K. Kishio

Anomalous peak effect in single crystal Bi2Sr2CaCu2O8+y studied by Hall probe magnetometry

Abstract We have studied an anomalous peak effect in a Bi 2 Sr 2 CaCu 2 O 8+ y single crystal using a specially designed Hall probe magnetometer. In order to investigate the origin of the anomalous peak effect which manifests itself in the magnetic hysteresis loop, the relaxation (10 -2 s t 4 s), and the local field imaging experiments were performed. Decoupling of 2D vortices in neighboring CuO layers, namely, a dimensional crossover in the pinning mechanism, is proposed as a possible mechanism of the peak effect.

Evidence for an Energy Scale for Quasiparticle Dispersion in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8}

Quasiparticle dispersion in Bi2Sr2CaCu2O8 is investigated with improved angular resolution as a function of temperature and doping. Unlike the linear dispersion predicted by the band calculation, the data show a sharp break in dispersion at 50+/-15 meV binding energy where the velocity changes by a factor of 2 or more. This change provides an energy scale in the quasiparticle self-energy. This break in dispersion is evident at and away from the d-wave node line, but the magnitude of the dispersion change decreases with temperature and with increasing doping.

Bilayer Splitting in the Electronic Structure of Heavily Overdoped Bi2Sr2CaCu2O8 + delta

The electronic structure of heavily overdoped Bi(2)Sr(2)CaCu(2)O(8+delta) is investigated by angle-resolved photoemission spectroscopy. The long-sought bilayer band splitting in this two-plane system is observed in both normal and superconducting states, which qualitatively agrees with the bilayer Hubbard model calculations. The maximum bilayer energy splitting is about 88 meV for the normal state feature, while it is only about 20 meV for the superconducting peak.

Universal relationship between crystallinity and irreversibility field of MgB2

The relationship between irreversibility field, Hirr, and crystallinity of MgB2 bulks including carbon substituted samples was studied. The Hirr was found to increase with an increase of full width at half maximum (FWHM) of MgB2 (110) peak, which corresponds to distortion of honeycomb boron sheet, and their universal correlation was discovered even including carbon substituted samples. Excellent Jc characteristics under high magnetic fields were observed in samples with large FWHM of (110) due to the enhanced intraband scattering and strengthened grain boundary flux pinning. The relationship between crystallinity and Hirr can explain the large variation of Hirr for MgB2 bulks, tapes, single crystals, and thin films.

Study on chemical diffusion of oxygen in Ba2YCu3O7−δ

Abstract Chemical diffusion in the high- T c oxide superconductor Ba 2 YCu 3 O 7−δ system was thermogravimetrically studied in a temperature range from 550 to 850°C under an oxygen pressure of 1 to 10 −2 atm, using highly densified polycrystalline specimens. Isothermal weight relaxation after stepwise changes of the oxygen pressure were measured and analyzed in terms of oxygen diffusion-controlled kinetics. The chemical diffusion coefficient was found to be strongly sensitive to the oxygen composition. Its absolute value sharply increased with decreasing oxygen deficiency, δ, although it did not change appreciably at the structural transition between orthorhombic and tetragonal phases. Such a compositional dependence of the chemical diffusion is not expected from a simple vacancy diffusion mechanism. It is interpreted in terms of composition-sensitive activation energy for the two-dimensional motion of oxygen within the unique crystal structure of this oxide system.

A new mercury-based superconductor: (Hg, Cr)Sr2CuOy

Abstract A new mercury-based cuprate superconductor, (Hg 1- x Cr x )Sr 2 CuO y (0⩽ x ⩽0.8), having T c of 58 K ( x =0.3 has been synthesized. While a chromium-free sample ( x =0) was multi-phased and the superconducting volume fraction was less than 1%, samples for x =0.3 and 0.4 were of nearly single phase and their superconducting shielding volume fraction reached 100% of perfect diamagnetism. X-ray diffraction analysis revealed that its crystal symmetry was tetragonal with lattice parameters a 0 =3.840−3.854 A and c 0 =8.639−8.705 A which slightly varied with x . The observed c 0 value, corresponding to the interlayer distance of superconducting CuO 2 planes, was found to be the shortest among all the known mercury-based “12( n -1) n ” superconductors. Compared to the barium-containing compounds, the present barium-free materials have been found to be insensitive to the ambient air atmosphere during the synthesis and also to be quite stable after storage in air.

Strong flux pinning up to liquid nitrogen temperature discovered in heavily Pb-doped and oxygen controlled Bi2212 single crystals

Abstract Heavily Pb-doped Bi2212 single crystals with carefully controlled oxygen compositions were prepared and their magnetization curves were studied as a function of carrier doping level from heavily overdoped (Tc = 67 K) to near optimally-doped (Tc = 96 K) states. All the Pb-doped samples showed much improved Jc-H properties compared to the conventional Pb-free Bi2212 crystals. In particular, the moderately overdoped crystals maintained high Jc and Hirr up to high temperatures and these values surpassed those for the Pb-ion irradiated Bi2212 with an optimized fluence. Dramatically improved flux pinning properties in the present material are possibly due to generation of temperature and field-induced new pinning centers, as suggested by anomalous dependence of Jc both on temperature and magnetic field.

Charge transfer associated with iodine intercalation in Bi2Sr2Can−1CunOy, n=1−3

Abstract Intercalation of iodine into the Bi 2 Sr 2 Ca n -1 Cu n O y ( n =1,2,3) superconductors has been studied. A variety of measurements indicate charge transfer is associated with the intercalation process, both directly to the BiO layers and also to the superconducting CuO 2 sheets. Changes in T c on intercalation can be understood in terms of hole doping to the CuO 2 planes. Hall coefficient measurements on a single crystal n =2 sample supported an increase in hole concentration, and an XPS study clearly showed the ionic character for the intercalated iodine, accompanied by changes in the Fermi level. Investigation of the c -axis resistivity, also on n =2 single crystals, indicated a change from the semiconductive behaviour of the parent crystal to an apparently metallic character, though the c -axis resistivity remained high.

Bulk superconductivity in both tetragonal and orthorhombic solid solutions of (La1−xSrx)2CuO4−δ

Comparative measurements of low-field Meissner and shielding signals of both powder and sintered pellet samples of (La[sub 1[minus][ital x]]Sr[sub [ital x]])[sub 2]CuO[sub 4[minus][delta]] have revealed that bulk superconductivity is present over a wide range of Sr composition, 0.0425[le][ital x][le]0.125, extending over the orthorhombic-to-tetragonal phase boundary at [ital x][congruent]0.105, except for a narrow anomaly region [ital x]=0.0625. Samples were prepared by three different methods through a combination of spray-drying or powder-mixing preparation with either slow cooling or quenching from the sintering temperature of 1100 [degree]C down to 500 [degree]C, at which oxygenation annealing was performed. Homogeneous solid solutions were obtained only for specimens prepared by spray drying and slow cooling. Dependences of the Meissner and shielding signals on the Sr content, on the intensity of magnetic field, and on the sample size were examined in terms of pinning effect and penetration depth.

Motion of vortices in superconductors

A dissipation-free current can be achieved in a superconductor only when tiny magnetic vortices, which penetrate the superconductor when a magnetic field is applied, are pinned down against current-induced force. To investigate the mechanism by which such vortex pinning occurs, we have made real-time observations of the onset of vortex motion in high-temperature Bi2Sr2CaCu2O8+δ(Bi-2212) superconductors.