Search

A large magnetoresistance change (ΔR/RH) of −550% has been observed at 270 K in (La0.8Ca0.2)MnO3 thin films. The films were prepared in situ on LaAlO3 substrates by single-liquid-source metal-organic chemical vapor deposition. M(thd)n (M = La, Ca, and Mn, and n = 2, 3) were dissolved together in an organic solution and used as precursors for the deposition of (La0.8Ca0.2)MnO3 thin films. Deposition was conducted at an oxygen partial pressure of 1.2 Torr and a substrate temperature ranging from 600 °C to 700 °C. The mechanism for the large magnetoresistance change in this manganese oxide is briefly discussed.

By means of high-temperature deformation, stacking faults, dislocations and dislocation pairs have been induced abundantly in the Al70Co15Ni15 decagonal phase. Burgers vector b , their directions, their senses and their magnitudes, of dislocations were determined by large-angle convergent-beam electron diffraction (LACBED). Moreover, three kinds of small-angle grain boundaries (SAGBs) were observed in an annealed Al70Co15Ni15 decagonal phase: (1) a tilt boundary consisting of an edge dislocation wall; (2) a twist boundary consisting of parallel screw dislocation pairs; and (3) a small-angle grain boundary consisting of a mixed dislocation network.

The temperature dependences of the positron annihilation lifetime and Doppler broadening have been measured in the Bi-Sr-Ca-Cu-O superconducting system from 77 to 300 K. The results show the existence of two superconducting phases: Tc=90 K 2212 phase and Tc=107 K 2223 phase, with the latter being dominant. Two anomalies were detected around 145 and 250 K, respectively. The role of long-time sintering and adding Pb and Nb in Bi-Sr-Ca-Cu-O is demonstrated in stabilizing the 2223 phase.

H c2 ( T ) has been studied in four Bi-(Pb)-Sr-Ca-Cu-O superconductors with the 110 K type of structure under magnetic fields up to 12 T. It is found that the curvature of the critical field close to T c is strongly sample dependent. By a number of measurements which characterize the samples, such as X-ray diffraction, SEM analysis, chemical analysis, AC susceptibility and normal state and superconducting resistive properties, we find that the curvature of H c2 ( T ) can be empirically correlated with the purity of the samples. It is suggested that the intrinsic behavior of the high- T c Bi-based polycrystalline superconductors is a linear critical fiels curve close to T c .

Cleaved specimens of layered compounds Bi 2 Sr 2( CaCuO 2)n CuO 6, n = 0, 1 and 2 phases have been studied by convergent beam electron diffraction. The identity and the symmetry of the phases have been determined from the higher order Laue zone (HOLZ) rings and the symmetry of their [001] patterns. In the case of Bi 2 Sr 2 CuO 6, the [001] axial periodicity corresponds to the component of the wave vector of the monoclinic modulation along the c axis. Widely different axial periodicities have been obtained. This is interpreted as the manifestation of the variation of the monoclinic modulation. The presence of basal stacking fault lowers the symmetry of the [001] pattern.

The resistivity of superconducting Pb-doped Bi–Sr–Ca–Cu–O single crystal, with 2212 structure, has been measured using Montgomery technique. The resistivity, like in other high T c oxide superconductors, shows very strong anisotropic characteristic, with a typical metallic behavior but remains a residual resistivity at T = 0 K in the direction parallel to the a − b plane and a "semiconducting-like" temperature dependence in the c direction. The anisotropy (ρc/ρab) is about 104. But the value of resistivity in a − b plane is larger than that of Bi 2 Sr 2 CaCu 2 O y crystal. This may imply that the Bi–O plane plays an important role in superconductivity in this kind of structure.

New data and analysis show that the Hall conductivity σ xy of cuprate superconductors consists of two additive contributions: a quasiparticle excitation term and a vortex motion term. Competition between the two leads to a sign reversal in YBCO but not in LSCO, where they both have the same sign.

The magnetoresistance for charge transport along the c -axis is negative in Bi 2 Sr 2 CaCu 2 O 8+ δ , unlike the in-plane transport. We show that Δϱ c 0 / ϱ c is thermally activated with a gap Δ that varies from 520 K to 997 K as the doping proceeds from under-to over-doped. We suggest that the applied field lowers the barrier impeding c -axis charge transport, by coupling to the Zeeman energy.

Diamagnetic susceptibility and the magnetization curves at 1.5 K and 77 K were measured on the single crystal of Bi 2 Sr 2 Ca 1 Cu 2 O y. The results show that the Meissner fraction is anisotropic and the susceptibility reduces slightly with temperature decreasing below 20 K. The magnetization curves exhibit the hysteresis at 1.5 K but no hysteresis is observed at 77 K. Critical current J c at 1.5 K and 77 K are estimated using critical state model.

