Jun Okamoto

High-temperature thermoelectric properties of the double-perovskite ruthenium oxide (Sr1−xLax)2ErRuO6

We have prepared polycrystalline samples of (Sr1−xLax)2ErRuO6 and (Sr1−xLax)2YRuO6, and have measured the resistivity, Seebeck coefficient, thermal conductivity, susceptibility, and x-ray absorption in order to evaluate the electronic states and thermoelectric properties of the doped double-perovskite ruthenates. We have observed a large Seebeck coefficient of −160u2009μV/K and a low thermal conductivity of 7 mW/cmK for xu2009=u20090.1 at 800u2009K in air. These two values are suitable for efficient oxide thermoelectrics, although the resistivity is still as high as 1 Ωu2009u2009cm. From the susceptibility and x-ray absorption measurements, we find that the doped electrons exist as Ru4+ in the low spin state. On the basis of the measured results, the electronic states and the conduction mechanism are discussed.

Magnetic circular dichroism of X-ray emission for gadolinium in 4d-4f excitation region

We have measured magnetic circular dichroism of x-ray emission spectra (XES) for gadolinium in the 4 d –4 f excitation region. At a pre-threshold excitation energy, a large magnetic circular dichroism (MCD) signal appeared in a Raman scattering and the dramatic dependence of the MCD spectra on the excitation energy was observed. Theoretical calculation shows that the magnetic moment estimated with total photoelectron yield (TEY) spectra was much smaller than that with the emission spectra. This indicates that the MCD for the TEY reflects the magnetic state on the surface whereas the MCD for the XES reflects that in the bulk. We also observed the MCD spectra for total photon yield (TPY) and found the great difference of the MCD spectra for the TEY and TPY.

Antiferromagnetic order of the Co2+ high-spin state with a large orbital angular momentum in La1.5Ca0.5CoO4

The antiferromagnetic insulator La1.5Ca0.5CoO4 has been investigated by Co L2,3-edge and O K-edge X-ray absorption spectroscopy (XAS) measurements and Co L2,3-edge resonant soft X-ray magnetic scattering (RXMS) measurement to determine the Co electronic structures associated with magnetic ordering. Co L2,3-edge linear-dichroic XAS shows that Co2+ takes a high-spin (HS) state and Co3+ takes a low-spin (LS) state. Using Co L2,3-edge RXMS, we directly determined that an antiferromagnetic order is formed with a HS state of Co2+ ions. Moreover, the spin and orbital angular momenta of the Co2+ HS state are quantitatively estimated to be 1.1 ± 0.1 and 1.0 ± 0.1, respectively, and to align parallel in the ab plane by utilizing the cluster model calculation. The large orbital angular momentum of the Co2+ HS state originates from the small D4h-symmetry crystal field splitting of t2g levels, which is comparable with the spin–orbit coupling constant of the Co 3d orbital.

Surface Ordering of Orbitals at a Higher Temperature in LaCoO3 Thin Film

We report on the distinct surface state of electronic orders, including spin, orbital, and spin-state degrees of freedom of Co3+ ion, in an epitaxially strained thin film of LaCoO3 grown on (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT) substrate. The surface ordered state was detected by the grazing-incidence resonant soft x-ray scattering at Co L-edge, where the probing depth is less than the topmost 4 nm of surface. Comparing with the result of bulk sensitive x-ray diffraction, we revealed that the transition temperature of the orbital order (spin order) at the surface region is about 30 K (20 K) higher (lower) than that of the bulk. A novel phenomenon of the surface order and bulk disorder of the orbital degree of freedom can be attributed to its collective and lattice-coupled nature which is strongly affected by the translational/inversion symmetry breaking at the surface.

Microscopic Examinations of Co Valences and Spin States in Electron-Doped LaCoO3

We studied the Co valences and spin states in electron-doped LaCo1−yTeyO3 by measuring X-ray absorption spectra and electron spin resonance. The low-temperature insulating state involves the low-spin Co3+ state (S = 0) and the high-spin Co2+ state, where the latter is described by g = 3.8 and jeff = 1/2. The results, in concurrence with the electron–hole asymmetry confirmed in the electrical resistivity, coincide with the spin-blockade phenomenon in this system. Furthermore, we discuss the g factor in terms of the strong covalent-bonding nature and consider multiple origins of this phenomenon.

Orbital correlations and dimensional crossover in epitaxial Pr0.5Ca0.5MnO3/La0.5Sr0.5MnO3 superlattices

We studied the charge-orbital ordering in the superlattice of charge-ordered insulating Pr0.5Ca0.5MnO3 and ferromagnetic metallic La0.5Sr0.5MnO3 by resonant soft x-ray diffraction (RSXD) and hard x-ray photoemission spectroscopy (HXPES). A temperature-dependent incommensurability is found in the orbital ordering by RSXD. In addition, a large hysteresis is observed that is caused by phase competition between the insulating charge ordered and metallic ferromagnetic states. No magnetic phase transitions are observed in contrast to pure Pr0.5Ca0.5MnO3 thin films, confirming the unique character of the superlattice. Mn HXPES spectra revealed a hysteresis in the metalicity, supporting the picture of phase competition. The deviation from the commensurate orbital order can be directly related to the decrease of ordered-layer thickness that leads to dimensional crossover from three-dimensional to two-dimensional orbital ordering.We studied the charge-orbital ordering in the superlattice of charge-ordered insulating Pr

Diffractometer for small angle resonant soft X-ray scattering under magnetic field

_{0.5}

Pseudogap-related charge dynamics in the layered nickelate R 2-xSr xNiO 4 (x∼1)

Ca

Resonant soft X-ray scattering study of the magnetic structures in La1.5Ca0.5CoO4 using a high vacuum diffractometer with a 4-blade-slit detector system

_{0.5}

Construction of a soft X-ray diffractometer with a 7.5-T superconducting magnet

MnO