Vortex dynamics in two-dimensional Josephson junction arrays with asymmetrically bimodulated potential

On the basis of resistively-shunted junction dynamics, we study vortex dynamics in two-dimensional Josephson junction arrays with asymmetrically single and bimodulated periodic pinning potential for the full range of vortex density f. The ratchet effect occurring at a certain range of temperature, current, and f, is observed in our simulation. We explain the microscopic behavior behind this effect by analyzing the vortex distribution and interaction. The reversal of the ratchet effect can be observed at several f values for a small driven current. This effect is stronger when the asymmetric potential is simultaneously introduced in two directions.     

无序二维约瑟夫森结阵列中磁场引起的涡旋玻璃态相变

采用电阻阻错结的无序二维约瑟夫森结阵列模型,数值研究超导薄膜中垂直磁场引起的涡旋运动.通过分析磁场激发产生的涡旋度Ne及低频电压噪声S₀的变化特性,得到如下结论：在无序超导体中固定温度不变,随着磁场的减弱涡旋液态经过准有序的布拉格相,涡旋玻璃相重新进入到低磁场下的钉扎稀磁液相. 由于在涡旋玻璃相中,电流驱动下的噪声值表现出一个峰,表明系统处于无序与有序相互竞争的亚稳态,并且临界电流应有峰值效应. 计算得到噪声值的变化与Okuma等得到的无序超导Mo_xSi_1-x膜实验现象一致,并能解释磁场降低引起的重新进入钉扎的稀磁液相行为. 关键词： 约瑟夫森结阵列 磁通玻璃 重新进入 峰值效应     

Computer simulations of the anisotropic Josephson junction arrays

Using complementary methods, we numerically investigate the anisotropic Josephson junction arrays (AJJAs). For various anisotropic strengths (λ$\lambda$), the Monte Carlo simulation gives a precise measurement of specific heat, magnetization, and magnetic susceptibility; while the resistively shunted-junction dynamical simulation produces the current–voltage characteristics. The critical temperatures obtained from the two approaches are well consistent with each other. We find that, except for the anisotropic limit (λ=0)$(\lambda =0)$, the quasi-long-range order is always established at a finite temperature. Further, the algebraically decaying spin–spin correlations in the low-temperature region are analyzed in detail. Finally, the full phase diagram of the AJJAs, which sheds some lights to the crossover of the XY model from one dimension to two, is constructed. These predictions are to be confronted with future experiments.     

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.     

Critical behavior of frustrated Josephson junction arrays with bond disorder

The scaling behavior of the current-voltage (IV) characteristics of a two-dimensional proximity-coupled Josephson junction array (JJA) with quenched bond disorder was investigated for frustrations f = 1/5, 1/3, 2/5, and 1/2. For all these frustrations including 1/5 and 2/5 where a strongly first-order phase transition is expected in the absence of disorder, the IV characteristics exhibited a good scaling behavior. The critical exponent nu indicates that bond disorder may drive the phase transitions to be continuous but not into the Ising universality class, contrary to what was observed in Monte Carlo simulations. The dynamic critical exponent z for JJA's was found to be only 0.60-0.77.     

高温超导约瑟夫森结阵列的相位锁定

根据交流约瑟夫森效应,研究了嵌入Fabry-Perot谐振腔里的约瑟夫森结阵列在微波辐照下的相位锁定问题。结阵列是由生长在双晶钇稳定氧化锆(YSZ)基片上的YB2Cu3O7(YBCO)超导薄膜光刻成微桥得到的。通过优化设计,在温度为79.2K辐照频率为77.465GHz的条件下,包含620个串联约瑟夫森双晶结的结阵列在外加微波辐照下实现了相位锁定,得到了陡峭的夏皮罗台阶。其第一级夏皮罗台阶的电压约为0.1V,台阶高度约为0.17mA。试验结果表明,这种结阵列结构在电压基准的应用上有极大的优势,而且这种准光学耦合方法在太赫兹信号发生和检测方面有很好的应用前景。     

Spin-size disorder model for granular superconductors with charging effects

A quantum pseudo-spin model with random spin sizes is introduced to study the effects of charging-energy disorder on the superconducting transition in granular superconducting materials. Charging-energy effects result from the small electrical capacitance of the grains when the Coulomb charging energy is comparable to the Josephson-coupling energy. In the pseudo-spin model, randomness in the spin size is argued to arise from the inhomogeneous grain-size distribution. For a particular bimodal spin-size distribution, the model describes percolating granular superconductors. A mean-field theory is developed to obtain the phase diagram as a function of temperature, average charging energy and disorder.     

