Meissner state in finite superconducting cylinders with uniform applied magnetic field

We study the magnetic response of superconductors in the presence of low values of a uniform applied magnetic field. We report measurements of DC magnetization and AC magnetic susceptibility performed on niobium cylinders of different length-to-radius ratios, which show a dramatic enhance of the initial magnetization for thin samples, due to the demagnetizing effects. The experimental results are analyzed by applying a model that calculates the magnetic response of the superconductor, taking into account the effects of the demagnetizing fields. We use the results of magnetization and current and field distributions of perfectly diamagnetic cylinders to discuss the physics of the demagnetizing effects in the Meissner state of type-II superconductors.Comment: Accepted to be published in Phys. Rev. B; 15 pages, 7 ps figure

Magnetic-field and current-density distributions in thin-film superconducting rings and disks

We show how to calculate the magnetic-field and sheet-current distributions for a thin-film superconducting annular ring (inner radius a, outer radius b, and thickness d<<a) when either the penetration depth obeys lambda < d/2 or, if lambda > d/2, the two-dimensional screening length obeys Lambda = 2 lambda^2/d << a for the following cases: (a) magnetic flux trapped in the hole in the absence of an applied magnetic field, (b) zero magnetic flux in the hole when the ring is subjected to an applied magnetic field, and (c) focusing of magnetic flux into the hole when a magnetic field is applied but no net current flows around the ring. We use a similar method to calculate the magnetic-field and sheet-current distributions and magnetization loops for a thin, bulk-pinning-free superconducting disk (radius b) containing a dome of magnetic flux of radius a when flux entry is impeded by a geometrical barrier.Comment: 10 pages, 13 figure

History effects and pinning regimes in solid vortex matter

We propose a phenomenological model that accounts for the history effects observed in ac susceptibility measurements in YBa2Cu3O7 single crystals [Phys. Rev. Lett. 84, 4200 (2000) and Phys. Rev. Lett. 86, 504 (2001)]. Central to the model is the assumption that the penetrating ac magnetic field modifies the vortex lattice mobility, trapping different robust dynamical states in different regions of the sample. We discuss in detail on the response of the superconductor to an ac magnetic field when the vortex lattice mobility is not uniform inside the sample. We begin with an analytical description for a simple geometry (slab) and then we perform numerical calculations for a strip in a transverse magnetic field which include relaxation effects. In calculations, the vortex system is assumed to coexist in different pinning regimes. The vortex behavior in the regions where the induced current density j has been always below a given threshold (j_c^>) is described by an elastic Campbell-like regime (or a critical state regime with local high critical current density, j_c^>). When the VS is shaken by symmetrical (e.g. sinusoidal) ac fields, the critical current density is modified to j_c^) at regions where vortices have been forced to oscillate by a current density larger than j_c^>. Experimentally, an initial state with high critical current density (j_c^>) can be obtained by zero field cooling, field cooling (with no applied ac field) or by shaking the vortex lattice with an asymmetrical (e.g. sawtooth) field. We compare our calculations with experimental ac susceptibility results in YBa2Cu3O7 single crystals.Comment: 11 pages, 7 figures. To be published in PR

Exact Solution for the Critical State in Thin Superconductor Strips with Field Dependent or Anisotropic Pinning

An exact analytical solution is given for the critical state problem in long thin superconductor strips in a perpendicular magnetic field, when the critical current density j_c(B) depends on the local induction B according to a simple three-parameter model. This model describes both isotropic superconductors with this j_c(B) dependence, but also superconductors with anisotropic pinning described by a dependence j_c(theta) where theta is the tilt angle of the flux lines away from the normal to the specimen plane

Overcritical states of a superconductor strip in a magnetic environment

A current-carrying superconducting strip partly penetrated by magnetic flux and surrounded by a bulk magnet of high permeability is considered. Two types of samples are studied: those with critical current controlled by an edge barrier dominating over the pinning, and those with high pinning-mediated critical current masking the edge barrier.It is shown for both cases that the current distribution in a central flux-free part of the strip is strongly affected by the actual shape of the magnetic surroundings. Explicit analytical solutions for the sheet current and self-field distributions are obtained which show that, depending on the geometry, the effect may suppress the total loss-free transport current of the strip or enhance it by orders of magnitude. The effect depends strongly on the shape of the magnet and its distance to the superconductor but only weakly on the magnetic permeability.Comment: 20 pages, 20 figure

Температурная зависимость критического тока гетероструктур YBCO-STO-LCMO вблизи Tc

Проведены исследования комплексной дифференциальной восприимчивости пленки YBCO и структур YBCO-STO-LCMO с различной толщиной STO (0; 2; 7 нм). Показано, что для структуры YBCO-LCMO наблюдается снижение температуры сверхпроводящего перехода Tonset и плотности критического тока jc по сравнению с пленкой чистого YBCO. В структуре YBCO-STO-LCMO, напротив, Tonset повышается, а jc увеличивается для структуры с STO толщиной 2 нм и снижается для 7 нм. Высказано предположение о смене механизма пиннингования вихрей при понижении температуры.Проведено дослідження комплексної диференціальної сприйнятливості плівки YBCO та структур YBCO-STO-LCMO з різною товщиною STO (0; 2; 7 нм). Показано, що для структури YBCO-LCMO спостерігається зниження температури надпровідного переходу Tonset та густини критичного струму jc в порівнянні з плівкою чистого YBCO. В структурі YBCO-STO-LCMO, навпаки, Tonset підвищується, а jc збільшується для структури з STO завтовшки 2 нм та знижується для 7 нм. Висловлено припущення про зміну механізма пінінговання вихрів при зниженні температури.The complex differential susceptibility a pure YBCO film and YBCO–STO–LCMO structures of different STO width (0; 2; 7 nm) is studied. It is shown, that a decrease of superconducting transition temperature Tonset and critical current density jc is observed, as expected for the structure YBCO–LCMO in comparison with the pure YBCO film. Conversely, the YBCO–STO–LCMO structures display an increase in Tonset; as for jc, it increases in the structure with 2 nm STO and decreases in that with 7 nm STO. It is suggested the pinning mechanism of vortices undergoes a change with decreasing temperatur