qubits are provided. Special attention is paid to the necessity of entangled states in the various implementations. Further, we demonstrate explicitly that the gates to implement the Bernstein-Vazirani algorithm can be realized by using a system of uncoupled qubits.

Magnetic properties of two-dimensional Josephson networks: Self-organized criticality in magnetic flux dynamics

The field dependence of the magnetic moment of square (100×100) Josephson networks was examined with the use of a SQUID magnetometer. The field dependence of the magnetic moment was found to be regular with features corresponding to integer and half-integer numbers of flux quanta per cell. At temperatures below 5.8 K, jumps in the magnetization curves associated with the entry and exit of avalanches of tens and hundreds of fluxons were observed. It was shown that the probability distribution of these processes corresponded to the theory of self-organized criticality. An avalanche character of flux motion was observed at temperatures at which the size of the fluxons did not exceed the size of the cell, that is, when a discrete vortex structure occurred.

Magnetic flux penetration mechanisms in inductive three-dimensional Josephson-junction network

The time evolution of magnetic flux distribution in a 8×8×8 Josephson-junction network is numerically followed, after the network's lower threshold field is exceeded. According to results, Josephson network vortices first form at network edges as bent. With time the vortices straighten and find their stationary positions inside the network.

Phase diagram for a Josephson network in a magnetic field

Phase diagram for a Josephson network in a magnetic field

Particle size effect of a.c. superconducting properties in compound

Using a new starting material of Ba2Cu3O5 and a three step heat treatment, single phase Tl2Ca1Ba2Cu2O8 high- superconducting samples have been prepared, possessing the onset- and critical temperatures K and K. The morphology dependent value of is 17 Oe, 8 Oe and 5 Oe at 77 K in the case of bulk, crushed and powdered materials, respectively. The a.c. susceptibility, r.f. susceptibility and microwave absorption properties show a significant dependence on the particle size with a sharp change in the interval between 750 μm and 1200 μm. These experiments provide characteristic parameters for intergrain material (treated as 3D Josephson network) as mm, Oe and A/cm2 at 77 K. The data are controlled by modulated microwave absorption measurements. The results obtained can be explained well both by the finite size junction model and cavity mode absorption model. The Josephson network is determined unambiguously by metallic S-N-S weak links.

Inhomogeneous magnetic flux focusing in superconducting Josephson networks

The bending effects of a transverse magnetic field on planar superconducting structures in the Meissner state are investigated. Systematic measurements are performed on arrays made of four and six small Josephson junctions connected in parallel by superconducting films. By applying an external uniform magnetic field H, we measure the Josephson critical current ${I}_{c},$ which is sensitive to the local magnetic-field distribution, as a function of H. We find that the experimental data cannot be accounted for by the external magnetic field, and a strong focusing effect is present. Moreover, only by using a nonuniform magnetic-field distribution with a maximum in the center of the array are we able to fit the experimental data by numerical simulations. We show that, in the absence of self-field effects resulting from the Josephson currents, the nonuniform magnetic-field distribution is caused by the diamagnetic shielding produced by Meissner currents flowing in the superconducting electrodes. Three-dimensional finite element numerical calculations are performed on several planar geometries in a transverse external magnetic field to compute the magnetic-field distribution and the screening current patterns. In this way, we find a quantitative agreement with the observed inhomogeneous field distribution. The multiloop geometry has also been compared with equivalent networks, which considerably reduce the numerical calculations and substantially give the same inhomogeneous flux focusing.

Two-dimensional Josephson junction network architectures for maximum microwave radiation emission

We have investigated experimentally a novel type of self-synchronizing two-dimensional Josephson junction array based on special symmetry breaking network architectures. The measurements confirm the theoretical prediction that in such Josephson networks the Josephson oscillators mutually synchronize into a coherent in-phase state and that such arrays are capable of emitting maximum coherent microwave power in the entire theoretically expected frequency range. The DC biased oscillator array operates coherently even without a reinjection of the generated microwave by an external load or by a resonator cavity. The selector network possesses a much higher tolerance against imperfections, perturbations and load parameter variations than conventional regular two-dimensional or one-dimensional arrays. The experimental sample has been fabricated in an industrial process using Nb-technology, in effect providing relatively large spreads in the array junction parameters. The measured frequency range goes from 85 GHz up to 380 GHz with a maximum microwave power of 0.16 /spl mu/W matched to a coupled load for an array with only 100 active Nb-AlO/sub x/-Nb junctions.

