Leonardo R.E. Cabral

Vortex configurations and metastability in mesoscopic superconductors

Abstract The vortex dynamics in mesoscopic superconducting cylinders with rectangular cross section under an axially applied magnetic field is investigated in the multivortex London regime. The rectangles considered range from a square up to an infinite slab. The flux distribution and total flux carried by a vortex placed in an arbitrary position of the sample is calculated analytically by assuming Clems solution for the vortex core. The Bean–Livingston energy barrier is also analytically calculated in this framework. A Langevin algorithm simulates the flux penetration and dynamical evolution of the vortices as the external field is slowly cycled. The simulated magnetization process is governed by metastable states. The magnetization curves are hysteretic, with paramagnetic response in part of the downward branch, and present a series of peaks corresponding to the entry or expulsion of a single vortex. For elongated rectangles, the vortices arrange themselves into parallel vortex chains and an additional modulation of the magnetization, corresponding to creation or destruction of a vortex chain, comes out.

Structural and superconducting properties of MS (M = Fe, Ni or Zn)-substituted

The effect of CuO substitution by MS ( M = Ni, Fe or Zn) in (Y-123) samples is studied by DC resistivity, AC susceptibility and x-ray emission spectroscopy and diffractometry. All samples show sharp x-ray diffraction patterns with no impurity phase lines for M = Ni, Fe or Zn-doped samples. The cell of Ni-, Zn- or Fe-substituted Y-123 samples remains orthorhombic for all concentrations studied. X-ray emission spectroscopy results show that sulphur atoms enter the lattice of Y-123 compounds and form oxyanion groups. The diamagnetic transitions are sharp for all the small concentrations and become broad for MS doping above 10%. The transition temperature decreases with increasing MS concentration when compared with the undoped sample.

Vortices in superconducting strips: interplay between surface effects and the pinning landscape

Abstract Vortices in a narrow superconducting strip with a square array of pinning sites are studied. The interactions of vortices with other vortices and with external sources (applied magnetic field and transport current) are calculated via a screened Coulomb model. The edge barrier is taken into account and shown to have an important role on the system dynamics. Numerical simulations in this approach show that the field dependent magnetic moment presents peaks corresponding to history dependent configurations of the vortex lattice. Some effects of the edge barrier on the I – V characteristics are also reported.

Superconducting properties of mesoscopic squares

We apply the complete nonlinear Time Dependent Ginzburg Landau (TDGL) equations to study the vortex dynamics in a mesoscopic type II superconductor using the numerical method based on the technique of gauge invariant variables. The solution of these equations shows how the vorticity penetrates into and goes out of the superconductor through the surface boundary. We calculate the spatial distribution of the superconducting electron density and the phase of the superconducting order parameter in a mesoscopic superconducting square sample containing two holes in the presence of a uniform perpendicular magnetic field. The dynamics of different vortex states are studied as a function of the external magnetic field.

Pinning-force measurements in Bi2Sr2CaCu2O8+ y

Abstract We report on the applied field and temperature dependence of the pinning force in Bi 2 Sr 2 CaCu 2 O 8+ y single crystals. The pinning force was obtained from the hysteresis curves in magnetization measurements. The field dependence of the pinning force presents a maximum at nonzero applied field. The temperature dependence of the field for maximum pinning force shows three different behaviours: For temperatures between 10 and 25 K it decreases steeply with temperature; this behaviour is changed for a smooth temperature dependence for temperatures in the range 25–40 K and then it approaches to zero above 40 K. This behaviour suggest the existence of different pinning mechanisms for different temperature regions.

Magnetization curves and geometric barrier in BSCCO-2212

We present magnetization curves measured with a BSCCO-2212 single crystals for external magnetic fields applied along the sample thickness at temperatures between 16 and 80 K. The data at high temperature (T/T c > 0.5) is compared with the numerically calculated magnetization, in which the specimen geometry and an explicit equilibrium relation H = H(B) are taken into account. The reversible experimental and calculated magnetization show good agreement, allowing to estimate H c1 (T) at the 50-80 K temperature range. The results show that the geometric barrier is mainly responsible for the magnetic hysteresis observed at high temperatures (T ≥ 70 K). The data at intermediate temperatures (0.2 < T/T c < 0.5) can be fitted by a non-monotonic critical current dependence on B.

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.

Flux field profiles and magnetization curves for bent thin strips in transverse field

The flux penetration in superconducting bent strips is calculated and compared to those calculated for flat strips. Using a modified Bean critical state model, the current and flux field profiles are calculated as a function of the coordinate parallel to the strip surface. It is shown that the flux penetration is delayed in bent strips compared to flat strips.

Magnetic response of hard type-II superconductors with a semicircular indentation

Abstract The magnetic response of superconductors with a semicircular indentation in parallel field is studied. The indentation is placed in a long superconductor wedge. The Meissner and vortex fields are calculated for any indentation radius in the London background. This allowed us to obtain the energy and the forces acting on a vortex in a given position inside the superconductor. The entrance magnetic field dependence on the indentation radius is calculated and compared with the entrance field for an indentation free semi-infinite superconductor. The role of the indentation on the surface barrier and on the vortex dynamics is also discussed.

Flux field profiles and magnetization curves for superconducting thin films: disks and conical shells

In this work, thin superconductors disks and thin conical shells in an applied magnetic field parallel to their symmetry axes are considered. In order to study the flux field profiles and magnetization curves, a critical state approach is adopted, in which flux creep effects are also taken into account through a constitutive relation between the electrical field and the current density. The critical current density is assumed to be both B independent (Bean state) and B dependent. The results for thin disks and thin conical shells show that the latter presents delayed flux penetration compared to the disk.