Observation of momentum-resolved charge fluctuations proximate to the charge-order phase using resonant inelastic x-ray scattering.

In strongly correlated electron systems, enhanced fluctuations in the proximity of the ordered states of electronic degrees of freedom often induce anomalous electronic properties such as unconventional superconductivity. While spin fluctuations in the energy-momentum space have been studied widely using inelastic neutron scattering, other degrees of freedom, i.e., charge and orbital, have hardly been explored thus far. Here, we use resonant inelastic x-ray scattering to observe charge fluctuations proximate to the charge-order phase in transition metal oxides. In the two-leg ladder of Sr(14-x)Ca(x)Cu24O41, charge fluctuations are enhanced at the propagation vector of the charge order (qCO) when the order is melted by raising temperature or by doping holes. In contrast, charge fluctuations are observed not only at qCO but also at other momenta in a geometrically frustrated triangular bilayer lattice of LuFe2O4. The observed charge fluctuations have a high energy (~1 eV), suggesting that the Coulomb repulsion between electrons plays an important role in the formation of the charge order.

Revisit to diffraction anomalous fine structure.

The diffraction anomalous fine structure (DAFS) method that is a spectroscopic analysis combined with resonant X-ray diffraction enables the determination of the valence state and local structure of a selected element at a specific crystalline site and/or phase. This method has been improved by using a polycrystalline sample, channel-cut monochromator optics with an undulator synchrotron radiation source, an area detector and direct determination of resonant terms with a logarithmic dispersion relation. This study makes the DAFS method more convenient and saves a large amount of measurement time in comparison with the conventional DAFS method with a single crystal. The improved DAFS method has been applied to some model samples, Ni foil and Fe3O4 powder, to demonstrate the validity of the measurement and the analysis of the present DAFS method.

Role of valence fluctuations in the superconductivity of Ce122 compounds.

Pressure dependence of the Ce valence in CeCu(2)Ge(2) has been measured up to 24 GPa at 300 K and to 17 GPa at 18-20 K using x-ray absorption spectroscopy in the partial fluorescence yield. A smooth increase of the Ce valence with pressure is observed across the two superconducting (SC) regions without any noticeable irregularity. The chemical pressure dependence of the Ce valence was also measured in Ce(Cu(1-x)Ni(x))(2)Si(2) at 20 K. A very weak, monotonic increase of the valence with x was observed, without any significant change in the two SC regions. Within experimental uncertainties, our results show no evidence for the valence transition with an abrupt change in the valence state near the SC II region, challenging the valence-fluctuation mediated superconductivity model in these compounds at high pressure and low temperature.

High-energy spin and charge excitations in electron-doped copper oxide superconductors.

The evolution of electronic (spin and charge) excitations upon carrier doping is an extremely important issue in superconducting layered cuprates and the knowledge of its asymmetry between electron- and hole-dopings is still fragmentary. Here we combine X-ray and neutron inelastic scattering measurements to track the doping dependence of both spin and charge excitations in electron-doped materials. Copper L3 resonant inelastic X-ray scattering spectra show that magnetic excitations shift to higher energy upon doping. Their dispersion becomes steeper near the magnetic zone centre and they deeply mix with charge excitations, indicating that electrons acquire a highly itinerant character in the doped metallic state. Moreover, above the magnetic excitations, an additional dispersing feature is observed near the Γ-point, and we ascribe it to particle-hole charge excitations. These properties are in stark contrast with the more localized spin excitations (paramagnons) recently observed in hole-doped compounds even at high doping levels.

Orientation dependence of Pd growth on Au electrode surfaces.

The structure of thin Pd films grown on Au(111) and Au(001) electrodes in a solution containing PdCl(4)(2 -) and SO(4)(2 -) has been investigated by surface x-ray scattering. This technique provided structural information on the Pd films in the lateral direction as well as in the surface normal direction. Comparison of Pd/Au(111) and Pd/Au(001) growth modes shows similarity in the first layer deposition. On Au(111) and Au(001) substrates, Pd follows the crystal structure of the substrates and forms a pseudomorphic monolayer. Beyond 2 ML, however, Pd films grown on Au(111) are relaxed, although there are still pseudomorphic layers at the interface. In contrast, Pd films on Au(001) continue to grow pseudomorphically over 10 ML. The difference in the growth mode between (111) and (001) surfaces is not ascribable only to anisotropy in the elasticity of the film. The relationship between a growing surface and an allowed gliding plane in misfit dislocations is presented as a crucial factor determining the critical thickness of the film.

Anomalous metallic state in the vicinity of metal to valence-bond solid insulator transition in LiVS2.

We investigate LiVS2 and LiVSe2 with a triangular lattice as itinerant analogues of LiVO2 known for the formation of a valence-bond solid (VBS) state out of an S=1 frustrated magnet. LiVS2, which is located at the border between a metal and a correlated insulator, shows a first order transition from a paramagnetic metal to a VBS insulator at Tc approximately 305 K upon cooling. The presence of a VBS state in the close vicinity of insulator-metal transition may suggest the importance of itinerancy in the formation of a VBS state. We argue that the high temperature metallic phase of LiVS2 has a pseudogap, likely originating from the VBS fluctuation. LiVSe2 was found to be a paramagnetic metal down to 2 K.

