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Single crystal x-ray diffraction measurements on both as-grown as well as oxygenated single crystals of an aluminium doped high temperature superconductor YBa2Cu3−xAlxO6+δ revealed the crystal structure to be orthorhombic with space group Pmmm, in contrast to, tetragonal crystal structures corresponding to space group P4/mmm, previously reported for as-grown YBa2Cu3−xAlxO6+δ, and conflicting structures on oxygenated YBa2Cu3−xAlxO6+δ. The orthorhombic crystal structure was confirmed by powder x-ray diffraction that showed the presence of two peaks corresponding to (020) and (200) reflections associated with orthorhombic structures of space group Pmmm, instead of a single (200) reflection corresponding to tetragonal crystal structures with space group P4/mmm. All the as-grown crystals were found to be superconducting. An oxygen-vacancy cluster distribution model is proposed to explain the differences in the obtained magnetisation hysteresis loop and the broad superconducting transition temperature. The model proposes the existence of two oxygen deficient clusters of (Al-..-Cu-O-Cu)n and (Cu-O-Cu-..)n juxtaposed with each other whose number and size vary as the as-grown single crystals of YBa2Cu3−xAlxO6+δ are subjected to oxygenation. X-ray photoelectron spectroscopy measurements showed the existence of two distinct peaks in each of the spectrum of O, Cu, Y and Ba in YBa2Cu3−xAlxO6+δ crystals corresponding to the two different types of clusters. The relative intensities of each XPS peak was found to decrease in the oxygenated crystals as compared to the as-grown ones confirming the change in the number and size of clusters in the as-grown crystals after oxygenation. [ABSTRACT FROM AUTHOR]

We report low-temperature magnetization measurements on a copper-containing coordination polymer [Cu( μ-C2O4)(4-aminopyridine)2(H2O)] n. According to these measurements and previous results (Prokofiev et al. in Cryst. Res. Technol. 42, 394, ), the material is a very good realization of an S=1/2 antiferromagnetic Heisenberg chain with a small saturation field B s=4.2 T, which marks the endpoint of a quantum-critical line in the B– T plane. Measurements of the magnetocaloric effect at low temperatures T=0.47 K across the saturation field yields a very large magnetothermal response with a characteristic sign change close to the quantum-critical point. The data are in good agreement with the quantum-critical behavior calculated on the basis of exact diagonalization and quantum Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

Abstract: The dHvA oscillations in crystals of LuNi2B2C are known to persist deep inside the vortex state, across the region of the peak effect located at the edge of the irreversibility field normal phase boundary. This makes the identification of H c2(T) ambiguous in magnetization hysteresis loops at temperatures at which the dHvA signal is prominent. One can overcome this difficulty if H c2(T) value is ascertained from the measurement of isothermal quadrupole moment (Q) vs. field values using a vibrating sample magnetometer. In these crystals, one can witness dHvA oscillations up to about 8K and we show that data in LuNi2B2C can be used to construct the vortex phase diagram over the entire (H, T) range. [Copyright &y& Elsevier]

Single crystals of a high temperature superconductor Bi 2 Sr 2 CaCu 2 O 8+ x (2212) were grown using two different techniques, namely, regrowth technique as well as a pressure technique and the results are compared. Single crystal X-ray diffraction (XRD) performed on crystals grown using both techniques show no impurity or intergrowth phase. However, powder XRD reveals the presence of only Bi 2 Sr 2 CuO 6+ x (2201) intergrowth phase in crystals grown using pressure technique, with additional impurity phases also found in the crystals grown using regrowth technique. Rietveld refinement showed the weight fraction of 2212 as 93 % in crystals grown from pressure technique, confirming the excellent quality of the as-grown crystals. The transition temperature of crystals grown using both techniques was ∼ 90 K. It was found that pressure technique gives crystals that have a higher superconducting volume fraction ( ∼ 100) than that of crystals grown using regrowth technique. Structural and magnetization measurements reveal that the as-grown crystals grown using pressure technique are of high quality, comparable to those grown using traveling solvent floating zone technique. Crystals grown by both the techniques exhibit second magnetization peak anomaly. [ABSTRACT FROM AUTHOR]