M. Suenaga

Neutron-diffraction experiments on ceramic powders of $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+x}$ (nonsuperconducting, with $x=0.0 \mathrm{and} 0.15$ have confirmed the existence of long-range, three-dimensional, antiferromagnetic order of the Cu spins. The structure determination was aided by the use of uniaxially oriented powders. The ordering wave vector within a Cu${\mathrm{O}}_{2}$ plane is (\textonehalf{} \textonehalf{}) and the planes are coupled antiferromagnetically along the $c$ axis. The N\'eel temperature is 400\ifmmode\pm\else\textpm\fi{}10 K for the $x=0.15\ifmmode\pm\else\textpm\fi{}0.05$ sample and \ensuremath{\gtrsim}500 K for $x=0.0$.

We show that the superconducting transition temperature ${T}_{c}$ of ${\mathrm{La}}_{2\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Ba}}_{\mathrm{x}}$${\mathrm{CuO}}_{4}$ as a function of barium content (0\ensuremath{\le}x\ensuremath{\le}0.25, determined by mutual inductance measurements) has two maxima, both ${T}_{c}$\ensuremath{\approxeq}25 K, near compositions x=0.09 and x-0.15. Between these two maxima is a local minimum (${T}_{c}$ about 5 K) for x=0.12. dc magnetization data are also reported for six compositions. Anomalies in electrical resistance appear near T=50--60 K at compositions 0.10\ensuremath{\le}x\ensuremath{\le}0.125. Many samples clearly show a second resistive superconducting transition near 30 K in addition to the bulk transition most clearly observed magnetically. The variation of ${T}_{c}$ with composition is discussed in relation to the occurrence of resistance anomalies. The tetragonal lattice parameters at room temperature are consistent with previous work, and show no obvious anomalies. The sample preparation procedures are discussed in detail.

Cerium oxide nanoparticles ${\mathrm{CeO}}_{2\ensuremath{-}x}$ (\ensuremath{\sim}3--20 nm in diameter) made by a vapor phase condensation method, have been studied by several methods of transmission electron microscopy (TEM): electron energy loss spectroscopy (EELS), high resolution imaging, and electron diffraction. The white-line ratios ${M}_{5}{/M}_{4}$ of the EELS spectra were used to determine the relative amounts of cerium ions ${\mathrm{Ce}}^{3+}$ and ${\mathrm{Ce}}^{4+}$ as a function of particle size. The fraction of ${\mathrm{Ce}}^{3+}$ ions in the particles rapidly increased with decreasing particle size below \ensuremath{\sim}15 nm in diameter. The particles were completely reduced to ${\mathrm{CeO}}_{1.5}$ at the diameter of \ensuremath{\sim}3 nm. This reduced cerium oxide has a fluorite structure which is the same as that of bulk ${\mathrm{CeO}}_{2}.$ Also, EELS spectra taken from the edge and center of the particle indicated that for larger particles the valence reduction of cerium ions occurs mainly at the surface, forming a ${\mathrm{CeO}}_{1.5}$ layer and leaving the core as essentially ${\mathrm{CeO}}_{2}.$ A micromechanical model based on linear elasticity was used to explain the lattice expansion of the ${\mathrm{CeO}}_{2\ensuremath{-}x}$ nanoparticles. Comparing our results with previously published works indicates that the amount of ${\mathrm{CeO}}_{1.5}$ in ${\mathrm{CeO}}_{2\ensuremath{-}x}$ nanoparticles is a strong function of the particular synthesis methods used to make these particles.

Thermodynamic quantities in quasi-2D type-II superconductors exhibit characteristic scaling behavior for high fields in the critical region around ${\mathit{H}}_{\mathit{c}2}$(T). Using a nonperturbative approach to the Ginzburg-Landau free energy functional, the scaling functions for the free energy, magnetization, entropy, and specific heat are evaluated in a closed form. The experimental data for ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{Ca}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{10}$ are presented which are in agreement with the theoretical results.

Iron was substituted for copper in ${\mathrm{YBa}}_{2}$(${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Fe}}_{\mathrm{x}}$${)}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ for 0\ensuremath{\le}x\ensuremath{\le}0.33. Superconducting transition temperatures and lattice parameters have been determined as a function of Fe content. The orthorhombic distortion \ensuremath{\Delta}${a}_{0}$=2(b-a)/(a+b) decreased to zero near x=0.03. Beyond x=0.03 the structure appears tetragonal. For compositions 0\ensuremath{\le}x\ensuremath{\le}0.15 the superconducting critical temperature ${T}_{c}$ decreased smoothly from ${T}_{c}$=90 K to below 4.2 K. Transmission electron microscopy (TEM) revealed that the spacing between the twin boundaries decreased from \ensuremath{\sim}200 nm for x=0 to \ensuremath{\sim}20 nm for x=0.02 before the twins disappeared for x>=0.03. The sharp streaks in TEM diffraction patterns were interpreted as due to the twin boundary layers whose thickness varied from \ensuremath{\sim}1 nm for the pure to \ensuremath{\sim}2 nm for the Fe (x=0.02) containing specimen. A tweed structure in the TEM image and associated diffuse streaks along the 〈110〉 directions in electron diffraction pattern appeared for x>0.015. This observation suggests that the structure consists of fine (<5 nm) orthorhombic domains, each domain having the twinlike crystallographic relation with its neighbors. Rietveld refinement of powder neutron diffraction for x=0.10 and 0.167 determined that Fe substituted primarily on the Cu ``chain'' site, and for every two atoms of Fe substituted, approximately one extra oxygen is incorporated in the Cu-O plane.