Crossover between channeling and pinning at twin boundaries in YBa2Cu3O7 thin films

The critical current (Jc) of highly twinned YBa2Cu3O7 films has been measured as a function of temperature, magnetic field, and angle. For much of the parameter space we observe a strong suppression of Jc for fields in the twin boundary (TB) directions; this is quantitatively modeled as flux-cutting-mediated vortex channeling. For certain temperatures and fields a crossover occurs to a regime in which channeling is blocked and the TBs act as planar pinning centers so that TB pinning enhances the overall Jc. In this regime, intrinsic pinning along the TBs is comparable to that between the twins.     

Weak to strong pinning crossover

Material defects in hard type II superconductors pin the flux lines and thus establish the dissipation-free current transport in the presence of a finite magnetic field. Depending on the density and pinning force of the defects and the vortex density, pinning is either weak collective or strong. We analyze the weak to strong pinning crossover of vortex matter in disordered superconductors and discuss the peak effect appearing naturally in this context.     

STM imaging of flux line arrangements in the peak effect regime

We present the results of a study of vortex arrangements in the peak-effect regime of 2H-NbSe2 by scanning tunneling microscopy. By slowly increasing the temperature in a constant magnetic field, we observed a sharp transition from collective vortex motion to positional fluctuations of individual vortices at the temperature which coincides with the onset of the peak effect in ac susceptibility. We conclude that the peak effect is a disorder driven transition, with the pinning energy winning against the elastic energy.     

Evolution of the vortex state in the BCS-BEC crossover of a quasi two-dimensional superfluid Fermi gas

We use the path-integral formalism to investigate the vortex properties of a quasi-two dimensional(2D) Fermi superfluid system trapped in an optical lattice potential.Within the framework of mean-field theory,the cooper pair density,the atom number density,and the vortex core size are calculated from weakly interacting BCS regime to strongly coupled while weakly interacting BEC regime.Numerical results show that the atoms gradually penetrate into the vortex core as the system evolves from BEC to BCS regime.Meanwhile,the presence of the optical lattice allows us to analyze the vortex properties in the crossover from three-dimensional(3D) to 2D case.Furthermore,using a simple re-normalization procedure,we find that the two-body bound state exists only when the interaction is stronger than a critical one denoted by G_c which is obtained as a function of the lattice potential's parameter.Finally,we investigate the vortex core size and find that it grows with increasing interaction strength.In particular,by analyzing the behavior of the vortex core size in both BCS and BEC regimes,we find that the vortex core size behaves quite differently for positive and negative chemical potentials.     

Depinning dynamics of driven colloids with random pinning： a numerical study

Using Langevin simulations, we have investigated numerically the depinning dynamics of driven two-dimensional colloids subject to the randomly distributed point-like pinning centres. With increasing strength of pinning, we find a crossover from elastic to plastic depinnings, accompanied by an order to disorder transition of state and a substantial increase in the depinning force. In the elastic regime, no peaks are found in the differential curves of the velocity－force dependence (VFD) and the transverse motion is almost none. In addition, the scaling relationship between velocity and force is found to be valid above depinning. However, when one enters the plastic regime, a peak appears in the differential curves of VFD and transverse diffusion occurs above depinning. Furthermore, history dependence is found in the plastic regime.     

Unusual resistivity broadening in the mixed state of NdBa2Cu3O7−δ crystals grown by a crystal-pulling technique

In order to clarify the origin of a strong peak effect in the magnetization curve of NdBa₂Cu₃O_7−δ crystals, we investigated the superconductivity transition behavior of the in-plane resistivity in static magnetic fields up to 8 T. Comparing the results for the samples exhibiting and not exhibiting the peak effect, we found that the former (peak effect sample) shows lower resistivity above the vortex melting temperature. This implies that the pinning force is effective in the vortex liquid state. We also found that the normal state resistivity behaviors of some samples were unusual, indicating inhomogeneous current flow. These results suggest an existence of sheet-like pinning centers perpendicular to the conduction planes.     

