Effect of Artificial Defects on Electric and Magnetic Transport Properties in YBa2Cu3O7−δ Thick Film (original) (raw)
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Physica C-superconductivity and Its Applications, 2002
We report measurements at zero applied magnetic field of the normal state resistivity against temperature, qðT Þ, and of the characteristic curves (E-J) in YBa 2 Cu 3 O 7Àd thin films. The E-J curves are measured at different temperatures, between 77 K and the superconducting transition temperature, and up to current densities one order of magnitude larger than the critical current density, J c , with the samples submerged in a liquid nitrogen bath, controlled in temperature by its vapor pressure. These E-J curves allow us to observe the abrupt electric-field jump from the superconducting state to an ohmic highly dissipative state appearing at a current density J Ã ) J c . Our results confirm the absence of any scaling between q and J c for thin films having different structural quality. In contrast, we have found that J Ã scales with the slope of the E-J curves, q n ð¼ E=J Þ, for J > J Ã , which is very close to the extrapolation of the normal-state resistivity to the range of temperatures where the E-J curves are measured.
Dynamic investigation of the transport current in YBa2Cu3O7−δ thin films
The current density evolution in YBa 2Cu 3O 7−δ thin films is studied by time-resolved magneto-optical imaging as a function of the phase of an ac current applied simultaneously with a perpendicular dc magnetic field. We present a new empirical method to separate the total current distribution into a circulating current, which screens the applied field, and the applied transport current. The latter shows an asymmetric profile with pronounced peaks at the edges of the sample and its phase-dependent self-field is contained in the flux region bound by the circulating current. Threading dislocations provide the necessary pinning sites for the observed high local values of the transport current.
Journal of Applied Physics, 1998
Spatially-resolved studies of a YBa2Cu3O 7−δ thin film bridge using electron probe microanalysis (EPMA), low-temperature scanning electron microscopy (LTSEM), and magneto-optical flux visualization (MO) have been carried out. Variations in chemical composition along the bridge were measured by EPMA with 3 µm resolution. Using LTSEM the spatial distributions of the critical temperature, Tc, and of the local transition width, ∆Tc, were determined with 5 µm resolution. Distributions of magnetic flux over the bridge in an applied magnetic field have been measured at 15 and 50 K by magneto-optical technique. The critical current density jc as a function of coordinate along the bridge was extracted from the measured distributions by a new specially developed method. Significant correlations between jc, Tc, ∆Tc and cation composition have been revealed. It is shown that in low magnetic fields deviation from the stoichiometric composition leads to a decrease in both Tc and jc. The profile of jc follows the Tc-profile on large length scales and has an additional fine structure on short scales. The profile of jc along the bridge normalized to its value at any point is almost independent of temperature.
Critical currents in vicinal YBa2Cu3O7−δ films
Physical Review B, 2004
Most measurements of critical current densities in YBa2Cu3O 7−δ thin films to date have been performed on films where the c-axis is grown normal to the film surface. With such films, the analysis of the dependence of jc on the magnetic field angle is complex. The effects of extrinsic contributions to the angular field dependence of jc, such as the measurement geometry and disposition of pinning centres, are convoluted with those intrinsically due to the anisotropy of the material. As a consequence of this, it is difficult to distinguish between proposed FLL structure models on the basis of angular critical current density measurements on c-axis films. Films grown on mis-cut (vicinal) substrates have a reduced measurement symmetry and thus provide a greater insight into the critical current anisotropy. In this paper previous descriptions of the magnetic field angle dependence of jc in YBa2Cu3O 7−δ are reviewed. Measurements on YBa2Cu3O 7−δ thin films grown on a range of vicinal substrates are presented and the results interpreted in terms of the structure and dimensionality of the FLL in YBa2Cu3O 7−δ . There is strong evidence for a transition in the structure of the flux line lattice depending on magnetic field magnitude, orientation and temperature. As a consequence, a simple scaling law can not, by itself, describe the observed critical current anisotropy in YBa2Cu3O 7−δ . The experimentally obtained jc(θ) behaviour of YBCO is successfully described in terms of a kinked vortex structure for fields applied near parallel to the a-b planes.
Low Temperature Physics, 2010
The temperature dependence of the critical density of superconducting current j c ͑͒ ͓ = ͑1−T / T c ͒, and T c is the critical temperature͔ of two epitaxial c-oriented YBa 2 Cu 3 O 7−␦ thin films grown on LaAlO 3 and SrTiO 3 substrates has been investigated by the method of dynamic magnetic susceptibility. In both samples, the j c ͑͒ dependence has two sections with power dependences j c ϳ 3/2 and j c ϳ 2 in the temperature region from 77 K to T c , with crossover between them. Measurements were made of the temperature dependences of the dynamic relaxation rate Q͑͒ ͑magnetic flux creep͒, which turned out to be unconnected with the observed crossovers. It is shown that the observed features of j c ͑͒ are not caused, as was assumed earlier, by the limitation of the critical depairing current density in the interdislocation gaps of the interblock boundaries of YBa 2 Cu 3 O 7−␦ epitaxial thin films. Based on numerical calculations, it is shown that the existence of crossovers and the breakdown of H / scaling detected in one of the samples ͑H is the dc magnetic field͒ in the j c ͑ , H͒ magnetic-field dependences close to T c can be explained by the manifestation of macroscopic inhomogeneity of the samples in the local values of the critical temperature T c ͑T c inhomogeneity͒. The absence of observable breakdown of the H / scaling in the second sample is explained by the smaller width of the distribution over T c in it by comparison with the first sample.
