A study of the d.c. susceptibility of granular superconductors by means of Josephson-junction array models

Il Nuovo Cimento D - The field-cooled d.c. susceptibility of granular superconductors is studied by means of simple Josephson-junction array models. It is shown that a paramagnetic behaviour after...     

Polarization effects induced by a magnetic field in intrinsically granular superconductors

Based on the previously suggested model of nanoscale dislocation-induced Josephson junctions and their arrays, we study the magnetic-field-induced electric polarization effects in intrinsically granular superconductors. In addition to the new phenomenon of chemomagnetoelectricity, the model also predicts a few other interesting effects, including charge analogs of Meissner paramagnetism (at low fields) and a “fishtail” anomaly (at high fields). The conditions under which these effects can be experimentally measured in nonstoichiometric high-T_c superconductors are discussed.     

Odd triplet superconductivity in superconductor-ferromagnet multilayered structures

We demonstrate that in multilayered superconductor-ferromagnet structures a noncollinear alignment of the magnetizations of different ferromagnetic layers generates a triplet superconducting condensate which is odd in frequency. This triplet condensate coexists in the superconductors with the conventional singlet one but decays very slowly in the ferromagnet, which should lead to a large Josephson effect between the superconductors separated by the ferromagnet. Depending on the mutual direction of the ferromagnetic moments, the Josephson coupling can be both of 0 and of pi type.     

On the cryocooler-based cooling of Josephson microchips fabricated from cuprate superconductors for use in voltage standards

The features of the cooling of arrays of Josephson junctions fabricated from cuprate superconductors, as well as their interaction with microwave radiation during treatment in a closed-cycle cryocooler, are investigated. Estimation of microchip heating sources is carried out by measuring the current–voltage characteristics (IVCs) of the Josephson junctions. On the developed the developed experimental setup, Shapiro steps with a voltage of about 25 mV are obtained on the IVCs of the structure exposed to microwave radiation at a frequency of 75 GHz.     

Simulation of two-dimensional vortex dynamics

The two-dimensional XY model is simulated with a time-dependent Ginzburg-Landau type dynamics. The data are, in the limit of small driving force, well described by the Minnhagen phenomenology for vortex dynamics of a two-dimensional superfluid. This phenomenology is different and distinguishable from the conventional AHNS phenomenology. The Minnhagen phenomenology has been observed in recent experiments on Josephson-junction arrays and high-T_c BSCCO films. The present simulations suggest that this reflects an intrinsic property of the vortex dynamics for a two-dimensional superfluid.     

Superconducting qubit storage and entanglement with nanomechanical resonators

We propose a quantum computing architecture based on the integration of nanomechanical resonators with Josephson-junction phase qubits. The resonators are GHz-frequency, dilatational disk resonators, which couple to the junctions through a piezoelectric interaction. The system is analogous to a collection of tunable few-level atoms (the Josephson junctions) coupled to one or more electromagnetic cavities (the resonators). Our architecture combines desirable features of solid-state and optical approaches and may make quantum computing possible in a scalable, solid-state environment.     

Quantum phase transitions and vortex dynamics in superconducting networks

Josephson-junction arrays are ideal model systems to study a variety of phenomena such as phase transitions, frustration effects, vortex dynamics and chaos. In this review, we focus on the quantum dynamical properties of low-capacitance Josephson-junction arrays. The two characteristic energy scales in these systems are the Josephson energy, associated with the tunneling of Cooper pairs between neighboring islands, and the charging energy, which is the energy needed to add an extra electron charge to a neutral island. The phenomena described in this review stem from the competition between single-electron effects with the Josephson effect. They give rise to (quantum) superconductor–insulator phase transitions that occur when the ratio between the coupling constants is varied or when the external fields are varied. We describe the dependence of the various control parameters on the phase diagram and the transport properties close to the quantum critical points. On the superconducting side of the transition, vortices are the topological excitations. In low-capacitance junction arrays these vortices behave as massive particles that exhibit quantum behavior. We review the various quantum–vortex experiments and theoretical treatments of their quantum dynamics.     

