The resistive transition of a preferentially oriented thin film of Y2Ba4Cu8O16‐δ has been measured in fields up to 20 T oriented parallel to the c‐axis. The data can be analysed in terms of thermally activated flux flow and flux pinning due to dislocation lines. The analysis yields the upper critical field Bc2 which shows an unusual upward curvature at low temperatures. Furthermore an estimate can be made of the correlation length Lc of the flux line lattice in the field direction. Lc is found to be very small, however, remaining larger than the Cu02 plane distance.

The electrical transport properties of a series of Y1Ba2Cu307.x thin films deposited onto single crystal substrates by thermal coevaporation and laser ablation were investigated. For fully epitaxial, c‐oriented films on (001) substrate surfaces, the critical current density Jc showed large high‐field enhancements for the film aligned with H parallel to the copper‐oxygen planes. With H||c, a precipitous decay of Jc with increasing field is described by thermally activated flux motion. Films on (110) SrTiO3 substrates were comprised of finely‐divided arrays of grains having [110], and <103>‐type orientations, and exhibited granular behavior. For these “triaxial” films, the transport dissipation was also thermally activated but with much smaller energy barriers.

We have performed transport critical current density, Jc, measurements on epitaxial superconducting thin films which were in‐situ patterned during the laser deposition process. Shadow masks of various dimensions were placed close to the substrate to generate different patterns. Epitaxial films of YBa2Cu3O7 on (100) SrTiO3, (100) YSZ, and (100) LaA1O3 substrates were fabricated at low processing temperatures (500‐650°Q by the biased laser deposition technique in an oxygen ambient of 200 mtorr. Excellent quality superconducting thin films were formed on patterned areas. The critical temperature of the films was found to be in the range of 88 to 90 K, and the best critical current density values (at 77K, and zero magnetic field) greater than 6.5 x106 Amps/cm2 were obtained for silver doped YBa2Cu3O7 films on (100)LaAlO3 substrates.

The variations of critical current density Jc of narrow Y‐Ba‐Cu‐O films as a function of temperature and magnetic field are investigated. Films with thickness of 150‐200 nm and width of 20‐100 urn were prepared by laser ablation and laser etching. For t (=T/TC) < 0.75, the temperature dependence of Jc shows a (1‐1) linear behavior, in sharp contrast to that of a Josephson junction. Near Tc, Jc varies with (1‐ t)n where n=3/2 or 2 depending on the morphology of films. The magnetic field dependence of Jc above a characteristic field value Hc’ can be described by a model based on flux creep.

We have reproducibly prepared thin films of Bi‐Sr‐Ca‐Cu‐O with Tc ≥ 105K. Depositions were done at ambient temperature with a subsequent post‐deposition anneal, and did not include lead substitution. X‐ray diffraction data indicates a majority fraction of the 2223 phase. These films possess very large grains of the order of 20‐30 u.m in size. Post‐deposition annealing conditions are a sensitive function of composition. Detailed transport measurements as a function of temperature and magnetic field have been obtained.

Thin films of YBa2Cu3O7-x have been produced in situ by laser ablation. As the condensation of the superconducting film is done onto hot substrates (600 - 780°C) at high oxygen pressures (~ 1 mbar), this techniques does not require any special postannealing step. Thus the total process time amounts to less than 10 min. Films were found to be polycrystalline with the c-axis preferentially oriented normal to the substrate plane on single crystalline (001) SrTiO3 and (random) ZrO2, as measured by ion channeling (Xmin = 5.5 % on SrTiO3), x-ray scattering and TEM. Complete superconducting transitions above 90 K with transition widths of about 1 K have been observed even on YSZ substrates. Critical current densities, jc (77 K), of more than 2 x 106 A/cm2 could be obtained on SrTiO3-substrates. The high quality of the thin films over large areas was demonstrated by the measurement of persistent superconducting current densities at 77 K in the order of 105 A/cm2 in a 1.5 mm broad ring of 9 mm diameter. No dissipation of this persistent current could be observed during one hour. Measurements of the same film after 8 months in normal air showed a 40 % decrease of jc and a stronger decrease of jc with an externed magnetic field, suggesting that in the meantime the macroscopic ring current density is dominated by intergranular weak links.

The magnetic hysteresis was studied for YBa2Cu3O7 films of different epitaxial orientations on single-crystal SrTiO3 and KTaO3 substrates. Deposition by coevaporation of Y, BaF2, and Cu was followed by annealing in flowing, wet oxygen at 800-850°C. The critical current density Jc deduced from magnetic hysteresis loops was compared with four-terminal transport-current results. Thermally activated magnetic flux flow was manifest in the audio-frequency data. Jc fits a scaling law similar to one used for describing Nb3Sn and other A15 materials.

The diamagnetic properties of NdCeCuO films with electron carriers were studied. The,critical current density ( Jc ) was calculated to be 0.73 x 106 A/cm2 . These films indicated the very strong magnetic relaxation with small activation energy, the significant resistive broadening in the critical region and the largely fluctuated diamagnetization. These results suggested the very weak pinning forces to cause the easily activated flux motion in these specimens.

The critical current density, Jc, of single crystals of Ba2YCu3O7 is comparable to that of melt-textured-growth materials as well as to the intragrain Jc of bulk polycrystalline Ba2YCu3O7. Typical values at 77 K and 9 kOe are near 5x 103 A/cm2, and are presumably limited by weak pinning. We have obtained a significant enhancement of this pinning-limited Jc by three techniques: 1) a short reannealing procedure, 2) fast neutron irradiation, and 3) proton irradiation. Interestingly, the combination of the first technique followed by either irradiation is not as effective as the iradiation alone. That is, the best candidates for irradiation are not necessarily those with the highest initial Jc.

