The decrease of the critical current of coated conductors when a perpendicular magnetic field is applied : a Josephson effect point of view

A large decrease is observed in the critical current density of YBCO coated conductors (CC) and related compounds when a strong perpendicular magnetic field is applied. While measurements are generally carried out at 77K only, here we present a magnetic technique permitting to determine the critical current per unit width of conductor (Icr/w) in a large temperature range. We report measurements carried out on various CCs that show that, in addition to the reduction in the critical temperature that can be attributed to the low value of the irreversibility field near Tc, the field application results in a large decrease of Icr/w at all temperatures. We ascribe this reduction to the Josephson behaviour of the twin boundaries included in the YBCO layer

Screening magnetic fields by a superconducting disk: a simple model

We introduce a simple approach to evaluate the magnetic field distribution around superconducting samples, based on the London equations; the elementary variable is the vector potential. This procedure has no adjustable parameters, only the sample geometry and the London length, $\lambda$, determine the solution. The calculated field reproduces quantitatively the measured induction field above MgB$_2$ disks of different diameters, at 20K and for applied fields lower than 0.4T. The model can be applied if the flux line penetration inside the sample can be neglected when calculating the induction field distribution outside the superconductor. Finally we show on a cup-shape geometry how one can design a magnetic shield satisfying a specific constraint

Screening magnetic fields by superconductors: A simple model

We introduce a simple approach to evaluate the magnetic field distribution around superconducting samples, based on the London equations; the elementary variable is the vector potential. This procedure has no adjustable parameters, only the sample geometry and the London length determine the solution. This approach was validated by comparing the induction field calculated to the one measured above MgB2 disks of different diameters, at 20K and for applied fields lower than 0.4T. The model can be applied if the flux line penetration inside the sample can be neglected when calculating the induction field distribution outside the superconductor. We conclude by showing on a cup-shape geometry how one can design a magnetic shield satisfying a specific constraint

Transport properties of ybco thin films near the critical state with no applied field

Transport measurements carried out on twinned ybco films are compared to the predictions of a previously proposed model suggesting that the vortices move along the films twin boundaries that behave as rows of Josephson weak links [P.Bernstein and J.F.Hamet, J.Appl.Phys.95 (2004) 2569]. The obtained results suggest that, except if the films are very thin, the twin boundaries consist of superimposed rows of weak links with mean height,ds, whose mean length along the TBs is an universal function of T/Tc, the reduced temperature. This conclusion yields a general expression for the critical surface current density of the films as a function of T/Tc and of the number of superimposed weak links rows, while the critical current density depends on ds. A comparison of the measurements reported by various authors shows that the nature of the substrate and the growth technique have both a strong effect on ds . The existence of superimposed weak links rows is attributed to extended defects generated by y2o3 inclusions.Comment: 33 pages, 13 figures; accepted for publication in Physica

Critical current reduction in coated conductors when in-plane fields are applied

International audienceWe report measurements of the critical current of two commercial coated conductors at low temperatures when in-plane magnetic fields up to 15 T are applied. The comparison between the two samples suggests that the presence of a magnetic substrate may have a detrimental effect, at least at low and medium fields. Nevertheless, the critical current density at 30 K and 15 T is in the range of that measured in self field at 77 K. We ascribe its reduction in presence of the field to inter-vortex forces

Ga doped ZnO thin films deposited by RF sputtering for NO2 sensing

International audienceGa doped ZnO thin films have been deposited by Radio-Frequency (RF) magnetron sputtering on fused silica substrates. The structural analysis of the n-type sensitive material showed a preferential orientation in the [00l] direction. The microstructure of the thin film indicated an increasing grain size with the increase of the thicknesses. The micro sensor platforms have been fabricated with ZnO:Ga thin film deposited using a reliable stencil mask onto interdigitated electrodes containing micro-hotplates. The as fabricated micro sensor allows to sense sub-ppm concentration (500 ppb) of nitric dioxide. This system reveals promising sensing performance with a sensitivity Rg/Ra up to 75 at low temperature (50 °C)

Measurements of the surface critical current of YBa2Cu3O7-delta thin films: Probing the nonuniformity of their superconducting critical temperature along the c-axis

International audienceThe existence of nonsuperconducting zones in YBa2Cu3O7-delta (YBCO) films below the transition temperature to the nonresistive state can limit the performances of YBCO devices. In this work, we present detailed measurements of the temperature dependence of the critical surface current density of c-axis oriented epitaxial YBCO films, that suggest that their superconducting critical temperature, T-c, is modulated along the c-axis. This nonuniformity of T-c, that could be present in all YBCO films, seems also to be supported by measurements of both the field cooled in-plane magnetic moment and the oxygen content (by non-Rutherford elastic resonance scattering) of some samples. (C) 2010 American Institute of Physics. [doi:10.1063/1.3431383

Measurements of the surface critical current of YBa2Cu3O7-delta thin films: Probing the nonuniformity of their superconducting critical temperature along the c-axis

International audienceThe existence of nonsuperconducting zones in YBa2Cu3O7-delta (YBCO) films below the transition temperature to the nonresistive state can limit the performances of YBCO devices. In this work, we present detailed measurements of the temperature dependence of the critical surface current density of c-axis oriented epitaxial YBCO films, that suggest that their superconducting critical temperature, T-c, is modulated along the c-axis. This nonuniformity of T-c, that could be present in all YBCO films, seems also to be supported by measurements of both the field cooled in-plane magnetic moment and the oxygen content (by non-Rutherford elastic resonance scattering) of some samples. (C) 2010 American Institute of Physics. [doi:10.1063/1.3431383