Effect of pinning and driving force on the metastability effects in weakly pinned superconductors and the determination of spinodal line pertaining to order—disorder transition

We explore the effect of varying drive on metastability features exhibited by the vortex matter in single crystals of 2H-NbSe₂ and CeRu₂ with varying degree of random pinning. The metastable nature of vortex matter is reflected in the path dependence of the critical current density, which in turn is probed in a contact-less way via AC-susceptibility measurements. The sinusoidal AC magnetic field applied during AC susceptibility measurements appears to generate a driving force on the vortex matter. In a nascent pinned single crystal of 2H-NbSe₂, where the peak effect (PE) pertaining to the order—disorder phenomenon is a sharp first-order-like transition, the supercooling feature below the peak temperature is easily wiped out by the reorganization caused by the AC driving force. In this paper, we elucidate the interplay between the drive and the pinning which can conspire to make the path-dependent AC-susceptibility response of different metastable vortex states appear identical. An optimal balance between the pinning and driving force is needed to view the metastability effects in typically weakly pinned specimen of low temperature superconductors. As one uses samples with larger pinning in order to differentiate the response of different metastable vortex states, one encounters a new phenomenon, viz., the second magnetization peak (SMP) anomaly prior to the PE. Supercooling/superheating can occur across both the PE and the SMP anomalies and both of these are known to display non-linear characteristics as well. Interplay between the path dependence in the critical current density and the non-linearity in the electromagnetic response determine the metastability effects seen in the first and the third harmonic response of the AC susceptibility across the temperature regions of the SMP and the PE. The limiting temperature above which metastability effects cease can be conveniently located in the third harmonic data, and the observed behavior can be rationalized within the Bean’s critical state model. A vortex phase diagram showing different vortex phases for a typically weakly pinned specimen has been constructed via the AC susceptibility data in a crystal of 2H-NbSe₂ which shows the SMP and the PE anomalies. The phase space of coexisting weaker and stronger pinned regions has been identified. It can be bifurcated into two parts, where the order and disorder dominate, respectively. The former part continuously connects to the reentrant disordered vortex phase pertaining to the small bundle pinning regime, where the vortices are far apart, interaction effects are weak and the polycrystalline form of flux line lattice prevails.     

Peak effect in the critical current of type II superconductors with strong magnetic vortex pinning

We perform 2D Langevin simulations studying the peak effect (PE) of the critical current taking into account the temperature dependence of the competing forces. We observe and report that the PE results from the competition of vortex-vortex interactions and vortex-pin interactions which have different temperature dependencies. The simulations reveal that the PE can take place only for certain pinning strengths, densities of pinning centers, and driving forces, which is in good agreement with experiments. No apparent vortex order-disorder transition is observed across the PE regime. In addition, the PE is a dynamical phenomenon, and thermal fluctuations can speed up the process for the formation of the PE.     

受驱无序胶体动力学

利用Langevin分子动力学，本数值研究点钉扎中心随机分布的二维胶体动力学．随着钉扎中心强度的提高，我们发现了从弹性脱钉到塑性脱钉的渡越，并伴随临界钉扎力在渡越区的明显提高，类似于超导体中的峰值效应．另外，我们首次发现：当塑性流动发生时，高速运动胶体粒子感受到的平均钉扎力在从玻璃态到液态的转变过程中会出现峰值效应，并伴随有速度-驱动力曲线的交叠．     

Line vortices in a three-dimensional ordered Josephson medium at a small pinning parameter

The structure and energy of a line vortex whose axis is aligned with the symmetry axis of a finite-thickness slab indefinitely long in two directions is calculated by solving a set of linear finite-difference equations. Fluxoid quantization conditions in cells near the center of the vortex serve as boundary conditions. An exact solution is approached by iterations in phase stepwise discontinuities that cannot be considered small. A close similarity between the configuration under study and a periodic sequence (chain) of vortices makes it possible to allow for the effect of the domain boundary on the structure and energy of the vortex. It is shown that, at any width of the slab, one can find a pinning parameter value so small that the vortex cannot be viewed as solitary and contributions from other vortices should be taken into account in calculation. Proceeding in this way, one can find the structure and energy of the vortex however small the pinning parameter is. The total energy of the vortex is its intrinsic energy plus the sum of its energies of interaction with other members of the chain. In turn, the intrinsic energy is the sum of the energies of the small discrete core and quasi-continuous outer shell. It is demonstrated that the energy of the core is a linear function of the pinning parameter and is comparable to the energy of the shell.     

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     

Flux jump avalanches in torque studies of single crystal YBa2Cu3O7−δ

A series of avalanche-like jumps are observed in the mixed state of single crystal YBa₂Cu₃O_7−δ (YBCO) superconductors. Emerging as a saw-tooth pattern in torque vs. sample orientation in magnetic field, these jumps are discontinuous on our most resolute angular scale. While reminiscent of the classical flux jump instability, the present jumps are instead proposed to be associated with the layered nature of the material and twin boundary (TB) pinning, the combination of which promotes a crossover from a tilted to a kinked vortex structure.     

