C. C. de Souza Silva

Vortex-rectification effects in films with periodic asymmetric pinning

We study the transport of vortices excited by an ac current in an Al film with an array of nanoengineered asymmetric antidots. The vortex response to the ac current is investigated by detailed measurements of the voltage output as a function of ac current amplitude, magnetic field, and temperature. The measurements revealed pronounced voltage rectification effects which are mainly characterized by the two critical depinning forces of the asymmetric potential. The shape of the net dc voltage as a function of the excitation amplitude indicates that our vortex ratchet behaves in a way very different from standard overdamped models. Rather, the repinning force, necessary to stop vortex motion, is considerably smaller than the depinning force, resembling the behavior of the so-called inertia ratchets. Calculations based on an underdamped ratchet model provide a very good fit to the experimental data.

Dipole-induced vortex ratchets in superconducting films with arrays of micromagnets

We investigate the transport properties of superconducting films with periodic arrays of in-plane magnetized micromagnets. Two different magnetic textures are studied: a square array of magnetic bars and a close-packed array of triangular microrings. As confirmed by magnetic force microscopy imaging, the magnetic state of both systems can be adjusted to produce arrays of almost pointlike magnetic dipoles. By carrying out transport measurements with ac drive, we observed experimentally a recently predicted ratchet effect induced by the interaction between superconducting vortices and the magnetic dipoles. Moreover, we find that these magnetic textures produce vortex-antivortex patterns, which have a crucial role in the transport properties of this hybrid system.

Local mapping of dissipative vortex motion

We explore, with unprecedented single vortex resolution, the dissipation and motion of vortices in a superconducting ribbon under the influence of an external alternating magnetic field. This is achieved by combining the phase sensitive character of ac susceptibility, allowing us to distinguish between the inductive and dissipative responses, with the local power of scanning Hall probe microscopy. Whereas the induced reversible screening currents contribute only inductively, the vortices do leave a fingerprint in the out-of-phase component. The observed large phase-lag demonstrates the dissipation of vortices at time scales comparable to the period of the driving force (i.e., 13 ms). These results indicate the presence of slow microscopic loss mechanisms mediated by thermally activated hopping transport of vortices between metastable states.

Irreversible matching effects in homogeneous and layered superconducting films

Abstract Numerical simulation is used to study the vortex dynamics in thin superconducting homogeneous films and multilayers. The films are assumed to have thickness less than or equal to the penetration depth and subjected to a parallel magnetic field. The structure of the multilayers is treated as a periodic pinning potential. We use Langevin dynamics and London theory to calculate the vortex configurations when the external field is increased from zero to a maximum value and turned back to zero. The results show that the vortex lattice consists of parallel linear chains of vortices and goes through transitions from n to n +1 chains in particular values of the increasing field. The strong surface effects yield an irreversible magnetization curve even for the homogeneous films. Moreover, the pinning structure of the multilayers presents only small influence on the vortex lattice.

Structure, microstructure, magnetic properties and chemical stability of HoBa2SbO6 with Yba2Cu3O7−δ superconductor

Abstract A complex perovskite oxide HoBa 2 SbO 6 in the Ho–Ba–Hf–O, rare-earth based oxide system, has been synthesised and sintered as a single phase material. The structure, microstructure, quantitative elemental analysis and magnetic properties of this material have been studied by X-ray diffraction, scanning electron microscopy (SEM), energy dispersive X-ray (EDX) techniques and magnetic susceptibility measurements. Significant presence of superstructure reflection lines in the XRD spectra of HoBa 2 SbO 6 reveals that it is an ordered complex perovskite oxide. Magnetic susceptibility measurements show that HoBa 2 SbO 6 presents a paramagnetic behaviour at low temperature (5–300 K). Microstructure and EDX analysis show that sintered HoBa 2 SbO 6 has homogeneous surface morphology with uniform particle size (1–2 μm) distribution and there is no evidence of impurity in the single phase sintered HoBa 2 SbO 6 . The chemical stability of HoBa 2 SbO 6 with YBa 2 Cu 3 O 7− δ superconductor was studied. This study shows that HoBa 2 SbO 6 is chemically compatible with YBa 2 Cu 3 O 7− δ and it did not have deleterious effect on the superconducting transition temperature of YBa 2 Cu 3 O 7− δ . Also, HoBa 2 SbO 6 melts congruently and single crystals could be grown from the melt. In view of these favourable characteristics, HoBa 2 SbO 6 could be used as a potential substrate material for YBa 2 Cu 3 O 7− δ superconductor film.

