John Bear

Effect of density, magnetic field modulation and magnetic field penetration depth on the microwave absorption properties of YBa2Cu3O7 –δ superconductors

Magnetic field sweep hysteresis of the low-field microwave absorption (LFMA) in YBa2Cu3O7 –δ is stronger for low magnetic field modulation, low temperatures and low microwave power. The field-sweep hysteresis is related to the pinning of magnetic fluxons. Under conditions favouring substantial fluxon pinning, noise-like structures are superimposed on the LFMA signal. The LFMA signal amplitude depends on magnetic flux pinning, the relative contributions of intergranular and intragranular currents, the distribution of Josephson junctions and the variation in the magnetic field penetration depth. The hysteresis behaviour is similar for samples with different densities at low and high modulation amplitudes. However, the LFMA amplitude decreases with increasing density. The effective magnetic field penetration depth is measured by electron paramagnetic resonance line broadening of spin probes adsorbed on the sample surface. The results are discussed in terms of several current models for the LFMA effect.

Microwave absorption characterization of the YBa2Cu3O7–δ high-temperature superconductor prepared by different sintering and oxygen annealing times

Low-field microwave absorption (LFMA) has been used to characterize and monitor subtle structural changes produced during the sintering and oxygen annealing process in the preparation of high-temperature superconductors such as YBa2Cu3O7–δ. LFMA measurements are compared with Meissner effect and resistance measurements and to the sample morphology. The LFMA technique as a probe is found to be more sensitive and discriminatory. It allows for an accurate determination of the onset transition temperature and is sensitive to identify multi-superconducting phases. Oxygen stoichiometry, grain size and inter-, intra-granular contacts are found to affect the LFMA intensity.

Intensity of the microwave absorption in high-temperature superconductors

Absorption of microwaves vs. magnetic field near zero field by high-temperature superconductors shows a large number of equally spaced lines in the superconducting state. The intensity of these lines vs. magnetic field is similar to that of Josephson currents which are induced in such oxide superconductors. There is a strong temperature dependence of the intensity with a peak somewhat below the superconducting transition temperature, Tc. The lines and microwave absorption completely vanish above Tc. The intensity of these low-field lines has been examined in four different samples, YBa2Cu3O7, YBa2Cu3O7–Ag, BiSrCaCu2Oy and Bi1.6Pb0.4Sr4Ca2Cu3Oy. In each case the intensity as a function of temperature shows a shallow minimum well below Tc. The temperature, T0, at which this shallow occurs was determined. A discussion of the liquid-like behaviour related to the flux liquid is given. Motional narrowing of the lines was detected.

Determination of the flux creep time from the microwave absorption in the high-temperature superconductor YBa2Cu3O7−δ

Abstract Absorption of microwave versus magnetic field near zero field by the high-temperature superconductor YBa2Cu3O7−δ shows a large number of equally spaced lines in the superconducting state. The intensity in the progression of these lines is similar to that of Josephson currents which are thought to be induced in such oxide superconductors. There is strong temperature dependence with a peak somewhat below the superconducting transition temperature (Tc) and the lines completely vanish at Tc. The lines are interpreted to arise from magnetic-flux quantization. This interpretation allows determination of the magnetic flux, hopping or creep time of 3.4s in YBa2Cu3O7−δ.

Influence of sintering temperature and lead content on the formation of the high temperature superconducting phase in Bi2−x PbxSr2Ca2Cu3Oy

Abstract The formation of the high temperature superconducting phase in Bi 2− x Pb x Sr 2 Ca 2 Cu 3 O y ( x = 0, 0.2, 0.4, 0.6, 0.8) with a superconducting transition temperature ( T c near 110 K is studied as a function of sintering temperature and lead content. The optimum range of sintering temperature is based on differential thermal analysis (DTA) results. DTA shows two endothermic peaks near 1135 and 1155 K. The temperature difference between these two peaks varies with lead content and is maximum at x = 0.4. The x = 0.4 sample also shows the highest fraction of high t c phase based on X-ray diffraction (XRD) and resistance measurements. Thus DTA, XRD and resistance measurements all independently support the x = 0.4 composition as having a unique response which seems characteristic of the high t c phase. XRD and DTA results also suggest that the high t c phase is formed by a reaction between the low T c phase and the products of decomposition of Ca 2 PbO 4 which is produced during the synthesis.

Phase Discrimination of High Temperature Superconducting Phases in the BiSrCaCuO System by Microwave Absorption

The formation of the so-called 80 K and 120 K superconducting phases in the BiSrCaCuO system has been investigated by microwave absorption. This material is characterized by an intense low field microwave absorption (LFMA) below the superconducting transition temperature. LFMA is shown to discriminate between the formation of the two high temperature superconducting phases in this system. Various heating stages in the sample preparation are studied

Microwave absorption investigation on the formation of YBa2Cu3O7−σ by a binary metal oxide route involving interaction of BaCuO2 and Y2Cu2O5

Low-field microwave absorption has been used to discriminate between the YBa2Cu3O7−σ high temperature superconductor synthesized by reacting the binary metal oxides, BaCuO2 and Y2Cu2O5, versus synthesis with a single metal oxide route. This discrimination is more clearly seen by microwave absorption than by resistance measurements. Supplementary data was obtained by electron spin resonance in the g=2 region, scanning electron microscopy, thermal analysis and x-ray diffraction techniques. The scanning electron micrographs indicate a more separated granular structure for the material synthesized from the binary oxides. The low field non-resonant microwave absorption, which is characteristic of the superconducting phase, is twice as intense for the binary metal oxide preparation as compared to the single metal oxide preparation. Also, the superconducting transition temperature seems to be several degrees higher for the binary oxide preparation.