Gilson Carneiro

Equilibrium vortex-line configurations and critical currents in thin films under a parallel field

Vortex-line equilibrium configurations and critical currents for type-II superconducting films at zero-temperature are studied theoretically. The films are assumed to be of thickness less than or equal to the penetration depth, free of pinning by imperfections, and to be subjected to a magnetic field parallel to the film surfaces and to a transport current perpendicular to the field. By numerical minimization of the exact London theory energy expression, using simulated annealing techniques, the equilibrium configurations are determined with great accuracy over a wide range of fields and currents. These consist of chains of straight vortex-lines whose number depends both on the field and on the current. Transitions involving a change by one in the chain number are found to take place either if the field is increased at zero transport current or if the transport current is increased at constant field. At the transitions, there is a considerable rearrangement in the vortex-line positions and a small, discontinuous, change in their number. The equilibrium configurations with chain numbers that are not too small form a nearly perfect triangular lattice, centered with respect to the film surfaces. However, small deviations from this arrangement are found to be important in determining the behavior near the transition and when the transport current approaches the critical current. It is found that the critical current has a a non-monotonic dependence on the field. The zero-temperature equilibrium phase diagram in the field-transport current plane is reported.

Pinning and creation of vortices in superconducting films by a magnetic dipole

Interactions between vortices in planar superconducting films and a point magnetic dipole placed outside the film, and the creation of vortices by the dipole, are studied in the London limit. The exact solution of London equations for films of arbritrary thickness with a generic distribution of vortex lines, curved or straight, is obtained by generalizing the results reported by the author and E.H. Brandt (Phys. Rev. B {\bf 61}, 6370 (2000)) for films without the dipole. From this solution the total energy of the vortex-dipole system is obtained as a functional of the vortex distribution. The vortex configurations created by the dipole minimize the energy functional. It is shown that the vortex-dipole interaction energy is given by

Tunable ratchet effects for vortices pinned by periodic magnetic dipole arrays

- {\bf m}\cdot {\bf b^{vac}}

Tunable pinning of a superconducting vortex by a magnetic vortex

, where

Tunable critical current for a vortex pinned by a magnetic dipole

{\bf m}

Vortex-loop fluctuations and vortex-line-lattice melting in layered superconductors

is the dipole strength and

Dynamical phase diagrams for moving vortices interacting with periodic pinning

{\bf b^{vac}}

Dynamical phases of driven vortices interacting with periodic pinning

is the magnetic field of the vortices at the dipole position, and that it can also be written in terms of a magnetic pinning potential acting on the vortices. The properties of this potential are studied in detail. Vortex configurations created by the dipole on films of thickness comparable to the penetration depth are obtained by discretizing the exact London theory results on a cubic lattice and minimizing the energy functional using a numerical algorithm based on simulated annealing. These configurations are found to consist, in general, of curved vortex lines and vortex loops.

Numerical method for zero-temperature vortex-line phase diagrams

The ratchet effect is demonstrated theoretically for the simple model of a vortex in a thin superconducting film interacting with a periodic array of magnetic dipoles placed in the vicinity of the film . The pinning potential for the vortex is calculated in the London limit and found to break spatial inversion symmetry and to depend on the orientation of the magnetic dipole moments. The motion of the vortex at zero temperature driven by a force oscillating periodically in time is investigated numerically. Drift vortex motion consisting of displacements by a translation vector of the dipole array during each period of oscillation is obtained and studied in detail. The direction of drift differs in general from that of the driving force, except if the driving force oscillates in a direction of high symmetry of the dipole array. The vortex drift velocity depends on the orientation of the magnetic moments, and can be tuned by rotating the dipoles. It is pointed out that if the magnetic moments are free to rotate, the ratchet effect can be produced and tuned by a magnetic field applied parallel to the film surfaces.

Dynamics of Driven Vortices in the Presence of a Regular Array of Columnar Defects

The interaction between a straight vortex line in a superconducting film and a soft magnetic nanodisk in the magnetic vortex state in the presence of a magnetic field applied parallel to the film surfaces is studied theoretically. The superconductor is described by London theory and the nanodisk by the Landau-Lifshitz continuum theory of magnetism, using the approximation known as the rigid vortex model. Pinning of the vortex line by the nanodisk is found to result, predominantly, from the interaction between the vortex line and the changes in the nanodisk magnetization induced by the magnetic field of the vortex line and applied field. In the context of the rigid vortex model, these changes result from the displacement of the magnetic vortex. This displacement is calculated analytically by minimizing the energy, and the pinning potential is obtained. The applied field can tune the pinning potential by controlling the displacement of the magnetic vortex. The nanodisk magnetization curve is predicted to change in the presence of the vortex line