Tc Oxide Research Articles

ChemInform Abstract: A Study of High Tc Oxide Superconductors by NQR and NMR Measurements

ChemInform Abstract: A Study of High Tc Oxide Superconductors by NQR and NMR Measurements

High Tc Oxide Superconductors

This issue of the MRS BULLETIN is devoted to high Tc superconductivity. It is the sequel to a previous series of articles on the same subject which appeared in the MRS BULLETIN in January 1989. While the articles in the January 1989 issue emphasized the families of high Tc superconducting oxides known at that rime, as well as novel processing techniques and thin films, the papers in this issue focus on the physical properties of high Tc oxide superconductors.The quality of polycrystalline and single-crystal bulk and thin-film materials has improved to the point where researchers can now make reliable measurements of many physical properties representative of the intrinsic behavior of these materials. As a result, a broad spectrum of important issues such as the nature of the electronic structure, the type of superconducting electron pairing, the magnitude and temperature dependence of the superconducting energy gap, the behavior of fluxoids in the vortex state, etc., can be addressed meaningfully. Presently emerging is a consistent picture of the physical properties of the high Tc oxides, which will form the foundation to eventually developing an appropriate theory for the normal and superconducting states of these remarkable materials.

Increase in the flux-pinning energy of YBa2Cu3O7−δ by shock compaction

Magnetic flux relaxation data on a composite specimen of YBa2Cu3O7−δ +Ag shock compacted at 167 kbar show that the flux-pinning energy at 70 K and 10 kOe is enhanced by a factor of 2–3. This enhancement persists after annealing at 890 °C in oxygen for 24 h. The increased pinning energy is apparently caused by shock-induced 〈100〉 and 〈110〉 line defects having a density up to 1012 cm−2. Annealing replaces line defects with (001) extrinsic stacking faults having displacement vectors of 1/6[031]. The average separation of the stacking faults in the [001] direction is 0.04 μm, which is comparable to the separation between fluxoids at 10 kOe. These results indicate that intragranular critical current densities can be increased significantly in bulk materials by shock processing. High densities of flux-pinning sites could be induced efficiently in industrial-scale quantities of bulk high Tc oxides by explosives.

Critical currents and flux creep in melt processed high Tc oxide superconductors

YBa 2Cu 3O 7 crystals fabricated by a quench and melt growth process contain fine Y 2BaCuO 5 particles. These fine precipitates are considered to have three beneficial effects: they suppress crack formation; they promote oxygen diffusion; and they act as pinning centres. Such crystals exhibit larger flux pinning than single crystals and their magnetic behaviour can be understood in terms of the critical state model which has been developed for conventional type II superconductors. Both transport and magnetization measurements gave J c values exceeding 30 000 A cm −2 at 77 K and 1 T. Flux creep rate is also much smaller than that of single crystals and a value of pinning energy, U, comparable to that of conventional superconductors was obtained. A theoretical estimation of the contribution of 211 precipitates to flux pinning is also presented.

Fabrication of thin-film-type Josephson junctions using a Bi-Sr-Ca-Cu-O /Bi-Sr-Cu-O/Bi-Sr-Ca-Cu-O structure

Thin-film Josephson junctions with normal metal barriers using a Bi-based high Tc oxide superconductor were successfully fabricated. Bi2Sr2Ca1Cu2Ox films were used for both superconducting electrodes and Bi2Sr2Cu1Oy for the barrier layer. The junction area of 20×40 μm2 was defined by photolithography and Ar ion milling. These S/N/S-type junctions clearly exhibited the ac Josephson effect under the irradiation of radio frequency waves of 12 GHz and more than 20 Shapiro steps were observed.

An overview of the transport properties of high- Tc oxides

The present experimental situation concerning the transport properties of high T c oxides is reviewed, highlighting the controversial issues in the context of the debate over the nature of the electronic state in this class of materials.

Effects of flux creep on a superconducting solenoid with high Tc oxide superconductors

The normalized decay rate of a persistent current due to thermally activated flux creep is discussed theoretically for the case of a long superconducting solenoid wound with wires of superconducting tape. It is shown that the normalized decay rate of the transport current in the solenoid for the persistent current mode is smaller than the normalized decay rate for magnetization of the superconducting tape. Furthermore, these values are related by the ratio of the superconducting film thickness in the tape to the bore radius of the solenoid, as far as the initial creep process is concerned. The duration of the initial creep process, during which the total flux inside the solenoid is conserved, depends exponentially on the effective pinning potential, U c. Even at 77 K, therefore, the period can be increased to several years or more by a slight increase in the value of U c in high T c superconducting wires.

Pulsed current measurement of the resistive transition and critical current in high T c superconductors

A pulsed current technique for measuring the critical transport properties of superconductors as a continuous function of current or temperature is described. Greatly reduced sample heating permits investigation of both the ‘‘weak link’’ and intrinsic properties of granular superconductors. Results obtained for various high Tc oxide superconductors provide an example of the technique’s application. Both intergranular and intragranular critical current densities are extracted from the data. Current-voltage measurements using the pulse method are corroborated by traditional dc measurements.

Spin Correlation and Superconductivity in High-TcOxides

63Cu NQR and NMR and 170 NMR studies are presented for La.-rSrrCuO., YBa.Cu307 and TI.Ba.Cu06+Y· In the superconducting state, the T-dependence of the Knight shift has shown that the superconducting energy gap is larger in the Cu02 (2LI/ka Tc=4.4 -4.9) than the BCS prediction, being consistent with an s-wave model. In contrast, the T, result below Tc cannot be interpreted by a simple s-wave model. In the normal state, T,-' of Cu is strongly enhanced by antiferromagnetic (AF) spin correlation and its T-dependence is related to the T-dependence of the staggered suscep· tibility x Q( T) at the zone boundary. The large enhancement of x Q is remarkable for the superconducting compound, which is considered to play a key-role for the anomalous normal and superconducting properties in high- Tc oxides. The correlation between Tc and AF spin correlation is discussed.