Spin-charge separation in strongly correlated electronic systems

The spin-charge separation (SCS) in 1D and 2D are discussed from the viewpoint of gauge theory. For 1D I discuss the angle-resolved photoemission spectra (ARPES), which show clear evidence for the spin-charge separation. For 2D the underdoped cuprates are discussed, where the three classes of electronic state, i.e., the Néel state (N), the valence-bond solid (VBS) state and the resonating-valence-bond (RVB) state, are relevant. These states can be understood in terms of the competition between (i) magnetic ordering versus singlet formation and (ii) confinement versus deconfinement of the gauge field. It is fairly easy to understand the former, (i), but the latter, (ii), is more subtle and has not yet been established. I argue that the deconfining phase, i.e., the RVB state, is a new state of matter with SCS, and is realized when the sheet resistance is less than a critical value of the order of the quantum resistance . This condition is equivalent to that for superconductivity in the Josephson network model. The anomalous Kondo effect due to the non-magnetic impurities doped into the system reflects the non-Fermi-liquid nature of the host electronic state, and hence is the most promising experimental evidence for this new state of matter. We put special emphasis on the residual resistivity, and propose that its value provides a clear test for SCS.

Quasiclassical method for Josephson network in magnetic field

We propose a method for calculation of vortex distributions in a Josephson network. We apply the method to describe magnetic properties of Josephson junction arrays forming ladder structures. We compare the obtained results with Monte Carlo (MC) simulations. Quite specific logic in the flux penetration into the network is obtained, which depends on the geometry of the system.

Electrical properties of a normal contact to HTSC Josephson media

Electrical properties of normal metal contacts to percolation Josephson HTSC systems are studied by computer simulation. The current and voltage distributions, current–voltage characteristics (IVC), and temperature dependences Rc(T) of the resistance of the Josephson network with a limited contact are calculated. It is shown that macroheterogeneity of current distribution leads to broadening of the IVC and Rc(T) dependences of the contact in comparison with the bulk characteristics. In the vicinity of critical currents and temperatures, the contact IVC and Rc(T) obey the “3/2” law. A peak on the current and temperature dependences of the spreading resistance is observed. The applicability of the effective medium model for describing contact characteristics of the Josephson network is substantiated. The computer simulation results are in accord with experimental temperature dependences of the contact resistance.

Magnetic field induced charging effects in Josephson junction arrays

A magnetic field induced electric polarization (analog of magnetoelectric effect) and the corresponding change of an effective junction capacitance are considered within a 3D model of disordered Josephson junction arrays. At some threshold field (near the Josephson network critical field), the effective junction charge and the related capacitance are shown to reach a maximum and to change a sign, respectively. A possibility to observe the predicted effects in artificially prepared arrays of superconducting grains is discussed.

Nature of Resistive Transitions in Josephson Networks in a Magnetic Field

We study the depinning of the vortices induced in a Josephson network by a magnetic field. A vortex crystal can be pinned when it can be made commensurate with the network potential by rotation; this can occur only when the number of vortices per unit cell p/q is constructed from special values of the integers p and q. For these cases the transition can be of roughening type; however, melting of the vortex crystal is an important competing process that changes the nature of the transition for $q&lt;16$ (square lattice) or $q&lt;12$ (hexagonal lattice). For the other values of p and q there can be a spontaneous deformation of the vortex crystal to a commensurable structure if the network potential is sufficiently large.

Scaling of current-resistivity isotherms of indium films in a magnetic field

We have studied the nonlinear resistivity ( J- ϱ characteristics) of granular indium films with differennt thickness t and grain size d in various constant magnetic fields B down to 0.6 K. Films 1 and 2 with d=14 nm are viewed as a dirty superconductor while films 3 and 4 with d = 28 nm are modeled as a random Josephson network. The J- ϱ data are analyzed according to the vortex-glass (VG) theory. For film 1 with t = 100 nm , we have obtained the VG transition temperatures T g( B) and scaling functions, which are similar to those obtained in high- T c superconductors, suggesting that the VG transition also takes place in a conventional superconductor with sufficiently low T c (< 4 K). A finite-size effect is visible for thinner film 2 with t=20 nm , where T g appears to be zero. This is in accord with the two-dimensional (2D) VG theory. The VG behavior is not observed for films 3 ( t = 60 nm) and 4 ( t = 200 nm) with d = 28 nm . We ascribe it to the fact that the measuring current is too large to probe the film at lengths larger than d = 28 nm .