F.M. Mueller

Effect of lattice strain and defects on the superconductivity of MgB2

The influence of lattice strain and Mg vacancies on the superconducting properties of MgB2 samples has been investigated. High quality samples with sharp superconducting transitions were synthesized. The variations in lattice strain and Mg vacancy concentrations were obtained by varying the synthesis conditions. It was found that high strain (∼1%) and the presence of Mg vacancies (∼5%) resulted in lowering the Tc by only 2 K.

De haas-Van Alphen measurements in Ba0.6K0.4BiO3

Abstract dHvA measurements have been made on the 32 Kelvin cubic superconductor Ba 0.6 K 0.4 BiO 3 (BKBO) using a new and non-destructive 50 Tesla pulsed field magnet at the NHMFL at Los Alamos, as well as in an 18 T superconducting magnet at Louisiana State University. Data were taken with the magnetic field aligned along the [001] crystallographic direction. Sample temperatures were varied from 1.5 to 3.9 Kelvin. The pulsed fields employed were high enough to drive the system well into the normal state. An analysis of these results showed that the frequencies of the observed dHvA oscillations arose from a several Fermi surface sheets. One cross-section was nearly identical with that predicted by a new LDA band calculation.

Rietveld refinement of crystal chemistry of RBa4Cu3O8.5+δ (R=rare earth)

Abstract Rietveld refinements of X-ray powder diffraction data were performed on RBa 4 Cu 3 O 8.5+ δ (R143) compounds, where R=Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb, to determine their oxygen content and distribution, lattice parameter, and space group. The total oxygen content was found to oscillate with the atomic number of the rare earth elements. The compounds with R having even atomic numbers have higher total oxygen content than those with R having odd atomic numbers. With increasing atomic number of the rare earth element, the oxygen distribution on the O3 and O4 sites changes from uniform (e.g. Sm143) to primarily on the O3 sites. The lattice parameter decreases smoothly with increasing atomic number, which is caused by the monotonic decease in the ionic radii of the rare earth elements. However, the ionic radius does not have significant influence of the total oxygen contents. The refinement also revealed that the correct space group of Sm143 and Eu143 and P2 3 and Pm 3 , respectively.

Synthesis and crystal chemistry of the new compounds GdBa4Cu3O8.5+δ and DyBa4Cu3O8.5+δ

Two new compounds, GdBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}} (Gd143) and DyBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}} (Dy143), were synthesized from precursors Gd{sub 2}O{sub 3}, Dy{sub 2}O{sub 3}, BaO{sub 2}, and CuO at 1000h{degree}C in an oxygen atmosphere. The oxygen stoichiometric value {delta} was found to be 0.68 for Gd143 and 0.6 for Dy143 by iodometric titration. Rietveld refinement of x-ray powder diffraction data showed that Gd143 belongs to the space group {ital Pm}3 while Dy143 belongs to the space group {ital P}23. The two space groups, {ital Pm}3 and {ital P}23, are very similar. Their main difference is that {ital P}23 does not have the inversion symmetry of {ital Pm}3. Both compounds have a cubic unit cell with a lattice parameter of 8.16528thinsp{plus_minus}thinsp0.00006thinsp{Angstrom} for Gd143 and 8.10807thinsp{plus_minus}thinsp0.00010thinsp{Angstrom} for Dy143. Superconducting quantum interference device (SQUID) measurement indicated that neither compound was superconductive down to 5 K. {copyright} {ital 1999 Materials Research Society.}

Synthesis and crystal chemistry of the new compounds GdBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}} and DyBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}}

Two new compounds, GdBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}} (Gd143) and DyBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}} (Dy143), were synthesized from precursors Gd{sub 2}O{sub 3}, Dy{sub 2}O{sub 3}, BaO{sub 2}, and CuO at 1000h{degree}C in an oxygen atmosphere. The oxygen stoichiometric value {delta} was found to be 0.68 for Gd143 and 0.6 for Dy143 by iodometric titration. Rietveld refinement of x-ray powder diffraction data showed that Gd143 belongs to the space group {ital Pm}3 while Dy143 belongs to the space group {ital P}23. The two space groups, {ital Pm}3 and {ital P}23, are very similar. Their main difference is that {ital P}23 does not have the inversion symmetry of {ital Pm}3. Both compounds have a cubic unit cell with a lattice parameter of 8.16528thinsp{plus_minus}thinsp0.00006thinsp{Angstrom} for Gd143 and 8.10807thinsp{plus_minus}thinsp0.00010thinsp{Angstrom} for Dy143. Superconducting quantum interference device (SQUID) measurement indicated that neither compound was superconductive down to 5 K. {copyright} {ital 1999 Materials Research Society.}

Synthesis and crystal chemistry of the new compounds GdBa 4 Cu 3 O 8.5+δ and DyBa 4 Cu 3 O 8.5+δ

Two new compounds, GdBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}} (Gd143) and DyBa{sub 4}Cu{sub 3}O{sub 8.5+{delta}} (Dy143), were synthesized from precursors Gd{sub 2}O{sub 3}, Dy{sub 2}O{sub 3}, BaO{sub 2}, and CuO at 1000h{degree}C in an oxygen atmosphere. The oxygen stoichiometric value {delta} was found to be 0.68 for Gd143 and 0.6 for Dy143 by iodometric titration. Rietveld refinement of x-ray powder diffraction data showed that Gd143 belongs to the space group {ital Pm}3 while Dy143 belongs to the space group {ital P}23. The two space groups, {ital Pm}3 and {ital P}23, are very similar. Their main difference is that {ital P}23 does not have the inversion symmetry of {ital Pm}3. Both compounds have a cubic unit cell with a lattice parameter of 8.16528thinsp{plus_minus}thinsp0.00006thinsp{Angstrom} for Gd143 and 8.10807thinsp{plus_minus}thinsp0.00010thinsp{Angstrom} for Dy143. Superconducting quantum interference device (SQUID) measurement indicated that neither compound was superconductive down to 5 K. {copyright} {ital 1999 Materials Research Society.}