Tunable anisotropic nonlinearity in superconductors with asymmetric antidot array

The influence of the spatial asymmetry of the pinning potential on the spectral composition of the voltage, induced in perforated superconducting Al bridges by the injection of a sinusoidal bias current, was investigated. The loss of the mirror symmetry of the pinning potential leads to the appearance of even Fourier components in the induced voltage in the vicinity of the superconducting phase transition line on the H-T diagram (H is the external magnetic field, and T is the temperature). Artificially-introduced asymmetry for vortex motion makes it possible to create low-resistive materials, in which nonlinearity depends on the direction of injected electrical currents.

Diode effects in the surface superconductivity regime

In this letter we demonstrate that large periodic arrays of asymmetric antidots perforated in a superconducting Al film rectify very efficiently an ac drive into a collective unidirectional synchronous motion of millions of vortices. Moreover, the rectification process has two distinctly different regimes: delocalized high-temperature regime with collective inter-antidot guided vortex motion and localized low-temperature regime with the intra-antidot rectification controlled by the fluxoid quantization condition satisfied synchronously at the edge of each antidot. As a result, a single condensed-matter device can be used to produce a tunable reversible macroscopic quantum diode effect important for the realization of the guided motion of large vortex ensembles for fluxonics applications.

Matching effect and vortex instabilities in Nb/Al multilayers

Abstract We present the field dependence of the magnetic moment on Nb/Al multilayers for applied field parallel and perpendicular to the multilayer surface. Zero-field cooled magnetization versus applied magnetic field (ZFC M versus H) curves, for the parallel applied field geometry, presents a series of maxima at both virgin and downward branches, the number of which being diminished as Tc is approached. Strong evidence of matching effects of the vortex lattice to the layered structure of the multilayer is revealed by the presence of well-defined plateaus, at the magnetization peaks positions, in magnetic induction (B) versus H curves. The jumps in magnetic induction are associated with the sudden splitting up of the in-plane vortex rows during rearrangement. At the perpendicular field geometry, we have found an extremely noisy behavior in a specific region in the magnetization curve also for both upward and downward branches. This suggests the occurrence of instabilities in vortex penetration in that field range.

Vortex Dynamics in Superconducting Films: Comensurability and Surface Effects

The penetration and further evolution of stiff vortex lines in a superconducting film under a parallel magnetic field is numerically simulated. A square array of in-plane columnar defects is considered as a periodic pinning potential. It is shown that the strong surface effects play an important role in the commensurability between the vortex lattice and the pinning potential. The matching peaks of the magnetization curve are associated to a stable composite vortex lattice in which two distinc vortex lattices are commensurate with the pinning potential.

EDX analysis and microstructural properties of the Yba2Cu3O7−δ-Ba2HoSbO6 superconducting composites

We have fabricated and characterized Yba2Cu3O7−δ-Ba2HoSbO6a realized high Tcsuperconductor - ceramic insulator composite system in which particles of the superconductor and ceramic insulator could coexist with well defined separated phases left intact by stringent processing conditions. All the composites exhibit superconductivity at 92K. EDX analysis show that element% and atomic% of the composites are in a good agreement with those of their respective components. SEM studies show homogenous surface morphology and particle size distribution. There is no detectable interface interaction between component grains and Ba2HoSbO6grains are distinguishably distributed in the Yba2Cu3O7−δmatrix.