Depinning and creep motion in glass States of flux lines

Using dynamical computer simulation, we investigate vortex matter in glass states. A genuine continuous depinning transition is observed at zero temperature, which also governs the low-temperature creep motion. With the notion of scaling, we evaluate in high accuracy critical exponents and scaling functions; we observe a non-Arrhenius creep motion for weak collective pinning where the Bragg glass is stabilized at equilibrium, while for strong pinning, the well-known Arrhenius law is recovered. In both cases, a sharp crossover takes place between depinning and creep at low temperatures.     

Static and dynamic behaviours of multivortex states in a superconducting sample with mesoscopic pinning sites

This preliminary work has focused on the static transitions between the multivortex states interacting with square arrays of the mesoscopic pinning sites in superconducting samples. Our results were obtained from an extensive series of numerical simulations as functions of the magnetic field, pinning radius, and sample size. We have presented a wide range of multivortex configurations from commensurate dimer states to more concentric vortex shells at the matching fields. The stability of these states was also studied by means of the current-voltage V(I) curves which illustrate dynamic phase transitions as a function of applied driving force. These transitions manifested themselves as either a sudden jump in velocity or a nonlinear increase with velocity fluctuations in V(I) curves. We have investigated whether that the phase transitions between the pinned regime and the elastic flow regime are indicative of the stability of the initial vortex states. The variety of intermediate flow phases is attributed to large pinning size (reentrant behavior), strong commensurability and caging effects. In particular, three-shell vortex structures were obtained in the presence of larger pinning sites at adequate matching magnetic fields.     

Three-dimensional Ginzburg-Landau simulation of a vortex line displaced by a zigzag of pinning spheres

A vortex line is shaped by a zigzag of pinning centers and we study here how far the stretched vortex line is able to follow this path. The pinning center is described by an insulating sphere of coherence length size such that in its surface the de Gennes boundary condition applies. We calculate the free energy density of this system in the framework of the Ginzburg-Landau theory and study the critical displacement beyond which the vortex line is detached from the pinning center.     

Peak effect and dynamic melting of vortex matter in NbSe2 crystals

We present a mode locking (ML) phenomenon of vortex matter observed around the peak effect regime of 2H-NbSe2 pure single crystals. The ML features allow us not only to trace how the shear rigidity of driven vortices persists on approaching the second critical field, but also to demonstrate a dynamic melting transition of driven vortices at a given velocity. We observe the velocity dependent melting signatures in the peak effect regime, which reveal a crossover between the disorder-induced transition at small velocity and the thermally induced transition at large velocity. This uncovers the relationship between the peak effect and the thermal melting.     

Low field irreversibility in twinned and untwinned YBaCuO crystals

An abrupt collapse of the irreversibility line is observed in both twinned and untwinned YBaCuO crystals within a critical regime. The exponent of the power law describing the irreversibility line H_irr changes from at high fields to larger values (between 1.5 and 2) below the collapse. We propose that the observed low field anomaly, which extends down to temperatures well below the irreversibility line, is associated with the thermal softening boundary which increases as T² and crosses H_irr shifting it to lower fields. Below this crossover, which occurs when the mean-square thermal displacement √ u² of the vortices exceeds coherence length ξ, the strength of the pinning is strongly reduced.     

Vortex softening: origin of the second peak effect in Bi(2)Sr(2)CaCu(2)O(8+delta)

Magnetic hysteresis and transverse ac permeability measurements in Bi(2)Sr(2)CaCu(2)O(8+delta) allow a comparative analysis of the critical current with the elastic response of vortex structures, in equilibrium with their pinning potential, in the field and temperature region where the second peak is detected. This study provides strong evidence that the second peak has its origin in changes of the elastic equilibrium properties of the vortex structures.     

Pinning effects on the vortex critical velocity in type-II superconducting thin films

We study the influence of artificial pinning centers on the vortex critical velocity in Al thin films deposited on top of a periodic array of Permalloy (FeNi) square rings. We demonstrate that the field dependence of the flux flow velocity strongly depends on the particular magnetic state of the rings. In particular, we find that, even when the rings are in a flux closure state, i.e. with little stray field, the vortex critical velocity shows a non-monotonic magnetic field dependence. This behaviour is in sharp contrast with the results obtained in a reference plain film, with no rings underneath. A comparison with the intrinsic strong pinning Nb films previously studied, suggests an interpretation in terms of a channel-like motion of vortices, here induced by the artificial pinning structure.