Low temperature heat capacity of YBa2Cu3O7

We have measured the heat capacity of superconducting, single phase YBa₂Cu₃O₇ in the temperature range 2 to 18 K. An extrapolation of the data between 4 and 9 K gives aC/T (T → 0) of ∼ 25 mJ/mole K². The Debye temperature obtained from the high temperature linear portion ofC/T vsT ² plot is 325 K.     

Microstructural investigations of melt grown YBa2Cu3O7

The results of microstructural investigations on bulk YBa₂Cu₃O₇ prepared by melt growth process are reported.     

Geometry effect on the nonlinear magnetic response of YBa2Cu3O7−y

Results are presented for study of nonlinear magnetization of a sintered YBa₂Cu₃O_7−y pellet of different thickness of a sample subjected to very low magnetic field. On cooling the sample belowT _c in zero field a change, in the oscillatory structure of harmonics in increasing dc field is observed in very low ac magnetic field. The effect of finiteness of the sample on the oscillatory structure and on the hysteresis of harmonics is also studied. The results are explained qualitatively.     

YBa2Cu3O7−y composites for improved fabricability

Composites of YBa₂Cu₃O_7−y with silver powder have been investigated for their fabricability and superconductivity. Silver does not react with the oxide powder. The composites can be easily rolled into thin strips. There is no appreciable degradation inT _c either during sintering or rolling.     

Magnetic field dependence of critical current density in YBa2Cu3O7

We have measured the transport critical current densityJ _cof sintered YBa₂Cu₃O₇, in various applied fields up to 185 Oe at 77 K. We find a sharp decay ofJ _cwith magnetic field. We show that this sharp decay is consistent with the low field hysteresis results of Groveret al. We argue that the observed field dependence is not caused by intragranular weak links.     

Enhancement of thermopower in the highT c superconductor YBa2Cu3O7 and related compounds

The absolute thermopower of single phase YBa₂Cu₃O₇ and Y_0.8Er_0.2Ba₂Cu₃O₇ has been measured in the range 250 K to the superconducting transition temperature. It is found that these compounds show a large enhancement of thermopower in the range 150 K down toT _c. This enhancement shows a steep exponential drop as the temperature increases from the transition temperature. The temperature variation of the enhancement is too steep to be accounted for by electron-phonon or electron-local structural excitation mechanisms.     

Structure and oxygen content in the high-temperature superconductor YBa2Cu3O7−δ

The effects of oxygen deficiency on the processes of ordering the oxygen vacancies and of the formation of superstructures are discussed in the layered perovskite structure of YBa₂Cu₃O_7−δ. Samples subjected to different heat and gas treatments are studied with X-ray diffraction techniques to get this information.     

Magnetization studies in niobium and YBa2Cu3O7 samples

The results of experimental studies on hysteresis in magnetization, thermomagnetic history effects, anomalous variations in magnetic hysteresis curves and the decay rates of magnetization obtained under different thermomagnetic histories in specimens of conventional and high temperature superconductors are presented. The Bean’s critical state model is considered adequate to explain magnetic behaviour in conventional hard superconductors. The similarity in the general features of the results of different experiments on specimens of the two families of superconductors underscores the efficacy of the said model to understand some aspects of the macroscopic magnetic response of high temperature superconductors as well. For instance, the isothermal magnetization hysteresis loop which comprises of magnetization curves along forward (−H _max to +H _max) and reverse (+H _max to −H _max) paths define an envelop within which all isothermal magnetization data along different thermomagnetic histories lie. There exist inequality relationship between various field values identified asH _peak,H _I,H _II etc. in isothermal magnetization hysteresis as well as magnetic relaxation data. The entire field span of an isothermal magnetization hysteresis data set can be considered to comprise of three parts corresponding to (M _rem(H)−M _FC(H)+M _ZFC(H)) being equal to, less than or greater than zero, whereM _rem(H) are the remanent magnetization values obtained on reducing field to zero after having the specimen in different applied field (H) values. There are, however some situations amongst thermomagnetic history effects in specimens which show incomplete flux trapping on field cooling, where the critical state model has been found inadequate.     

Anomalous change in the photoacoustic phase near superconducting transition in YBa2Cu3O7

Photoacoustic amplitude and phase measurements have been carried out near the superconducting transition temperature in the highT _csuperconductor YBa₂Cu₃O₇. In both the samples investigated, the photoacoustic phase undergoes an anomalous step-like decrease aroundT _c≈ 96 K and increases at lower temperatures. A clear temperature hysteresis for the phase change has been noticed. No anomalous variation has been observed in the amplitude values. Neither phase nor amplitude shows any anomaly in a non-superconducting sample of the same composition.     

Stages of thermally induced gas-exchange in the high-temperature superconductor YBa2Cu3O7−δ

The oxygen stoichiometry in the YBa₂Cu₃O_7−δ high temperature superconductors is studied, using DSC/DTA techniques on samples subjected to different heat and gas treatments. Characteristic temperature regions of high temperature superconductivity exchange with oxygen in gas phase are identified and the resulting structural changes are discussed.     

Theory of high temperature superconductivity in YBa2Cu3O7_δ

A theory of high temperature superconductivity in YBa₂Cu₃O₇__δ compound has been developed on the basis of the momentum pairing of electrons through the relativistic Darwin interaction. The transport behaviour of electrons is explained in terms of a mechanism of correlated electron transfer arising from the electron-phonon coupling. A model Hamiltonian has been developed to describe the superconducting properties of the system. This gives an energy gap which is higher than the BCS value. Attempts have been made to explain the absence of isotope effect, the linear dependence of specific heat, the presence of larger temperature-independent paramagnetism in the normal phase and the softening of some of the optic phonon modes observed in this system.     