Effects of divalent impurities on the fluctuation conductivity of YBa 2Cu 3O 7 single crystals
Physica B-condensed Matter - PHYSICA B, 2009
We have studied experimentally the in-plane fluctuation conductivity near the superconducting transition in single crystal samples of YBa2Cu3O7, Y0.98Ca0.02Ba2Cu3O7, YBa1.9Sr0.1Cu3O7 and YBa2Cu2.97Zn0.03O7. In order to test the stability of the observed fluctuation regimes, low magnetic fields were applied perpendicular to the Cu–O2 atomic planes. When the transition is approached from above we first observe a three-dimensional (3D) Gaussian regime then a crossover to a genuine critical region where the exponent is consistent with the predictions of the 3D-XY-E universality class. Decreasing further the temperature towards Tc, our results systematically reveal the occurrence of a regime beyond 3D-XY characterized by a very small critical exponent. We propose that this regime is precursory to a weak first-order superconducting transition driven by antiferromagnetic excitations related to the pseudogap phenomenon. The dilution of divalent impurities in YBa2Cu3O7 does not affect the st...
Journal of Applied Physics, 1987
Measurements of the transport critical-current density (Jc), magnetization Jc, and magnetoresistance in a number of bulk sintered samples of Y1Ba2Cu3Ox from several different laboratories indicate that the transport Jc is limited by weak-link regions between high Jc regions. The weak-link Jc has a Josephson character, decreasing by two orders of magnitude as the magnetic field is increased from 0.1 to 10 mT at 77 K. An examination of the grain-boundary region in Y1Ba2Cu3Ox shows no observable impurities or second phases to the scale of the [001] lattice planes (∼12 Å). The effect of intrinsic conduction anisotropy is discussed. A current-transfer model is proposed in which weak conduction along the c axis plays a role in limiting Jc at grain boundaries. Orienting the grains in the powder state during processing may result in enhanced transport Jc in bulk conductors.
Defects in YBa2Cu3O7−δ thin films studied by magneto-optics
Physica C-superconductivity and Its Applications, 1996
By means of the magneto-optical Faraday effect, flux-density distributions are observed in laser-ablated YBa2Cu30 7_ 8 thin films. The samples investigated are found to contain numerous defects of two different classes; (1) extended linear defects of weakly superconducting material and (2) small non-superconducting particles. The influence of these defects on the flux distributions is analyzed in detail using the concept of discontinuity lines of the currents [Th. Schuster et al. Phys. Rev. B 49 (1994) 3443]. The magneto-optical flux patterns clearly reveal that an application of a critical-state model to calculate the critical current density from magnetization data is impossible when a sample contains such defects. It is shown that even in the best samples the flux fronts are not homogeneous but consist of a large number of defect-induced parabolic flux plumes.
Applied Physics Letters, 1991
We have measured the transport critical current density (Jc) in epitaxial quality films of YBa2Cu3O7−δ some of which were covered by thin (10 nm) Ag films. The films, both with and without Ag, had Jc values greater than 106 A/cm2 in liquid nitrogen. The effect of the Ag was to greatly reduce the dependence of Jc on external magnetic fields in the case where the field was oriented in the plane of the film, that is, perpendicular to the c axis. It is unlikely that the effect is simply due to altered surface pinning, although qualitative agreement with critical state models is observed.
Thin Solid Films, 2010
We have fabricated and studied quasi-multilayered thick YBa 2 Cu 3 O 7 − δ (YBCO) films composed of several YBCO layers interspaced with quasi-layers of non-superconducting YBa 2 Cu 3 O x nanodots, grown by Pulsed Laser Deposition on SrTiO 3 (100) substrates. Magnetization J c (B) at 77.3 K for these thick films showed significant improvement as compared to pure YBa 2 Cu 3 O 7 − δ films of same or even smaller thickness. A high J c (B) in our quite thick films (1 μm to 6 μm) provides a very high total critical current per centimetre of the film width, I c − w . Critical current as high as 830 A per cm width in self field and 77.3 K was achieved in 5 μm thick quasi-multilayer film with non-superconducting YBa 2 Cu 3 O x nanodots. Frequency-dependent susceptibility measurements showed also an increase in the pinning potential. The angular dependence of I c − w at 86.5 K, in 3 T shows a clear indication of anisotropic pinning centres aligned along the c-direction.