The Effects of Pinning on Critical Currents of Superconducting Films

Using molecular dynamics simulations, we analyze the effects of artificial periodic arrays of pinning sites on the critical current of superconducting thin films as a function of vortex density. We analyze two types of periodic pinning array: hexagonal and Kagomé. For the Kagome pinning network we make calculations using two directions of transport current: along and perpendicular to the main axis of the lattice. Our results show that the hexagonal pinning array presents higher critical currents than the Kagomé and random pinning configuration for all vortex densities. In addition, the Kagomé networks show anisotropy in their transport properties.     

Depinning of Two-Dimensional Vortex System with Square Pinning Array at the Second Matching Field

Depinning of a two-dimensional vortex system at the second matching field B=2B _? in the presence of square pinning array was studied by molecular dynamics simulations. The annealed ground structures and depinning properties are found to be dependent on pinning strength. For strong pinning, we find a two-step depinning transition with a lower depinning force F _c1 and a higher one F _c2. At F _c1 only the interstitial vortices start to move, while at F _c2 the vortices at pinning sites are depinned and all vortices move.     

Superfluidity and Vortex Lattice Formation in Quasi Two-Dimensional Exciton System

Bose-Einstein condensation(BEC) and superfluidity of excitons in type-II quantum wells are studied theoretically. As a typical example, we consider GaAs/AlAs quantum wells with each layer thickness L less than 37Å. First, we investigate the interaction between excitons, thus finding it repulsive for L less than about 15Å, which shows the possibility of BEC. Secondly, we study the stationary pattern of the phase of the BEC order parameter under the external current J. The interaction between excitons and the weak laser light violates conservation of number of excitons and fixes that phase. There appears a vortex lattice with net supercurrent when J is larger than a critical value. The experimental observation of this critical current will give an evidence that BEC and superfluidity occur in exciton system.     

Collective Pinning of a Flux-Line Lattice with Screw Dislocations

The energy of screw dislocations of a flux-line lattice in Type-II superconductors is calculated taking account of dispersion of the tilt modulus. On the basis of this calculation, the collective-pinning theory for the plastic case, proposed by Mullock and Evetts, leads to correlation lengths of a flux-line lattice much shorter than that for the elastic case leads, and much better theoretical fits to experimental results in Nb ₃ Ge and 2H-NbSe _2–x S _x are obtained. Furthermore, the intermediate state between three-dimensional disorder and two-dimensional disorder in a flux-line lattice is predicted when the screw dislocations trigger the dimensional crossover.     

Superconducting Vortex Lattice Configurations on Periodic Potentials: Simulation and Experiment

Nb films have been fabricated on top of array of Ni nanodots. The array of periodic pinning potentials modifies the vortex lattice for specific values of the external applied magnetic field. By means of an implemented code developed from scratch, computer simulations based only on the vortex?Cvortex and the vortex?Cnanodot interactions provide the total interaction between vortices and pinning sites as well as the position of the vortices in the array unit cell. This simulation approach could be performed on square, rectangular or triangular arrays of nanodefects of different size.     

Vortex Pinning to a Solid Sphere in Helium II

We report here the results of a computer simulation of quantized vortex pinning in He II at 0 K in the simple situation where a single sphere and a rectilinear vortex are considered. Our simulation shows that a vortex nearby a sphere is captured due to a velocity field produced by the sphere, exciting Kelvin waves. The dependence of the “pinning” on temperature is investigated as well. Finally, the possibility of pinning in PIV experiments, which visualize superfluid turbulence, is discussed.     

Relation Between Vortex Pinning Energy and Anderson's Theorem

We discuss the elementary vortex pinning in type-II superconductors in connection with the Anderson's theorem for nonmagnetic impurities. We address the following two issues. One is an enhancement of the vortex pinning energy in the unconventional superconductors. This enhancement comes from the pair-breaking effect of a nonmagnetic defect as the pinning center far away from the vortex core (i.e., the pair-breaking effect due to the non-applicability of the Anderson's theorem in the unconventional superconductors). The other is an effect of the chirality on the vortex pinning energy in a chiral p-wave superconductor. The vortex pinning energy depends on the chirality. This is related to the cancellation of the angular momentum between the vorticity and chirality in a chiral p-wave vortex core, resulting in local applicability of the Anderson's theorem (or local recovery of the Anderson's theorem) inside the vortex core.     

