Sign reversal of the Hall effect in YBa2(Cu1?x)3O7?δ single crystals

We have measured the in-plane Hall effect and in-plane resistivity of cobalt-doped YBa₂Cu₃O_7– (YBCO) single-crystal samples. The concentration of cobalt ranged from 1 to 3%. As the applied field was varied, a sign reversal of the Hall resistivity was seen for 1 and 2% Co-doped samples, but not for the 3% sample. Our results are expressed in terms of conductivities, as recently recommended by the work of Dorseyet al. and Kopninet al. In the mixed state just below the superconducting transition temperatureT _c, the Hall conductivity _xy is the sum of two terms,C ₁/H andC ₂H, whereC ₁ andC ₂ are field-independent but temperature-dependent. As previously observed by Ginsberg and Manson for undoped YBCO, the coefficientC ₁ is approximately proportional to ² andC ₂ is approximately linear in, where = 1–(T/T _c ). The values ofC ₁ andC ₂ are presented for each cobalt concentration.     

Disordered Vortex Phases in YBa2Cu3O7?δ

The disordered vortex phases induced by line and point pinning in YBa₂Cu₃O_7– are explored. At high defect densities, only a single disordered solid separated from the liquid phase by a melting line is observed. At low defect densities the topology of the phase diagram changes dramatically, with a vortex lattice phase adjoining disordered phases at high or low field. Critical points at the termination of the first-order melting line separate the lattice and the disordered phases. The line defect disordered phases follow the expected Bose glass behavior, while the point defect disordered phases do not exhibit the expected vortex glass behavior.     

Thermoelectric Power Studies on YBa2?xCaxCu3O7?δ System

The temperature dependence of the thermoelectric power of calcium substituted YBa_2–x Ca _x Cu₃O_7– pellets with 0x1.5 is presented between 60 K and 300 K. A metal–insulator transition was reported earlier by us in this system and was attributed to the ionic size-dependent localization effect. While the sign of thermoelectric power of all the calcium substituted samples was found to be positive, its magnitude increases significantly with calcium content in YBCO. The normal state thermoelectric power data of substituted YBa_2–x Ca _x Cu₃O_7– (0x1.5) are discussed in light of a two-band model originally proposed by Gottwick et al. for heavy fermion systems and later modified by Forro et al.     

Thermoelectric power measurements of YBa2Cu3O7?δ up to 950°C and their application to test the band structure near EF

Four-probe resistivity () and thermoelectric power (TEP) measurements were carried out on samples of YBa₂Cu₃O_7– up to 950°C, in air and in flowing oxygen at 1 bar. Below 700 K the TEP is small and increases rapidly above it, reaching, at 1200 K, +140V/K in air and +120V/K in oxygen. At the changeover temperature (700 K) the slope of log vs.T changes abruptly. These results are interpreted in terms of a model of transport of carriers in a narrow band, which is full for = 1 and half-filled for = 0. Possible origins for such a narrow band are discussed in detail.     

Fabrication of bulk superconductors YBa2Cu3?xTexO7?δ with highly oriented layer structure using the melt-texture growth process

We report on the successful fabrication of bulk samples satisfying the nominal composition YBa₂Cu_3–x Te _x O_7– and containing highly single-crystal-like texture. While this type of crystal growing process is not new, our process requires only the existence of spatial temperature gradient in some parts of the common tube furnace. The samples with tellurium were found to be much more stable and better superconductors than the Y-123 specimen. Large layer textures with cross-section 50×500m² were observed to grow inside the usual bulk sample. The critical currents were measured by the d.c. magnetization method and found to be easily greater than 6000 A cm^–2 for the samples with Te.     

Normal state and superconductivity of ErBa2(Cu1?xCrx)O7?δ(x=0–0.1)

The effects of Cr in ErBa₂(Cu_1–x Cr _x )O_7– (x=0–0.1) superconductor have been investigated. The critical temperature, which was determined by DC electrical resistance measurements, showed no suppression of the onset temperature (T _{c onset}) within the substitution range. The transition width (T _c ) broadened as the Cr content is increased. The normal state changes from the metal-like to semimetal/semiconductor-like for x0.03. Micrographs from the scanning electron microscope, X-ray diffraction pattern, and energy dispersive X-ray analysis results are used to describe the superconducting properties of these materials. The orthorhombic structure was preserved throughout the substitution range. Some possible roles of Cr in the system are discussed.     

On crystal alignment in polycrystalline YBCO materials based on TEM observation

Previously, we have observed that parallel lattice fringes show up regularly in MTG samples, indicative of good alignment of unit cells in strips of size 100 nm × a few times 100 nm. We have prepared films ( 80 nm) for TEM observation using YBCO material fabricated by the conventional solid-state reaction method in order to study the crystal texture in a length scale from a few 100 nm down to 3 Å. Parallel lattice fringes are observed to stretch across a cross-section 0.3×0.5 m². Cross lattice fringes intersecting at 90° and 45° are observed as rare events. This result suggests that the degree of alignment in the crystal texture is higher than expected inside a grain of size a few m.     

Anisotropic uniaxial pressure effects in YBa2Cu3O7?δ

We present measurements of the uniaxial pressure dependence ofT _c of untwinned YBa₂Cu₃O_7– crystals with various oxygen stoichiometries. For all samples investigated,T _c decreases for pressure alonga, increases for pressure alongb, and, in oxygen deficient samples, increases strongly for pressure alongc. These results are compared to the behavior found in the La_2–x Sr _x CuO₄ and YBa₂Cu₄O₈ systems. Neither the model of pressure-induced charge transfer nor coupling to orthorhombic distortions can explain all the data. However, the presence of singularities in the electronic density of states close to the Fermi energy is a possible origin of the observed behavior. Our preliminary data on the pressure dependence of thec-axis and in-plane resistivities in twinned crystals are consistent with this view.     

Improvement of the Superconducting Transport Properties of YBa2Cu3O7?δ by BaZrO3 Doping

We have doped the YBa₂Cu₃O_7– superconducting ceramics with BaZrO₃ up to 75wt.% and studied the changes of some physical properties. The most important finding is the enhancement of the critical current density, which has a maximum at around 5 wt.% doping level. Compared with the undoped samples, the critical current density is four times higher if the doping compound is introduced prior to the calcination treatment, or two times higher if BaZrO₃ is introduced prior to the sintering treatment. The dependence of the critical current density on the doping level is consistent with the dependence of the Seebeck coefficient. We also observed a very small decrease of the critical temperature. The real density of the samples decreases with increasing doping level as expected from the ratio between the theoretical densities of YBa₂Cu₃O_7– and BaZrO₃, the effect being important for doping levels higher than 25 wt.%.