M. Uehara

Percolative phase separation underlies colossal magnetoresistance in mixed-valent manganites

Colossal magnetoresistance—an unusually large change of resistivity observed in certain materials following application of magnetic field—has been extensively researched in ferromagnetic perovskite manganites. But it remains unclear why the magnetoresistive response increases dramatically when the Curie temperature (T C) is reduced. In these materials, T C varies sensitively with changing chemical pressure; this can be achieved by introducing trivalent rare-earth ions of differing size into the perovskite structure, without affecting the valency of the Mn ions. The chemical pressure modifies local structural parameters such as the Mn–O bond distance and Mn–O–Mn bond angle, which directly influence the case of electron hopping between Mn ions (that is, the electronic bandwidth). But these effects cannot satisfactorily explain the dependence of magnetoresistance on T C. Here we demonstrate, using electron microscopy data, that the prototypical (La,Pr,Ca)MnO3 system is electronically phase-separated into a sub-micrometre-scale mixture of insulating regions (with a particular type of charge-ordering) and metallic, ferromagnetic domains. We find that the colossal magnetoresistive effect in low-T C systems can be explained by percolative transport through the ferromagnetic domains; this depends sensitively on the relative spin orientation of adjacent ferromagnetic domains which can be controlled by applied magnetic fields.

Superconductivity in the Ladder Material Sr0.4Ca13.6Cu24O41.84

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.

Large room-temperature intergrain magnetoresistance in double perovskite SrFe1−x(Mo or Re)xO3

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.

Thermal and electronic transport properties and two-phase mixtures in La(5/8-x)Pr(x)Ca(3/8)MnO3

We measured thermal conductivity kappa, thermoelectric power S, and electric conductivity sigma of La(5/8-x)Pr(x)Ca(3/8)MnO3, showing an intricate interplay between metallic ferromagnetism (FM) and charge ordering (CO) instability. The change of kappa, S, and 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.

Spin dynamics of the spin-ladder dimer-chain material Sr14Cu24O41

We have performed inelastic neutron scattering on a single crystal sample of Sr14Cu24O41 to study the spin dynamics of the Cu2O3 spin ladder layers, and CuO2 chains. Data collected with incident energies of 50 meV, 200 meV, 350 meV and 500 meV are best fitted with a dispersion with a spin gap of 32.5+/-0.1 meV and a maximum of 193.52.4 meV, consistent with a coupling along the ladders, J|| = 130 meV and a rung coupling J^=72 meV. We find that excitations with an energy transfer of approximately 11.5 meV can be described solely in terms of a dimer chain with an antiferromagnetic intra-dimer coupling, J1 = 11.2 meV, between next-nearest-neighbour Cu ions and a ferromagnetic inter-dimer coupling, J2 = -1.1 meV. The dimers are separated by two Cu ions providing a periodicity for the dimer chain of five units.

Superconductivity in the new compound (Y1-xCax)0.95Sr2.05Cu2.4(CO3)0.6Oy

Abstract A new Cu-oxide superconductor (Y 1− x Ca x ) 0.95 Sr 2.05 Cu 2.4 (CO 3 ) 0.6 O y has been found by resistivity and magnetization measurements. The transition temperature is about 63 K. The crystal structure consists of the periodic replacements of the Cu(I) site with a Co 3 group in the basic YBa 2 Cu 3 O y structure.

Superconducting Properties of CdCNi3

CdCNi 3 is a newly synthesized superconductor with T c =2.5–3.2 K. The crystal has the same perovskite-type structure as MgCNi 3 but the lattice constant is ∼1% larger than that of MgCNi 3 . For th...

Giant 1 / f noise in perovskite manganites: Evidence of the percolation threshold

We discovered an unprecedented magnitude of the

New Antiperovskite-Type Superconductor ZnNyNi3

1/f

Superconductivity in the new compound Sr2CuO2(CO3)1−x(BO3)x: The new method for carrier doping on layered copper-oxycarbonates

noise near the Curie temperature