We have used magnetization measurements to study the effect of proton irradiation on the critical current density of a YBaCuO single crystal. We have obtained a maximum value of 1.5x105 A/cm2 at 77 K and 1 T at a fluence of 2 x 1016 cm-2, representing a 50-fold increase with respect to the unirradiated sample.

Significantly improved flux-pinning has been achieved in bulk YBa2Cu3O7-δ superconductor (‘123’ compound) containing fine-scale defects (<~50 A thick). The measured Jc(magn.) of ~l05 A/cm2 at 77K, H = 0.9T is about ten times higher than the typical values for bulk Y-Ba-Cu-O. The improved structure was produced by rapid decomposition at 920°C of the YBa2Cu4O8 (‘124’) precursor.

Critical current densities and upper critical fields were measured for a single crystal of the high Tc oxide superconductor, Bi2Sr2CaCu2O8+d, within the ab basal plane and along the c axis. The anisotropy in critical current densities was observed to be Jc⊥/Jc//=10 in agreement with the anisotropy in resistivity for the normal state. The magnetic field dependence of the critical current densities can be interpreted by the anisotropy in the upper critical fields. The scaling of the critical current density with the magnetic field was found. The critical current density scaled to zero at fields Bc2 in the ab plane and along the c axis which were in good agreement with the upper critical fields measured by transport.

The anisotropy in flux pinning force density along the b axis, Fp//b, and along the a axis, Fp//a, was found to be Fp//b/Fp//a=3. The data provided strong evidence for flux pinning by the modulated structure in this system.

The results of measurements of critical currents and magnetic relaxation on a Bi2 Sr2 Ca1 Cu2O8+x single crystal are reported. Activation energies for the flux motion have been determined from the temperature dependence of the critical current density and the time decay of the zero‐field‐cooled and the remanent magnetization. The effective activation energy increases with temperature, in agreement with the existence of a distribution of activation energies.

The kinetics of magnetization in two samples of YBa2Cu3O7 prepared by different methods, are studied. Magnetization vs. time data were obtained at various temperatures for a number of applied fields. The data display (after some short period of time) logarithmic behavior and (after a sufficiently long time) exponential behavior. The pinning parameter “activation volume” was determined from the experimental data for both samples.

We have measured the temperature and the magnetization of singlecrystalline YBaCuO samples in a variable magnetic field under quasi adiabatic conditions. By varying the sweep rate of the field, the thermal link to the bath, the size of the sample, the critical temperature and the sample temperature, we have measured the time dependence of the vortex relaxation (the penetration of the critical state, flux jumps, and the instability regime). This magnetothermal instability controls the value of the effective critical current.

The transport critical current density Jc has been measured in high quality poly-crystalline YBa2Cu3C7 as a function of temperature and for various sweeps of sub-Tesla applied magnetic fields. Large increases over the virgin response were observed in Jc at fields of a few hundred gauss and are attributed to trapped flux in the samples. The amount of trapped flux was measured and correlated to the change at Jc at various temperatures. The position of the magnetic irreversibility line was measured by field induced absorption shift in AC susceptibility. The various data are discussed in the context of the weak link conduction model.

We report a novel ac technique for characterizing the magnetic behavior of high-temperature superconductors. This technique is based on monitoring the spectrum of the magnetic response of these materials to small sinusoidal fields. A nonlinear magnetic behavior is indicated by the appearance of harmonic components in the spectrum. Measurements of these harmonics as a function of bias field, temperature and frequency provide valuable information about the processes of flux penetration, flux pinning and motion in these materials.

We have studied the variation of dissipative effects with doping in the high Tc superconductors La2-xSrxCuO4-y and YBa2Cu3O7-y by studying the lossy component of ac susceptibility as a function of temperature. We find relatively strong excess losses at low temperature in both systems, which indicate stronger flux creep at grain boundaries, in lower level doped ceramic samples produced by a similar process. This effect is more significant in the La2-xSrxCuO4-y (214) system than in the YBa2Cu3O7-y(123) system. We suggest that the microscopic mechanisms of weakening of the order parameter in the grain boundaries may be different in the two systems. In the 214 system, as the charge carrier density in the CuO2 planes is reduced, the spatial fluctuation of the order parameter in the CuO2 planes is enhanced. In the 123 system, the initial removal of O from the sample, which weakens the coupling between planes and increases the spatial fluctuation of the order parameter between CuO2 planes. The grain boundaries are degraded in both systems. The degradation in the 214 samples is more significant than in the 123 samples.

The transport critical current of bulk sintered YBa2Cu3Ox has been measured at 77 K in the presence of static and sinusoidally-time-dependent (10–100 Hz) magnetic fields. In all cases, the field required for a given reduction of critical current density was independent of frequency to within ~10%.

Flux pinning and weak link structure were studied on the effect of Ca doping in LnBa2Cu3O7-y system (Ln=La and Y). The magnitude and the magnetic field dependence of the critical current density were improved by Ca doping. A small amount of impurity phase of such as Ca2Cu1O3 may work as a desirable flux pinning center. Moreover, it was found that the current-temperature characteristics for Ca-doped samples showed the evidence of two kinds of superconducting phases which have different transition temperature Tc and Tc'. The experimental result agrees well with the Ambegaokar-Baratoff theory for asymmetric Josephson junctions (S-I-S') in the temperature range of T < Tc' and with the proximity junction theory (S-N-S) in the range of Tc'<T<Tc.