M. Uehara

We have observed superconductivity in the ladder material Sr 0.4 Ca 13.6 Cu 24 O 41.84 under pressures of 3 GPa and 4.5 GPa by means of electrical measurements. The superconducting transition temperatures T c (onset) are 12 K and 9 K at 3 and 4.5 GPa, respectively. The superconducting volume fraction was obtained to be about 5% from magnetization measurement under 3.5 GPa at 4.2 K, indicating the bulk nature of the superconductivity in this system.

We report significant intergrain magnetoresistance (IMR) in polycrystalline double perovskites of SrFe1−x(Mo or Re)xO3 at room temperature. Systematics of the temperature dependence of IMR indicates that the observed large room-temperature IMR in SrFe1/2Mo1/2O3 originates from the ferrimagnetic nature of insulating grain boundaries as well as the half-metallicity of this perovskite. Our results indicate that a new avenue for spin-polarized tunneling junctions is to utilize insulating interface layers with ferromagnetic or ferrimagnetic coupling.

We have performed inelastic neutron scattering on a single crystal sample of ${\mathrm{Sr}}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ to study the spin dynamics of the ${\mathrm{Cu}}_{2}{\mathrm{O}}_{3}$ spin ladder layers and ${\mathrm{CuO}}_{2}$ chains. Our data are best fitted with a dispersion which has a spin gap of $32.5\ifmmode\pm\else\textpm\fi{}0.1\mathrm{meV}$ and a maximum of $193.5\ifmmode\pm\else\textpm\fi{}2.4\mathrm{meV}$, consistent with a coupling along the ladders, ${J}_{\ensuremath{\parallel}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}130\phantom{\rule{0ex}{0ex}}\mathrm{meV}$ and a rung coupling ${J}_{\ensuremath{\perp}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}72\mathrm{meV}$. Excitations with an energy transfer of approximately 11.5 meV arise from a dimer chain with an antiferromagnetic intradimer coupling ${J}_{1}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}11.2\mathrm{meV}$ between next-nearest-neighbor Cu ions, and a ferromagnetic interdimer coupling ${J}_{2}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}1.1\mathrm{meV}$. The dimer chains have a periodicity of five units.

We measured thermal conductivity $\ensuremath{\kappa}$, thermoelectric power $S$, and electric conductivity $\ensuremath{\sigma}$ of ${\mathrm{La}}_{5/8\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Ca}}_{3/8}{\mathrm{MnO}}_{3}$, showing an intricate interplay between metallic ferromagnetism (FM) and charge ordering (CO) instability. The change of $\ensuremath{\kappa}$, $S$, and $\ensuremath{\sigma}$ with temperature $(T)$ and $x$ agrees well with the effective medium theories for binary metal-insulator mixtures. This agreement clearly demonstrates that with the variation of $T$ as well as $x$, the relative volumes of FM and CO phases drastically change and percolative metal-insulator transition occurs in the mixture of FM and CO domains.