Vortex Lattice Structures of a Bose-Einstein Condensate in a Rotating Lattice Potential

We study vortex lattice structures of a trapped Bose-Einstein condensate in a rotating lattice potential by numerically solving the time-dependent Gross-Pitaevskii equation. By rotating the lattice potential, we observe the transition from the Abrikosov vortex lattice to the pinned vortex lattice. We investigate the transition of the vortex lattice structure by changing conditions such as angular velocity, strength, and lattice constant of the rotating lattice potential.     

Pinning of the Vortex System and Magnetostriction of Superconductors

No Heading Hard type-II superconductors usually reveal a large, of the order of 10^–5 or 10^–4, magnetostriction in the external magnetic field of several Tesla. Such a strong magnetostriction can be well understood in the framework of the model of the pinning-induced magnetostriction. Although the assumptions of this model are easy to understand, the solution of the problem for realistic sample shapes is difficult, because the stress induced in the sample volume by the screening currents has non uniform distribution and it may also change the sample shape. The model of pinning-induced magnetostriction is insufficient to explain all magnetostriction results in superconductors. Sometimes, it is necessary to consider also other mechanisms of the magnetostriction. Magnetostriction of superconductors was analyzed theoretically and studied experimentally in a large number of both conventional and high temperature superconductors. In this paper we present a review of the most characteristic results. We also present the phenomenon of giant magnetostriction jumps, which is closely related to the phenomenon of giant flux jumps or thermomagnetic instabilities in type-II superconductors.PACS numbers: 74.25.Qt, 75.80.+q     

Oblique Vortex Lattice Implies Unconventional Pairing Symmetry in High Temperature Superconductors

The symmetry of order parameters of YBa₂Cu₃O_{7 –} high temperature superconductor was studied with the Ginzburg–Landau theory. The vortex lattice of a YBa₂Cu₃O₇ superconductor is oblique at a temperature well below the transition temperature T _c, where the mixed s– state is expected to have the lowest energy, whereas very close to T _c, the -wave is slightly lower in energy, and a triangular vortex lattice recovers. The coexistence and the coupling between the s- and d-waves would account for the unusual behaviors such as the upward curvature of the upper critical field curve H _C2(T).     

Dynamics of Vortex Lattice Formation in a Rotating Bose-Einstein Condensate with an Optical Lattice Potential

We study dynamics of quantized vortex lattice formation in a rotating Bose-Einstein condensate with a square blue-detuned optical lattice by solving the Gross-Pitaevskii equation. This dynamics depends on the depth of the optical lattice. Vortices tend to form a triangular lattice under the rotation, while an optical lattice likes to pin vortices at their peaks. Such a competition of two effects makes this system more interesting and complicated.     

Vortex Lattice Dynamics in a Dilute Gas BEC

We study the dynamics of large vortex lattices in a dilute gas Bose–Einstein condensate. Rapidly rotating condensates are created that contain vortex lattices with up to 300 vortices. The condensates are held in a parabolic trapping potential, and rotation rates exceeding 99% of the radial trapping frequency are achieved. By locally suppressing the density while maintaining the phase singularities, we create vortex aggregates. To illustrate the underlying Coriolis force driven dynamics, oscillation frequencies of the vortex aggregate area are measured. A related technique also enables us to excite and directly image Tkachenko modes in a vortex lattice. These modes provide evidence for the shear strength that a vortex lattice in a superfluid can support.     

Vortex Self-Organization in the Presence of Magnetic Pinning Arrays

Lateral nanostructuring is an efficient tool to control vortex confinement in superconductors. This will be illustrated by studying pinning phenomena in type-II superconducting Pb films with a lattice of submicron magnetic dots. We consider rectangular Co dots with in-plane magnetization and circular Co/Pt dots with out-of-plane magnetization. The domain structure of the Co dots can be changed from multi- to single-domain, resulting in an enhancement of their stray field. After covering this Co dot array with a Pb film, we demonstrate the influence of the local magnetic stray field of the dots on their flux pinning efficiency. The Co/Pt dots have a single-domain structure with their magnetic moment out of plane. Depending on the magnetic history, the magnetic moment of all dots can be aligned in positive or negative direction, or a random distribution of positive and negative magnetic moments of the dots can be achieved. For a Pb film covering this Co/Pt dot array, we observe an asymmetric magnetization loop due to the magnetic interactions between the vortices and the magnetic dots.     