Dynamics of semifluxons in Nb long Josephson 0-pi junctions

We propose, implement, and test experimentally long Josephson 0-pi junctions fabricated using conventional Nb-AlOx-Nb technology. We show that by using a pair of current injectors one can create an arbitrary discontinuity of the Josephson phase and, in particular, a pi discontinuity, just as in d-wave/s-wave or in d-wave/d-wave junctions, and study fractional Josephson vortices which spontaneously appear. Moreover, using such junctions, we can investigate the dynamics of the fractional vortices-a domain which is not yet available for natural 0-pi junctions due to their inherently high damping. We observe half-integer zero-field steps which appear on the current-voltage characteristics due to the hopping of semifluxons.     

Resonant transport through midgap states in voltage-biased Josephson junctions of d-wave superconductors

We study theoretically the ac Josephson effect in voltage-biased planar junctions of d-wave superconductors. For some orientations of the superconductors a current peak is found at finite voltage in the current–voltage characteristics. We pick out the relevant physical processes and write down an analytical formula for the current which clearly shows how the midgap state acts as a resonance and produces the peak. We present a possible explanation for the zero-bias conductance peak, recently found in experiments on grain boundary junctions of high-temperature superconductors, in terms of resonant transmission through midgap state of quasiparticles undergoing multiple Andreev reflections. We note that within our framework the zero-bias conductance peak appears in rather transparent Josephson junctions of d-wave superconductors.     

SNS与SINIS结阵制作工艺的比较

国家电压基准是基于约瑟夫森量子化效应的。传统的SIS结阵具有不能快速选择特定的台阶,相位锁定时间短的问题。I-V曲线无回滞的约瑟夫森结阵,其I-V曲线单值,电压台阶宽,解决了SIS结阵的问题。美国标准技术研究院的Hamilton提出并制作高度阻尼的可编程SNS约瑟夫森结阵电压标准;德国物理技术研究院则选择SINIS结来制作可编程约瑟夫森结阵电压基准。从结构、性能和制作方法等方面对SNS和SINIS可编程约瑟夫森结阵进行了比较。     

Smearing origin of zero-bias conductance peak in Ag-SiO-Bi2Sr2CaCu2O8+δ planar tunnel junctions: influence of diffusive normal metal verified with the circuit theory

We propose a new approach of smearing origins of a zero-bias conductance peak (ZBCP) in high-T_c superconductor tunnel junctions through the analysis based on the circuit theory for a d-wave pairing symmetry. The circuit theory has been recently developed from conventional superconductors to unconventional superconductors. The ZBCP frequently appears in line shapes for this theory, in which the total resistance was constructed by taking account of the effects between a d-wave superconductor and a diffusive normal metal (DN) at a junction interface, including the midgap Andreev resonant states (MARS), the coherent Andreev reflection (CAR) and the proximity effect. Therefore, we have analyzed experimental spectra with the ZBCP of Ag-SiO-Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) planar tunnel junctions for the {110}-oriented direction by using a simplified formula of the circuit theory for d-wave superconductors. The fitting results reveal that the spectral features of the ZBCP are well explained by the circuit theory not only excluding the Dynes's broadening factor but also considering only the MARS and the DN resistance. Thus, the ZBCP behaviors are understood to be consistent with those of recent studies on the circuit theory extended to the systems containing d-wave superconductor tunnel junctions.     

On the transmission of binary bits in discrete Josephson-junction arrays

In this work, we use supratransmission and infratransmission in the mathematical modeling of the propagation of digital signals in weakly damped, discrete Josephson-junction arrays, using energy-based detection criteria. Our results show an efficient and reliable transmission of binary information.     

Realization of fully frustrated Josephson-junction arrays with cold atoms

Fully frustrated Josephson-junction arrays (FF-JJA's) exhibit a subtle compound phase transition in which an Ising transition associated with discrete broken translational symmetry and a Berezinskii-Kosterlitz-Thouless transition associated with quasi-long-range phase coherence occur nearly simultaneously. In this Letter we discuss a cold-atom realization of the FF-JJA system. We demonstrate that both orders can be studied by standard momentum-distribution-function measurements and present numerical results, based on a successful self-consistent spin-wave approximation, that illustrate the expected behavior of observables.     

Quantum terahertz electrodynamics and macroscopic quantum tunneling in layered superconductors

We derive a quantum field theory of Josephson plasma waves (JPWs) in layered superconductors, which describes two types of interacting JPW bosonic quanta (one heavy and one lighter). We propose a mechanism of enhancement of macroscopic quantum tunneling (MQT) in stacks of intrinsic Josephson junctions. Because of the long-range interaction between junctions in layered superconductors, the calculated MQT escape rate Gamma has a nonlinear dependence on the number of junctions in the stack. We show that the crossover temperature between quantum and thermal escape increases when increasing the number of junctions. This allows us to quantitatively describe striking recent experiments in Bi2Sr2CaCu2O8+delta stacks.     

