J. Hejtmánek

Electrical resistivity and thermopower measurements of the hole- and electron-doped cobaltites Ln CoO 3

Two extreme members of the cobaltite series, LaCoO3 and DyCoO3, were investigated by the electrical resistivity and thermopower measurements up to 800-1000 K. Special attention was given to effects of extra holes or electrons, introduced by light doping of Co sites by Mg2+ or Ti4+ ions. The experiments on the La based compounds were complemented with magnetic measurements. The study shows that both kinds of charge carriers induce magnetic states on surrounding CoIII sites and form thus thermally stable polarons of large total spin. Their itinerancy is characterized by low temperature resistivity, which is of Arrhenius type r~exp(EA/kT) for the hole (CoIV) doped samples, while an unusual dependence r~1/Tn (n=8-10) is observed for the electron (CoII) doped samples. At higher temperatures, additional hole carriers are massively populated in the CoIII background, leading to a resistivity drop. This transition become evident at ~300 K and 450 K and culminates at TI-M=540 and 780 K for the La and Dy based samples, respectively. The electronic behaviours of the cobaltites are explained considering two excitation processes in parent compounds. The first one is related to a local excitation from the diamagnetic LS CoIII to close-lying paramagnetic HS CoIII state. Secondarily, a metallic phase of the IS CoIII character is formed through a charge transfer mechanism between LS/HS pairs. The magnetic polarons associated with doped carriers are interpreted as droplets of such IS phase.

Ordering phenomena and transport properties of Bi1/2Sr1/2MnO3 single crystals

Measurements of the electrical and thermal conductivities, thermopower, and paramagnetic susceptibility have been performed on single-crystal samples of Bi1/2Sr1/2MnO3, complemented with the x-ray powder diffraction data. A pronounced hysteretic behavior, observed below the cubic-to-orthorhombic transition at Tcrit=535u2009K, is related to the onset of long-range charge order (CO) at TCO=450u2009K and its further evolution down to about 330 K. The diffraction data suggest that the charge-ordered state is formed by Zener pairs, represented by Mn4+ dimers linked by one extra eg electron, and is possibly stabilized by cooperative Bi,Sr displacements. An extremely low thermal conductivity is observed down to the lowest temperatures, without any recovery at the antiferromagnetic ordering temperature TN=150u2009K. Such behavior points to a presence of strong scatterers of phonons. Their possible origin can be linked to “optical-like” oscillations that are associated with fluctuating charges within the Zener pairs.

