磁通格子的有序-无序相变和反向熔化

考虑了无序钉扎、热涨落和磁通互作用, 用Monte Carlo分子动力学模拟方法研究二维磁通格子在无序钉扎强度和温度空间的相图, 以及由布拉格玻璃相到非晶磁通玻璃相和到磁通液体相的有序-无序相变.为了决定磁通格子的序，计算了静态结构因子和磁通格子位形的有限尺寸指数.计算结果表明，Bragg玻璃相在低温的无序磁通玻璃相和高温的磁通液体相之间 , 表现出磁通格子的反向熔化行为.分析后认为，这一反向熔化行为起因于磁通之间互作用的温度效应. 关键词： Ⅱ类超导体 磁通格子 相图 结构因子 反向熔化     

The structure of the vortex lattice in anisotropic superconductors

The structure of the vortex lattice in anisotropic superconductors at an arbitrary temperature in magnetic fields close to critical was studied. Generally, a rhombohedral structure with a vertex angle depending on temperature, magnetic field, and material constants is formed. An important factor is the small (2%) difference of the free energies of the triangular and square lattices in the Ginzburg-Landau approximation. This factor also persists in anisotropic superconductors.     

Kinetics of vortex formation in superconductors with anisotropic pairing

The evolution of the vortex state in superconductors with anisotropic pairing is investigated on the basis of the time-dependent Ginzburg-Landau equations. Fiz. Tverd. Tela (St. Petersburg) 39, 35–38 (January 1997)     

Nonequilibrium phase transitions of vortex matter in three-dimensional layered superconductors

Large-scale simulations on the three-dimensional (3D) frustrated anisotropic XY model have been performed to study the nonequilibrium phase transitions of vortex matter in weak random pinning potential in layered superconductors. The first-order phase transition from the moving Bragg glass to the moving smectic is clarified, based on thermodynamic quantities. A washboard noise is observed in the moving Bragg glass in 3D simulations for the first time. It is found that the activation of the vortex loops plays the dominant role in the dynamical melting at high drive.     

Melting of the nanocrystalline vortex matter in highly anisotropic high-temperature superconductors

Multilevel Monte Carlo simulations of the vortex matter in the highly anisotropic high-temperature superconductor Bi2Sr2CaCu2O8 were performed. We introduced low concentration of columnar defects satisfying Bphi < or = B. Both the electromagnetic and Josephson interactions among pancake vortices were included. The nanocrystalline, nanoliquid, and homogeneous liquid phases were identified in agreement with experiments. We observed the two-step melting process and also noted an enhancement of the structure factor just prior to the melting transition. A proposed theoretical model is in agreement with our findings.     

无序超导体磁通运动的两次退钉扎效应和重新进入超导相

考虑平面内和不同平面磁通之间的相互作用力，计算了无序各向异性超导体中磁通运动的平均速度、微分电阻随驱动力F_l的变化规律，用层间关联函数C_z的值来判断2D塑性流动和3D关联流动的运动图像.观察到随着外驱动力的增大微分电阻出现两个尖峰，它对应着磁通运动存在两次退钉扎现象.在一定层间耦合条件下，在微分电阻双峰之间，可观察到重新进入微分电阻为零的钉扎相.这与最近实验上新发现的无序弱钉扎超导体有重新进入超导相的巨大峰值效应相吻合.同时，也可发现随着驱动电流的增大，磁通运动出现由2D塑性流动到3D弹性流动的相变，这一维度的变化对应着微分电阻dV/dI曲线中的二次峰位置. 并证明当层间耦合(即代表磁场的大小)在一定范围时，3D-2D相变对应的临界电流随磁场的增大而增大, 反映了第二磁化峰附近的磁通格子软硬度改变的微观图像. 关键词： 第Ⅱ类超导体 磁通线格子 钉扎 峰值效应     

Dynamic phase transitions in moving vortex structures in type II superconductors

A theoretical explanation for the I–V characteristic of type II superconductors in the vortex state is provided. The run of the experimental I–V curves is associated with discontinuities and depends on the experimental prehistory. It is conjectured that dynamic phase transitions occur in the network of mobile and immobile channels along which the vortices travel when a transport current is applied. This work has been done on the author’s own initiative.     

Nonequilibrium effects of anisotropic compression applied to vortex lattices in bose-einstein condensates

We have studied the dynamics of large vortex lattices in a dilute-gas Bose-Einstein condensate. While undisturbed lattices have a regular hexagonal structure, large-amplitude quadrupolar shape oscillations of the condensate are shown to induce a wealth of nonequilibrium lattice dynamics. When exciting an m=-2 mode, we observe shifting of lattice planes, changes of lattice structure, and sheetlike structures in which individual vortices appear to have merged. Excitation of an m=+2 mode dissolves the regular lattice, leading to randomly arranged but still strictly parallel vortex lines.     

Ordered, disordered, and coexistent stable vortex lattices in NbSe2 single crystals

The peak effect (PE) in the critical current density of type II superconductors has been related to an order-disorder transition in the vortex lattice (VL), but its underlying physics remains a controversial issue. Intrinsic to the PE are strong metastabilities that frequently mask the stationary VL configurations. We follow shaking and thermal protocols in NbSe2 single crystals to access these configurations and examine them by linear ac susceptibility measurements that avoid VL reorganization. We identify three different regions. For TT2(H), configurations are fully disordered and no metastability is observed. In the T1相似文献   

Delocalization in two-dimensional disordered Bose systems and depinning transition in the vortex state in superconductors

We investigate a two-dimensional (2D) Bose system with the long range interactions in the presence of disorder. Formation of the bound states at strong impurity sites gives rise to a depletion of the superfluid density. We predict the intermediate superfluid state where the condensate and localized bosons are present simultaneously. We find that interactions suppress localization and that with the increase of the boson density the system experiences a sharp delocalization crossover into a state where all bosons are delocalized. We map our results onto a 3D system of vortices in type II superconductors in the presence of columnar defects; the intermediate superfluid state maps to an intermediate vortex liquid where vortex liquid neighbors pinned vortices. We predict the depinning crossover within the vortex liquid and depinning induced vortex lattice-Bose glass melting.     

Equilibration and dynamic phase transitions of a driven vortex lattice

We report on the observation of two types of current driven transitions in metastable vortex lattices. The metastable states, which are missed in usual slow transport measurements, are detected with a fast transport technique in the vortex lattice of undoped 2H-NbSe2. The transitions are seen by following the evolution of these states when driven by a current. At low currents we observe an equilibration transition from a metastable to a stable state, followed by a dynamic crystallization transition at high currents.     

Vortex entanglement in disordered superconductors

Vortex entanglement and pinning in multiply connected disordered superconductors are studied. It is shown that the winding of vortices around repulsive obstacles is greatly enhanced by quenched columnar disorder and suppressed by point disorder, compared to the clean case. This leads to an additional contribution to the effective pinning force acting on vortices, which, unlike the conventional mechanisms of pinning, grows with temperature.