Vortex Matter in the Presence of Magnetic Pinning Centra

Within the non-linear Ginzburg-Landau (GL) theory, we investigate the vortex structure of a thin type II superconductor with a ferromagnetic dot on top of it. Spontaneous creation of vortex-antivortex pairs as well as specific arrangements of vortices are found. We show that the equilibrium vortex phase diagram depends on the size of the magnetic dot and its magnetization, and we discuss the shell structure of the multivortex states with magic numbers which are size and magnetization dependent.     

NMR Lineshape in Anisotropic Superconductors with Nonregular Vortex Lattice

The nuclear magnetic resonance line shapes within a primitive cell of the vortex lattice of the type II anisotropic superconductors in a case when a vortex is displaced on small distance a from a regular position in a primitive cell are constructed. The results of the numerical calculations show that displacement of the flux line lattice essentially changes the NMR lineshape. The derivative of the power of the absorption energy with respect to the magnetic field is calculated. It allows to obtain more detailed information about the real vortex lattice of a superconductor.     

Anisotropy in the Transport Properties of Type II Superconducting Films with Periodic Pinning

Using numerical simulations, we analyze the anisotropy effects in the critical currents and dynamical properties of vortices in a thin superconducting film submitted to hexagonal and Kagomé periodical pinning arrays. The calculations are performed at zero temperature, for transport currents parallel and perpendicular to the main axis of the lattice, and parallel to the diagonal axis of the rhombic unit cell. We show that the critical currents and dynamic properties are anisotropic for both pinning arrays and all directions of the transport current. The anisotropic effects are more significant just above the critical current and disappear with higher values of current and both pinning arrays. The dynamical phases for each case and a wide range of transport forces are analyzed.     

Transport Properties of a Tl-2201 Film Close to the Critical Temperature: The Vortex Glass Transition

We have studied the I-V characteristics of a Tl-2201 film at zero field. In the regime in which flux creep is the dominant dissipation mechanism, the J _c -T curve is divided into two parts at a temperature T _g (about 82 K), close to the critical temperature (84 K). The I-V characteristics around T _g are well described using a flux creep model. For T>T _g , J _c /J _c (0) =0.445x(l-0.525t-0.5t ² ); for T _g , J _c /J _c (0) = 0.9x(1-0.595t-0.44t ² ). Differential resistance (dV/dI) as a function of the measuring current shows a change in curvature close to T _g . The I-V curves collapsed nicely into two branches by plotting (V/I)/|T–T _g | ^v(z-1) vs. (I/T)/|T _g –T| ^2v , indicating a current–reduced vortex glass transition.     

Incommensurate and Commensurate Density-Wave in the Two-Dimensional Lattice

We have investigated anisotropic density-wave phase with arbitrary ordering wave vector (Q) and estimated the value of Q that corresponds to the minimum of the free energy. We have found that for a wide range of model parameters the commensurate d-density-wave (DDW) is actually the most stable density-wave state. However, for moderate doping the commensurate DDW state is stable only at finite temperatures and disappears when the temperature is sufficiently low. These features may assist in clarification of the mechanism of the pseudogap.     

Analysis of Photonic Band Gaps in a Two-Dimensional Triangular Lattice with Superconducting Hollow Rods

In this work, we use the plane wave expansion method to calculate photonic band structures in two-dimensional photonic crystals which consist of high-temperature superconducting hollow rods arranged in a triangular lattice. The variation of the photonic band structure with respect to both, the inner radius and the system temperature, is studied, taking into account temperatures below the critical temperature of the superconductor in the low frequencies regime and assuming E polarization of the incident light. Permittivity contrast and nontrivial geometry of the hollow rods lead to the appearance of new band gaps as compared with the case of solid cylinders. Such band gaps can be modulated by means of the inner radius and system temperature.