Structure and physical properties of YCoO{sub 3} at temperatures up to 1000 K

The crystal structure and oxygen stoichiometry in two-layer Na{sub 0.74}CoO{sub 2} and Na{sub 0.38}CoO{sub 2} at room temperature are analyzed by powder neutron diffraction. Two sets of diffraction data for each sample, taken at different incident neutron wavelengths, {lambda}=1.1968 A and {lambda}=1.5403 Ang , are analyzed simultaneously by the Rietveld method, allowing for the independent refinement of all structural parameters. The fractional oxygen site occupancies are found to be 1.01(1) for Na{sub 0.74}CoO{sub 2} and 0.99(2) for Na{sub 0.38}CoO{sub 2}, respectively. These results indicate that the oxygen content of these phases is stoichiometric to a precision of 1 to 2%, and therefore the formal cobalt oxidation state is determined solely by the sodium content. The analysis also reveals that both types of sodium ions in the structure are in off-center distorted trigonal prismatic geometry.We prepared a series of Mg{sub 1-x}(AlLi){sub x}B{sub 2} samples with 0{<=}x{<=}0.45 in order to compensate with Li the electron doping induced by Al. Structural characterization by means of neutron and x-ray diffraction confirms that Li enters the MgB{sub 2} structure even though in an amount less than nominal one. We performed susceptibility, resistivity, and specific heat measurements. Vibrational properties were also investigated by means of Raman spectroscopy. We compare these results with those obtained on a homologous series of Mg{sub 1-x}Al{sub x}B{sub 2} samples. The systematic success of scaling the relevant properties with the Al content rather than with the electron doping suggests that lattice deformation plays an important role in tuning the superconducting properties.High-resolution x-ray powder diffraction and extended x-ray-absorption fine-structure (EXAFS) measurements have been performed on the iso-structural framework crystals Cu{sub 2}O and Ag{sub 2}O as a function of temperature. According to diffraction, both compounds exhibit a negative thermal expansion (NTE) of the lattice parameter over extended temperature intervals (from 9 to 240 K for Cu{sub 2}O, up to 470 K for Ag{sub 2}O) and anisotropic thermal displacements of M atoms (M=Cu,Ag). EXAFS measures a positive expansion of the nearest-neighbors M-O pair distance and a perpendicular to parallel anisotropy of relative motion, much stronger than the anisotropy of the absolute M motion. The M-O bond is much stiffer against stretching than against bending. According to EXAFS, out of the 12 M-M next-nearest-neighbor pairs, the 6 connected via a bridging oxygen undergo negative expansion, while the 6 lacking the bridging oxygen undergo positive expansion. These results show a rather complex local behavior, which, while confirming the connection of NTE to strong perpendicular vibrations, is inconsistent with rigid unit modes models and suggests a more flexible model based on rigid M-O rods.Resonant x-ray scattering (RXS) at the Mn K edge has been used to characterize the low temperature ordered phase of Bi{sub 0.5}Sr{sub 0.5}MnO{sub 3}. Strong resonances were observed for the (030), (050), (05/20), (0 7/2 0), (4 (1/2) 0), and (3 (1/2) 0) reflections as the photon energy is tuned through the Mn K edge. The reported azimuthal and polarization dependence of the resonant intensities indicates that the low temperature phase is described as a checkerboard ordering of two types of Mn sites with different local geometrical structures. One of the sites is anisotropic, a tetragonal distorted oxygen octahedron and the other one is isotropic, a nearly undistorted oxygen octahedron. The distinction of these two Mn sites is accompanied by a displacement of the Mn atoms transverse to the b axis. We conclude that this checkerboard-ordered pattern is a common ground state in half-doped manganites. Intermediate valence states according to fractional charge segregation were deduced for the two nonequivalent Mn atoms, far from the localized Mn{sup 3+} and Mn{sup 4+} ionic species. Hence, the experimental data discard the ionic model of charge and orbital ordering of localized 3d (or 3d-like) states at the Mn site (or the MnO{sub 6} cluster).morexa0» We suggest a description of the RXS in terms of band structure effects and lattice dynamics.«xa0lessThe crystal structure of the PbMg{sub 1/3}Ta{sub 2/3}O{sub 3} relaxor ferroelectric was studied under hydrostatic pressure up to {approx}7 GPa by means of powder neutron diffraction. We find a drastic pressure-induced decrease of the lead displacement from the inversion center, which correlates with an increase by {approx}50% of the anisotropy of the oxygen temperature factor. The vibrations of the Mg/Ta are, in contrast, rather pressure insensitive. We attribute these changes being responsible for the previously reported pressure-induced suppression of the anomalous dielectric permittivity and diffuse scattering in relaxor ferroelectrics.The ground state energy and pairing gap of the interacting Fermi gases calculated by the ab initio stochastic method are compared with those estimated from the Bardeen-Cooper-Schrieffer pairing Hamiltonian. We discuss the ingredients of this Hamiltonian in various regimes of interaction strength. In the weakly interacting (1/ak{sub F} or approx. 0, it becomes part of the bare Hamiltonian. However, the bare BCS Hamiltonian is not adequate for describing atomic gases in the regime of weak to moderate interaction strength -{infinity}<1/ak{sub F}<0 such as ak{sub F}{approx}-1.

Stabilization of the high-spin state of Co3+in LaCo1−xRhxO3

The rhodium doping in the LaCo

Spin-state crossover and low-temperature magnetic state in yttrium-doped Pr0.7Ca0.3CoO3

_{1-x}

Structure and properties of novel cobaltates Ln0.3CoO2 (Ln = La, Pr, and Nd)

Rh

Simultaneous valence shift of Pr and Tb ions at the spin-state transition in (Pr1−yTby)0.7Ca0.3CoO3

_{x}

Glassy ferromagnetism and phase separation in Pr0.5Ca0.5CoO3

O

Thermally and field-driven spin-state transitions in (Pr1−yYy)0.7Ca0.3CoO3

_3

Magnetism and transport properties of layered rare-earth cobaltates Ln0.3CoO2

perovskite series (