Dynamical phase transition in two-dimensional fully frustrated Josephson junction arrays with resistively shunted junction dynamics

The dynamical phase transitions in two-dimensional fully frustrated Josephson junction arrays at zero temperature are investigated numerically with the resistively shunted junction model through the fluctuating twist boundary condition. The model is subjected to a driving current with nonzero orthogonal components i _x , i _y parallel to both axes of the square lattice. We find a roughly lattice size independent phase diagram with three dynamical phases: a pinned vortex lattice phase, a moving vortex lattice phase and a moving plastic phase. The phase diagram shows a direct transition from the pinned vortex to the moving vortex phase and the separation of the pinned vortex and the moving plastic phases. The time-dependent voltages v _x and v _y are periodic in the moving vortex lattice phase. But they are aperiodic in the moving plastic phase, resulting in non-monotonic characteristics and hysteresis in the current-voltage curves. It is found that the characteristic frequency is twice the time-averaged voltage in the moving vortex phase and around the time-averaged voltage in the plastic flow regime.Received: 29 May 2003, Published online: 2 October 2003PACS: 64.60.Ht Dynamic critical phenomena - 74.25.Sv Critical currents - 74.25.Fy Transport properties     

Application of nano-scale Josephson junction as phase sensitive detector for analysis of vortex states in mesoscopic superconductors

We study phase shifts in a Josephson junction induced by vortices in superconducting mesoscopic electrodes. The position of the vortices are controlled by suitable geometry of a nano-scale Nb–Pt_1−xNi_x–Nb junction of the overlap type made by Focused Ion Beam (FIB) sculpturing. The vortex is kept outside the junction, parallel to the junction plane. From the measured Fraunhofer characteristics the entrance and exit of vortices are detected. By changing the bias current through the junction at constant magnetic field the vortices can be manipulated and the system can be switched between two consecutive vortex states which are characterized by different critical currents of the junction. A mesoscopic superconductor thus acts as a non-volatile memory cell in which the junction is used both for reading and writing information (vortex). Furthermore, we observe that the critical current density of Nb–Pt_1−xNi_x–Nb junctions decreases non-monotonously with increasing Ni concentration. It exhibits a minimum at ∼40 at.% Ni, which is an indication of switching into the π state.     

Directed transport and complex dynamics of vortices in a Josephson junction network driven by modulated currents

Directed transport of vortices in a Josephson junction network (JJN) with structural disorder is studied using a numerical simulation. Using spatiotemporal modulation of driving currents, the directed transport of vortices occurs even in the presence of disorder under certain conditions. From the analyses of the current–voltage and local voltage characteristics, it is found that the collective motion of vortices has an importance for the occurrence of directed transport.     

Hall effect and geometric phases in Josephson junction arrays

Since effectively the local contact vortex velocity dependent part of the Magnus force in a Josephson junction array is zero in the classical limit, we predict zero classical Hall effect. In the quantum limit because of the geometric phases due to the finite superfluid density at superconductor grains, rich and complex Hall effect is found in this quantum regime due to the Thouless-Kohmoto-Nightingale-den-Nijs effect.     

Charging fluctuations and resistive transition in Josephson junction arrays

The density of a vortex plasma in a two-dimensional quantum planar model is calculated as a function of temperature. The results imply flux-flow resistance with tails extending down to T = 0 and allowing for the possibility of resistance minima.     

Directed motion of vortices in a current-driven Josephson junction network

Dynamics of directed motion of vortices in a Josephson junction network (JJN) with a ladder structure is studied using a numerical simulation. By applying spatial and temporal modulation of external bias currents, directed motion of vortices occurs in the absence of a ratchet-type asymmetric potential. In the present system, the asymmetry of the directed motion emerges as a dynamical effect due to the modulated bias current. Some dynamical effects such as mode-locking and vortex–antivortex excitation are relevant to the directed dynamics. We clarify the details of the directed motion of vortices in the JJN.     

Competing boundary interactions in a Josephson junction network with an impurity

We analyze a perturbation of the boundary Sine-Gordon model where two boundary terms of different periodicities and scaling dimensions are coupled to a Kondo-like spin degree of freedom. We show that, by pertinently engineering the coupling with the spin degree of freedom, a competition between the two boundary interactions may be induced, and that this gives rise to nonperturbative phenomena, such as the emergence of novel quantum phases: indeed, we demonstrate that the strongly coupled fixed point may become unstable as a result of the “deconfinement” of a new set of phase-slip operators — the short instantons — associated with the less relevant boundary operator. We point out that a Josephson junction network with a pertinent impurity located at its center provides a physical realization of this boundary double Sine-Gordon model. For this Josephson junction network, we prove that the competition between the two boundary interactions stabilizes a robust finite coupling fixed point and, at a pertinent scale, allows for the onset of 4e superconductivity.     

Vortex motion rectification in Josephson junction arrays with a ratchet potential

By means of electrical transport measurements we have studied the rectified motion of vortices in ratchet potentials engineered on overdamped Josephson junction arrays. The rectified voltage as a function of the vortex density shows a maximum efficiency close a matching condition to the period of the ratchet potential indicating a collective vortex motion. Vortex current reversals were detected varying the driving force and vortex density revealing the influence of vortex-vortex interaction in the ratchet effect.     

Core states and spectral flow in vortex dynamics

I review the semi-classical picture of how states bound in the core of a vortex in an S-wave superconductor respond to relative motion between the vortex and the condensate. I show how the momentum absorbed as a result of the Magnus force acting on the core leads to a change in the distribution of occupied states (“spectral flow”). In the simplest relaxation time approximation this modified distribution gives rise to the Kopnin–Kravtsov force on the vortex.