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Register a new userNeutron Diffraction Studies on La2-xDyxCa2xBa2Cu4+2xOz Superconductors
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Rayaprol Sudhindra Dr.
Publication date
2004
fields
Physics
and research's language is
English
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Structural studies on Dy-substituted La-2125 type superconductors have been carried out by neutron diffraction experiments at room temperature using a monochromatic neutron beam of wavelength lambda = 1.249 Angstroms. A series of samples with La2-xDyxCa2xBa2Cu4+2xOz stoichiometric composition, for x = 0.1 - 0.5, have been studied for their structural properties. A tetragonal Y-123 unit cell was taken as the starting model for the Rietveld analysis. All the samples fit into the starting model, exhibiting no structural transition taking place with increasing dopant concentration. The results of Rietveld analysis and structural properties are discussed in detail.
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The La-2125 type La2-xDyxCa2xBa2Cu4+2xOz (0.1 < x < 0.5; LDBO) compounds have been synthesized and studied for their structural and superconducting properties by room temperature neutron diffraction, high field dc magnetization, four-probe resistivity and iodometric double titration. The Rietveld analysis of the neutron diffraction data reveals tetragonal structure for all the samples, which crystallizes into La-123 type tetragonal structure in P4/mmm space group. Iodometric double titrations were performed to determine the oxygen content values and calculate mobile charge carrier (holes) density. The superconducting transition temperatures (Tc) increases from ~ 20 K for x = 0.1 to a maximum of 75 K for x = 0.5. Flux pinning force (Fp) and critical current density (Jc), calculated from the low temperature hysteresis loops, also increases with increasing dopant concentration. The paper presents the studies on structure and superconducting properties of all LDBO compounds in light of the role of calcium in inducing superconductivity in the tetragonal non-superconducting oxide.
Polycrystalline La2-xPrxCa2xBa2Cu4+2xOz (LPCaBCO) compounds with x = 0.1 - 0.5 were synthesized by solid-state reaction method and studied by room temperature X-ray diffraction, dc resistivity, dc magnetization and iodometry. The superconducting transition temperatures in these tetragonal triple perovskite compounds increases from 32 to 62 K (Tconset values) with increasing dopant concentration. The mixing of rare earth La3+ and Pr3+/4+ ions at rare earth site (La3+) along with substitution of divalent Ca2+ results in the shrinkage of unit cell volume. The contraction of unit cell volume due to larger ion being substituted by smaller ions, gives rise to creation of pinning centers in the unit cell leading to increase in critical current density and flux pinning
We have grown cubic centimetre-size crystals of YBa2Cu3Ox suitable for neutron studies, by a top-seeded melt-growth technique. Growth conditions were optimized with an eye toward maximizing phase purity. It was found that the addition of 2% Y2BaCuO5 and 0.5% Pt (by mass) were required to prevent melt loss and to obtain the highest crystallinity. A neutron diffraction study on a mosaic of six such crystals found that the final Y2BaCuO5 concentration was 5%, while other impurity phases comprised less than 1% by volume. The oxygen content was set to x=6.5, the crystals were detwinned and then carefully annealed to give the well-ordered ortho-II phase. The neutron study determined that 70% of the mosaics volume was in the majority orthorhomic domain. The neutron (0,0,6) and (1,1,0) rocking curve widths were ~1 degree per crystal and ~2.2 degrees for the mosaic, and the oxygen chain correlation lengths were >100 A in the a- and b-directions and ~50 A in the c-direction.
A review of high-pressure studies on Fe-pnictide superconductors is given. The pressure effects on the magnetic and superconducting transitions are discussed for different classes of doped and undoped FeAs-compounds, ROFeAs (R = rare earth), AeFe2As2 (Ae = Ca, Sr, Ba), and AFeAs (A = Li, Na). Pressure tends to decrease the magnetic transition temperature in the undoped or only slightly doped compounds. The superconducting Tc increases with pressure for underdoped FeAs-pnictides, remains approximately constant for optimal doping, and decreases linearly in the overdoped range. The undoped LaOFeAs and AeFe2As2 become superconducting under pressure although nonhydrostatic pressure conditions seem to play a role in CaFe2As2. The superconductivity in the (undoped) AFeAs is explained as a chemical pressure effect due to the volume contraction caused by the small ionic size of the A-elements. The binary FeSe shows the largest pressure coefficient of Tc in the Se-deficient superconducting phase.
We present local structural evidence supporting the presence of charge inhomogeneities in the CuO2 planes of underdoped La2-xSrxCuO4. High-resolution atomic pair distribution functions have been obtained from neutron powder diffraction data over the range of doping 0 < x < 0.30 at 10 K. Despite the average structure getting less orthorhombic we see a broadening of the in-plane Cu-O bond distribution as a function of doping up to optimal doping. Thereafter the peak abruptly sharpens. Complementary evidence is also evident from the observation of octahedral tilt disorder in the PDF at higher atomic separation. This suggests a crossover from a charge inhomogeneous state at and below optimal doping to a homogeneous charge state above optimal doping. The strong response of the local structure to the charge-state implies a strong electron-lattice coupling in these materials.
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