Measurement and understanding of magnetization in AC and DC fields and the determination of intragrainH c1 in highT c RBa2Cu3O7 superconductors

We present DC and low frequency AC magnetization measurements on various RBa₂Cu₃O₇ superconductors. We identify features intrinsic to these compounds, and establish the features originating from intergranular links in sintered pellets. The isothermal magnetization curves, and the temperature dependence of magnetization in field-cooled and zero field-cooled states are shown to be consistent with the calculations done following a recent extension of Bean’s model. Low field anomalies predicted within this model are observed, and yieldH _c1 values of a few Oe. These values are shown to be consistent with the temperature variation of magnetization. A comparison is made with the other existing data and it is demonstrated that earlier quoted values ofH _c1 are gross overestimates.     

The scaling of the current voltage characteristics of the YBa2Cu3O7:Y2BaCuO5 composites

We have investigated features in the current voltage (I—V) characteristics of YBa₂Cu₃O₇:Y₂BaCuO₅ composite sinters which indicate the occurrence of a second-order phase transition. For each sinter composition similar families of characteristics are found for a wide range of magnetic fields. The relation between V and I is seen to be of the form V = f(I, ζ(B, T)) where a single parameter ξ contains all of the field and temperature dependence. The locus of points in the B, T plane for which ζ(B, T) is a constant have been examined, and a low field and a high field régime have been identified. Samples of different composition show families of I—V characteristics which scale onto one another in a simple way, hence we are led to suppose the V = f(I, ζ(B, T, p)) where p is the dilution fraction. Data taken at temperatures below the phase transition agrees well with a simple scaling picture of dissipation close to a second order phase transition, with exponents which are independent of magnetic field and composition. Above the transition, however, the behaviour is more complex.     

YBa_2Cu_3O_7基面电子态特性计算

用散射理论描述了 YBa2 Cu3O7( 0 0 1)面的电子能带结构。用近似到第三近邻的紧束缚模型得到了 YBCO的体电子态密度 ,Cu O及 Cu O2 的表面态密度和表面投影能带 ,进而分析了 Cu O和 Cu O2 的表面态密度的特点及与体态密度的差别。     

Bilayer exchange coupling and neel temperature of YBa2Cu3O6.2

The present paper attempts to study the Neel temperature of bilayer antiferromagnetic cuprate YBa₂Cu₃O_6.2 within anisotropic Heisenberg model. The double time Green’s function formalism within random phase approximation (RPA) has been used to obtain various correlation functions. The magnetization and the Neel temperature (T _N) are evaluated. It is observed that the ratio of intrabilayer to inplane exchange coupling (r=J⊥/J‖) plays an important role in the magnetic dynamics of bilayer systems. The recent experimental data of bilayer system YBa₂Cu₃O_6.2 have been used to estimate the ratio r from the expression for Neel temperature. The estimated values of spin gap and the ratio of hopping matrix elements t⊥/t‖ are found to be in fairly good agreement with the existing experimental results.     

Study of single phase YBa2Cu3O7−δ compound around 1–2–3 stoichiometry

The structural changes accompanying small stoichiometric variations around 1–2–3 composition in the Y-Ba-Cu-O compound have been investigated. Ba-deficient composition is found to give considerable orientation along (001). This oriented powder gives a higher current density compared to the unoriented powder. The temperature has been correlated with stoichiometry.     

Critical current measurements on extruded YBa2Cu3O y wires

The successful extrusion of short lengths of brittle wires of YBa₂Cu₃O _y that are superconducting above 77 K is reported. Detailed critical current measurements are presented on wires having undergone different physical treatments. The highest critical current obtained from V-I data is ⋍ 90 A/cm², but this particular wire shows clear evidence of flux-pinning forces corresponding to ⋍ 130 A/cm².     

Critical current densities of high pressure oxygen sputtered thin films of YBa2Cu3O7−x by non-resonant rf absorption method

The critical current densities (J _c) have been measured at 77K in high pressure oxygen sputtered thin films of YBa₂Cu₃O_7−x superconductor using the non-resonant rf absorption technique. High values ofJ _c (∼ 10⁵ A/cm²) are observed in these relatively large area (∼ 1·2 cm²) films.     

The transition width and critical current density measurements on slow-cooled YBa2Cu3O7−x superconductor

It is shown that by adopting a very gradual programmed cooling procedure in oxygen environment one can obtain a sharp transition (ΔT _c⋍1 K) in YBa₂Cu₃O_7−x while retaining the high-T _c value (⋍105 K) of samples prepared by a semi-wet route. This is attributed to a maximum occupancy of oxygen at 0 1/2 0 sites and a near-perfect ordering of vacancies at 1/2 0 0 sites in the orthorhombic unit cell which maximizes the availability of conduction paths in the form of continuous CuO₄ chains. Critical current densities (J _c) of 204 A cm⁻² are obtained for bulk samples at 77 K. It is suggested that the intergrain coupling is weak and thus limits the transportJ _c-values.     

Magnetic measurements and microstructure studies of highly oriented Bi2Sr2Ca2Cu3O8 bulks

A highly oriented structure for Bi₂Sr₂Ca₂Cu₃O₈ bulk superconductors was successfully made by hot-forging. The J_c values were closely related to the degree of orientation. Microstructure observations and magnetization measurements revealed that the properties of the highly oriented BSCCO reflected the two-dimensionality in this layered structure material.