We have prepared two kinds of single crystals in Bi-Sr-Ca-Cu oxides by flux method;plate-like single crystals which exhibit metal to superconductor at 93 K, being the high-Tclayered compound BiSr (Ca 0.35 Sr 0.25 Bi 0.40)2 Cu 2Oy with orthorhombic structure with a=5.42 Å, b=5.44 Å and c=30.78 Å respectively, and another kind of bar-shapedbismuth-poor single crystals which is superconducting at the temperature below 82K whilebeing semiconducting above this temperature.

X-ray precession technique1 has been utilized to study the reciprocal space of the superconducting single crystal Bi 2 Sr 2 CaCu 2 O y. An average structure with space group of Pnnn and a real structure with the superspace group of [Formula: see text] or [Formula: see text] have been found for this one dimensional modulated incommensurate material. As well, a basic structure with the space group of Fmmm is derived from the experimental data.

The lower critical fields of single crystal Bi 2 Sr 2 CaCu 2 O y at 77 K have been found to be 99.0 G and 19.8 G for the field parallel and perpendicular to c-axis respectively. From H c2 −T curve, the slope d H c2 / d T near T c has also been estimated by linear estimation. The results exhibit an anisotropy of the sample. The observation of Meissner and shielding effect shows that the sample is bulk superconductor.

Following the observations of spin-pair excitations in La 2 CuO 41 and YBa 2 Cu 3 O 6+δ,2 similar results in Bi-Sr-Ca-Cu-O systems are obtained. The center frequencies of the excitations are about 1900–2300 cm −1, which correspond to the value of the exchange constant J~690–840 cm −1. The extraordinary similarity suggests the same origin of superconductivity of these three kinds of copper oxides superconductors. This means that any theory of superconductivity in copper oxides should account explicitly for these highly energetic spin fluctuations.

Pb-doped (Bi1 − xPbx)2Sr2Ca2Cu3O10 (2223) compounds with x = 0, 0.05, 0.10, 0.15, 0.20, 0.25 have been studied by scanning and transmission electron microscopy. It is found that the size of the lamella crystals increases significantly as the Pb-doping level increases. The 2223 phase, which appears as intergrowths among the Bi2Sr2CaCu2O8 (2212) phase in undoped and slightly doped samples, grows at the expense of the 2212 phase as x increases. With x = 0.20, single phase 2223 is obtained. Pb-doping also changes the modulation of the superconducting phases.

Electron spin resonance of a high-Tc Bi superconductor sample is reported. The d.c. susceptibility, d.c. resistivity and a.c. susceptibility show two superconducting transitions at 105 K and 75 K. The ESR spectra show a main resonance line whose temperature dependence is studied in detail. The g value shows a maximum of 2.24 at 230 K and decreases to 2.12 at 100 K. The line width also shows a maximum of 520 G at the same temperature and drops to 200 G at 100 K. An unusual behaviour is observed in the decrease of the integrated intensity from a maximum at 230 K to below noise at level 100 K. It is possible that the origin of this signal is due to impurity phases. However, the unusual behaviour of its intensity (disappearance of the signal below Tc) may indicate that it arises from pair formation much above Tc.

In the single crystal BiSr ( Ca 0.35 Sr 0.15 Bi 0.40)2 Cu 2 O y a characteristic defect-superdislocation has been observed by TEM. The twist boundary resulted from the interaction of two sets of mutually perpendicular superdislocations has been analysed.

Some magnetization curves of single crystal of Bi 2 Sr 2 CaCu 2 O y in low magnetic field H ‖ c axis are given. The magnetization loop is getting smaller with the temperature increase and then disappears at temperature even higher than T c . The experimental measurements of the susceptibility in zero-field limit show that the results can be treated with two-dimensional thermo-fluctuation theory.

The specific heat of Bi 2 Sr 2 CaCu 2 O y has been measured using non-adiabatic method between 10 K and 130 K. No specific heat jump related to superconducting transition was observed around T c . This may be due to the defects and disorder of atoms existed in the single crystal.

H c1 (T) values near T c for the field along the c direction of single crystal Bi 2 Sr 2 CaCu 2 O y are obtained from the magnetization measurements at low field. The nonlinear behavior of magnetization just above H cl are discussed with a model of pinning and dissipative flux creep.

We report Raman spectra of highly oriented crystals of Bi2Sr2CuO6+δ and Bi2Sr2CaCu2O8+δ. A factor group analysis of the k = 0 phonons is also given. Most phonons observed have Ag symmetry. A mode of B1g symmetry is observed at 282 cm−1 which is equivalent to the 337 cm−1 mode of YBa2Cu3O7.

The single particle tunnelling junctions consisting of single crystal BCSCO and Sn film counter electrode have been prepared. The I-V and dI/dV-V characteristics have been measured. Observed value of the energy gap is 23 meV which corresponds to 2Δ/kT c of 6.0, much larger than the BCS weak-coupling limit of 3.52.

The single crystal BCSCO superconductors have been investigated by electron tunneling with both point contacts and BCSCO/Sn junctions. A gap parameter of Δ=34meV and 2Δ/kTc=8.9 have been obtained. Various dI/dV curves including BCS-like and so-called harmonic structure have been observed. We propose to make use of proximity effect and quasiparticle interference to interpret these phenomena.