酸化物高温超伝導薄膜の形成法について

酸化物高温超伝導薄膜の形成法について

Is there a flux lattice in high temperature superconductors?

Abstract In type II superconductors, like the new high Tc oxide materials, if you cool acrystal below the superconducting transition in an applied magnetic field, B>Bc1, the system minimizes its free energy by distributing the flux, in units of single flux quanta, on a lattice. This should be a perfect experiment for the loveable neutron with its ability to detect variations of magnetic field in a sample. There exist several studies of these effects in conventional superconductors but, as usual for high Tc systems, weird and wonderful things are expected. The theoreticians predict: no flux lattice only a glassy state, a lattice at low T which will melt below Tc, entangled-disentangled configurations in the liquid state, a vortex-plasma of flux-lines (whatever that may be) and so on. Its the usual story, we need some experiments to separate fact from highly entertaining theoretical models. However, the experiments are not that easy…

Determination of Pb4+ and Pb2+ in high Tc oxide superconductors

Dosage, apres dissolution des echantillons, du Pb 4+ par spectrometrie d'emission-ICP ou par titrage avec EDTA. Dosage du plomb total egalement par spectrometrie d'emission-ICP et calcul de la concentration en Pb 2+ par difference

高Ｔｃ酸化物の延性

高Ｔｃ酸化物の延性

FLUX CREEP IN HIGH Tc YBa2Cu3Oy BULK SUPERCONDUCTOR

Flux creep in high Tc oxide YBa2Cu3Oy superconductor in critical state has been studied by observing variation of magnetization with time. Experimental results show that for the zero field cooling sample, the magnetization decays with time following the law M∝lnt over the observation time period, and this process accompanies by the occurrence of flux jumps. The rate of the magnetization decay dM/dlnt, as a function of the field, exhibits a maximum near H=3HP. It is believed to be related to weak link superconductivity. dM/dlnt decreases monoto-neously with the rising temperature in the temperature range above 77K. Finally, a discussion is made about the activation energy of the flux lines U0.

特集・酸化物高温超伝導体―現状と展望 層状構造酸化物超伝導体の固体化学

特集・酸化物高温超伝導体―現状と展望 層状構造酸化物超伝導体の固体化学

Anisotropy in transport critical current density of a Bi-Sr-Ca-Cu-oxide single crystal

Critical current densities and upper critical fields were measured for a single crystal of the high Tc oxide superconductor, Bi2Sr2CaCu2O8+d, by transport 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 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.

合成する立場よりみた高温酸化物超伝導体

合成する立場よりみた高温酸化物超伝導体

A comment upon the use of two criteria in the identification of intrinsic esr in high Tc oxides

An ESR signal detected in a Bi compound has recently been attributed to the SC phase, arguing that two criteria (value and thermal behaviour of the intensity) were met. We draw attention to the weakness of this argument, recalling a canonical example Un signal RPE observe dans un compose au bismuth a ete recemment attribue a la phase SC, en s'appuyant sur le fait que deux criteres (valeur et comportement thermique de l'intensite) sont verifies. Nous attirons l'attention sur la faiblesse de cette argumentation, et rappelons un exemple canonique

超電導体技術開発の動向

The application of superconductivity is a key to produce a number of breakthroughs in a vast area of science and technology covering new energy systems, computer electronics, medical diagnosis, high-speed transportations, high-energy particle accelerators, etc. The recent discovery of high Tc oxide superconductors may provide a more positive prospect for the future development of the superconductivity technology. In the area of metallic superconductors, the development of ultrafine filamentary wires for AC use, which may bring about a large impact in electrical engineering, is being carried out. The high-field performance of Nb3Sn wires has been significantly improved by the Ti-doping, and multifilamentary (Nb, Ti)3Sn wires capable of generating fields up to 17 T have become commercially available. The development of Nb3Al and other superconductors with high-field performance superior to that of (Nb, Ti)3Sn is also being proceeded using new fabrication technologies.The application of techniques elaborated the fabrication of metallic superconductors, e.g. powder process using metal sheath, solidification process, diffusion process and high-rate deposition process has been attempted for the processing of high-Tc oxide superconductors. Ag-sheathed BPSCCO tapes show a Jc (77K, OT) of 3×104/cm2, and keep nearly the same Jc at 25T and 4.2K. The solidification process has achieved an encouraging result for obtaining enough Jc in bulk high-Tc oxides. Superconducting tubes coated with a thick YBCO layer, promising for cavities and magnetic shieldings, have been fabricated by a plasma spraying process. Meanwhile, large Jc's at 77K and high magnetic fields have been obtained in thin oxide films prepared by different physical and chemical deposition processes. Various problems to be solved, e.g. homogenization, densification, crystal orientation and introduction of pinning centers, for improving Jc in bulk high-Tc oxides are also described in this article.

Cu spin dynamics in high Tc and relaxed oxides investigated by nuclear spin-lattice relaxation.

Abstract Temperature and concentration dependences of Cu spin dynamics have been investigated for high T c and related metallic oxides by the measurements of 63 Cu nuclear spin-lattice relaxation rates. The results suggest the gap opening in the Cu d-spin excitation spectrum at around 120 K, follwed by superconducting transition.