Chaotic behavior of a stack of intrinsic Josephson junctions at the transition to branching for overcritical currents

Phase dynamics of a stack of coupled intrinsic Josephson junctions was investigated in the framework of capacitively coupled Josephson junctions with diffusion current model. We study the transition from the current-voltage characteristic specific to Josephson junctions arrays with small dissipation and weak coupling between the junctions to the arrays with strong coupling between the junctions and high dissipation. Low dissipative arrays of Josephson junctions are characterized by the absence of branching for overcritical currents which appears for highly dissipative arrays. Described branching appears due to charging on the superconducting layers and charge traveling waves generation. Arrays of Josephson junctions with intermediate values of coupling and dissipation parameters are characterized by the chaotic behavior, confirmed by positive Lyapunov exponent, and branching on the current voltage characteristic for both sub- and overcritical currents.     

Phase Transition in Two-Dimensional Josephson-Junction Arrays

The self-charging model of two-dimensional Josephson-junction arrays at T = 0 is studied by the coupled cluster expansion method. The calculated results for the mass gap converge nicely. Besides the critical point, we also determine the critical parameters at T = 0. The system displays a second-order phase transition and the results are consistent with those obtained by other methods.     

Tunneling spectroscopy and order parameter symmetry of hole- and electron-doped cuprate superconductors

In tunneling spectroscopy of superconductors the density of states close to the surface or the interface to an insulating tunneling barrier is probed. For d-wave superconductors the particle–hole coherence results in interesting new phenomena at surfaces such as the formation of bound surface states at the Fermi level by Andreev reflection due to a sign change of the order parameter field in different k -directions. The probing of these states represents a phase-sensitive experiment allowing the determination of the order parameter symmetry in superconductors. We summarize the present experimental status with respect to the study of high-temperature superconductors (HTS). We discuss theoretically predicted consequences of a dominating d-wave order parameter in the hole-doped HTS on their tunneling spectra as well as on the physics of high-temperature superconductor Josephson junctions. A comparison of the tunneling spectra obtained for hole- and electron-doped HTS leads to the conclusion that the former have a d-wave, whereas the latter most likely have an anisotropic s-wave order parameter. We also address some unsettled questions related to the presence of a state with broken time-reversal symmetry at surfaces and interfaces of d-wave HTS and discuss specific features of d-wave tunnel junctions that have been predicted theoretically but still not been confirmed in experiments.     

Fluxon ratchet potentials in superconducting circuits

A fluxon in a Josephson-junction parallel array behaves like a single particle in a periodic pinning potential. Different configurations of critical currents and cell areas result in different profiles for the fluxon potential. We analyze the minimal conditions to achieve an effective potential in which mirror symmetry is absent, namely a fluxon ratchet potential. Following one of the configurations, we designed circular arrays and probed some of the fluxon properties. Theoretical predictions are nicely fulfilled by the experiments. Received: 20 October 2001 / Accepted: 14 January 2002 / Published online: 22 April 2002     

Theoretical analysis of coherent electron transport in structures containing multiband superconductors with different types of superconducting pairing

Coherent electron transport in structures with multiband superconductors described by models of intraorbital (the s _± model) and interorbital superconducting pairing has been theoretically considered. Conductivities of junctions of a single-band normal metal with superconducting pnictides for these pairing models have been calculated. Temperature and phase dependences of the Josephson current through junctions containing a conventional Bardeen-Cooper-Schrieffer superconductor and a superconducting pnictide have been calculated within the considered pairing models taking into account temperature dependences of superconducting order parameters.     

d-Wave-induced Josephson current counterflow in YBa2Cu3O7/Nb zigzag junctions

Well-defined zigzag-shaped ramp-type Josephson junctions between YBa2Cu3O7 and Nb have been studied. The magnetic field dependencies of the critical currents provide evidence for d-wave--induced alternations in the direction of the Josephson current between neighboring sides of the zigzag structure. The arrays present controllable model systems to study the influences of pi facets in high-angle high- T(c) grain boundaries. From the characteristics, we estimate a possible imaginary s-wave admixture to the order parameter of the YBa2Cu3O7 to be below 1%.