Akihiko Nishida

Irreversibility behavior investigated with microwave absorption in the high-Tc Bi(Pb)-Sr-Ca-Cu-O superconducting ceramics

Abstract Reversible and irreversible microwave absorption is observed as a function of temperature and field in the high-Tc phase of a Pb doped Bi-Sr-Ca-Cu-O superconductor. The difference between zero-field-cooled and field-cooled absorption behaviors is related to the flux-pinning property. The irreversibility temperature Tirr scales with the applied field as H 1 3 , in accord with an exponent for a Tl-based superconductor, but in contrast to H 2 3 power law in the Y-based superconductor or spin glasses.

Temperature and field dependence of magnetization of MgB2 polycrystals

Abstract We present magnetization properties of magnesium diboride (MgB 2 ). MgB 2 polycrystals were fabricated by using the vapor transport method inside a sealed stainless tube. The critical current density J c was estimated from the magnetization hysteresis Δ M by using the Bean model. The temperature dependence of J c ( T ) showed a linear behavior below 15 K and deviated from such behavior in the higher temperature region. This behavior indicates that a flux creep effect influences the wide temperature range for the MgB 2 polycrystals. Furthermore, small values of Δ M show that the pinning force density F p is very weak. The temperature dependence of the upper critical field H c2 ( T ) was also obtained and revealed a linear behavior far from T c . From our experimental results, we conclude that the MgB 2 polycrystals are of good quality with less pinning centers, compared to the commercially available MgB 2 powder.

Upper and Lower Critical Fields of TaS2(Pyridine)1/2

The temperature dependences of the upper critical fields, H c2//,⊥ , and the lower critical field, H c1// , have been measured on the single crystal of the layer structure TaS 2 intercalated with p...

Effects of Y substitution and oxygen deficiency on the superconducting transitions in YxGd1−xBa2Cu3O7−δ

Abstract A series of ceramic samples of Y x Gd 1− x Ba 2 Cu 3 O 7− δ with various Y substitution x and oxygen deficiency δ is prepared and investigated with scanning electron microprobe, X-ray diffraction (XRD) and electrical resistivity. The normal resistivity is almost independent of Y substitution except for the x =0.3 sample which indicated poor crystallinity. Although oxygen deficiency results in higher normal resistivity and lower superconducting transition temperature T c , higher T c is achieved in the Y-substituted sample. XRD analyses show that the increase in the crystal c -axis due to oxygen deficiency is suppressed by the Y substitution. The origin of improved T c is discussed in relation to the suppression of the c -axis stretching and the suppression of decrease in the hole concentration in the CuO 2 plane.

Transmission Electron Spin Resonance Investigations in Aluminium

Conduction electron spin resonance transmission in aluminium has been observed at a frequency of 22.6 GHz under the completely damped spin-wave condition. Line shape fittings based on two TESR theories have been performed. The theory of Walker has turned out to explain fairly well the data in the anomalous skin effect condition at 18 K. The probability e of a spin flip at the surface has been found to be about 1 ×10 -3 , which is in good agreement with that obtained from the reflection measurement. DC resistivity in the same samples has also been measured. The applicability of several TESR theories has been considered, referring to the skin effect condition, g anisotropy and electron-electron interaction.

Examinations of Superconducting Transition Width, Oxygen Deficiency and Crystallographic Quality in YBa2Cu3O7-δ

A series of high-T c YBa2Cu3O7-δ ceramics has been investigated through electrical resistivity measurements, least squares analyses and Rietveld refinements of X-ray powder diffractions; with emphasis on crystallographic and superconducting homogeneities. We have applied a simple method for estimating oxygen contents in the specimen through determination of unit cell parameters by X-ray diffraction peaks. Structural and stoichiometric examinations have also been performed through the Rietveld refinement. By the good agreement of our X-ray Rietveld analyses with the neutron diffraction results, we have confirmed the reliability of our X-ray studies. Nonlinear correlation between full widths at half-maximum of several diffraction peaks and superconducting transition widths has been observed, and the importance of local structural variation has been discussed.

Orientation of Pyridine in TaS2(Py)1/2

X-ray diffraction measurements on TaS 2 (Py) 1/2 single crystals of the single phase with the layer spacing 12.03 A showed that the intercalated pyridine molecules lay with aromatic ring planes perpendicular to the TaS 2 layers, and the nitrogen atom located midway between the TaS 2 layers with the long C–N axis parallel to the layers.

ESR absorption in a high-Tc Bi(PB)−Sr−Ca−Cu−O superconductor

ESR absorption at 22.9 GHz of the high-T c phase of a Bi(Pb)-Sr-Ca-Cu-O ceramic superconductor has been studied under two different spectrometer modes : a conventional field-modulation mode and a direct reflection-amplitude mode. Owing to the effect of hysteresis in the microwave response, the field modulation did not provide the proper field derivative of ESR absorption. By employing the direct mode, we were able to obtain more reliable data for the temperature dependence of the g value, linewidth and integral intensity. The spin susceptibility followed the Curie-Weiss behavior with the negative Weiss temperature around −4 K

In-field logarithmic relaxation of microwave absorption in superconducting Tl2Ba2Ca2Cu3O10 and flux pinning properties

Abstract Flux pinning and creep behaviors of Tl2Ba2Ca2Cu3O10 ceramics have been investigated through relaxation of microwave magnetoabsorption under various magnetic field and measuring time. Although magnetoabsorption has increased rapidly with increasing magnetic field, the absorption has unexpectedly decreased gradually with time under the constant magnetic field. This indicates that in the relaxation process under the magnetic field much larger than the lower critical field the magnetoabsorption is closely correlated to sample magnetization rather than flux density. The microwave absorption has decayed as logarithm of time. From the decay rate flux pinning potential U0 has been estimated between ∼0.1 and ∼1 eV, depending on the applied magnetic field Ha. The observed field dependence of U0 ∝ Ha− 1 2 is consistent with the previous remanence and irreversibility experiments, supporting the thermal activation model for vortices with intrinsic pinning layers in a quasi-two dimensional superconductor.

Effects of Y substitution on the temperature and field dependence of magnetic hystereses in YxGd1−xBa2Cu3O7−δ

Abstract Effects of Y substitution on the magnetic hystereses are investigated in Y-substituted Y x Gd 1− x Ba 2 Cu 3 O 7− δ ceramics. Magnetizations are first measured at respective constant temperatures, where the hysteresis Δ M is estimated by the magnetization difference between field increase and decrease. When the variations of Δ M for the identical field against temperatures are examined, Δ M is found to obey Δ M = A exp[− T / T 0 ] with the single T 0 for a given field. The characteristic temperature T 0 is in the range of 10–30 K, and the field dependence is compared with the function: H −1/ n . The field dependence of T 0 for the x =0.2 (nominal) sample is larger than that for the pure Gd123 ceramics of x =0, reflecting larger T 0 at lower magnetic fields. This is owing to larger Δ M at higher temperatures and lower fields in the Y-substituted system. Improvement of the intergrain coupling by the Y substitution is discussed in comparison with magnetic behaviors in the pure sample.