Charge excitations in the stripe-ordered La5/3Sr1/3NiO4 and La2-x(Ba,Sr)xCuO4 superconducting compounds.

Charge excitations in stripe-ordered 214 compounds La_{5/3}Sr_{1/3}NiO_{4} and 1/8-doped La2-x(Ba or Sr)xCuO4 are studied using resonant inelastic x-ray scattering in the hard x-ray regime. We observe = or approximately 1 eV excitation with a momentum transfer corresponding to the charge stripe spatial period both for the diagonal (nickelate) and parallel (cuprates) stripes. They are interpreted as collective stripe excitations or anomalous softening of the charge excitonic modes of the in-gap states.

Dual nature of a Ni dopant in the hole-type La2-xSrxCuO4 cuprate superconductor.

Local distortions around a Ni dopant in the hole-type La2-xSrxCuO4 superconductor system were studied by x-ray-absorption fine structure (XAFS) using single crystals over a wide hole-doping range. Two distinct interatomic distances between Ni and in-plane oxygen appear in the Ni K-edge extended XAFS. Combined with previous results on hole-localization effects by Ni doping, two types of charge states are strongly indicated for Ni. This duality disqualifies a magnetic-impurity picture for Ni dopant in the superconducting phase of cuprates.

X-ray topography of piezoelectric La(3)Ta(14)Ga(5.5)O(14) crystal grown by Czochralski Method.

We performed synchrotron X-ray topography on a La(3)Ta0(0.5)Ga (5.5)O(14) (LTG) crystal grown by the Czochralski method. Since a synchrotron X-ray source can provide high-energy X-rays, one can detect bulk structures by X-ray topography. LTG is one of the most attractive piezoelectric crystals along with La(3)Ga(5)SiO(14) (LGS) because of its excellent acoustic properties (temperature compensation of acoustic losses). Since LTG single crystals can be grown from a stoichiometric melt, it was expected that single crystals with better quality than the LGS crystal, which cannot be grown from a stoichiometric system but only from a congruent melt, can be obtained. However, 60 keV X-ray topography revealed that the LTG crystal quality was not as high as the LGS crystal quality. The crystal quality of the central region was lower than that of the surrounding region.

Momentum dependence of charge excitations in the electron-doped superconductor Nd1.85 Ce0.15 CuO4: a resonant inelastic x-ray scattering study.

We report a resonant inelastic x-ray scattering (RIXS) study of charge excitations in the electron-doped high-T(c) superconductor Nd1.85 Ce0.15 CuO4. The intraband and interband excitations across the Fermi energy are separated for the first time by tuning the experimental conditions properly to measure charge excitations at low energy. A dispersion relation with q-dependent width emerges clearly in the intraband excitation, while the intensity of the interband excitation is concentrated around 2 eV near the zone center. The experimental results are consistent with theoretical calculation of the RIXS spectra based on the Hubbard model.

Mott gap excitations in twin-free YBa2Cu3O7-delta (Tc=93 K) studied by resonant inelastic x-ray scattering.

Mott gap excitations in the optimally doped high-T(c) superconductor YBa(2)Cu(3)O(7-delta) (T(c)=93 K) have been studied by the resonant inelastic x-ray scattering method. Anisotropic spectra in the ab plane are observed in a twin-free crystal. The excitation from the one-dimensional CuO chain is enhanced at 2 eV near the zone boundary of the b* direction, while the excitation from the CuO2 plane is broad at 1.5-4 eV and almost independent of the momentum transfer. Theoretical calculations based on the one-dimensional and two-dimensional Hubbard model reproduces the observed spectra when different values of the on-site Coulomb energy are assumed. The Mott gap of the CuO chain site is found to be much smaller than that of the CuO2 plane site.

Charge-ordered state in single-crystalline CaFeO3 thin film studied by x-ray anomalous diffraction.

X-ray anomalous diffraction, together with a band-structure calculation, was employed to obtain a quantitative understanding of the charge-ordering state in a single-crystalline CaFeO3 thin film. The experimental result shows a characteristic energy dispersion of the nearly inhibited reflection at 150 K, implying Fe atoms split into two distinct states. The energy dispersion is in good agreement with the calculated spectrum based on the LDA+U scheme. The calculation also reveals an electronic structure of the system where holes in the oxygen orbital surround one of the distinct Fe atoms, in spite of the total electron number in both Fe atoms remaining unchanged.

Self-regeneration of a Pd-perovskite catalyst for automotive emissions control.

Catalytic converters are widely used to reduce the amounts of nitrogen oxides, carbon monoxide and unburned hydrocarbons in automotive emissions. The catalysts are finely divided precious-metal particles dispersed on a solid support. During vehicle use, the converter is exposed to heat, which causes the metal particles to agglomerate and grow, and their overall surface area to decrease. As a result, catalyst activity deteriorates. The problem has been exacerbated in recent years by the trend to install catalytic converters closer to the engine, which ensures immediate activation of the catalyst on engine start-up, but also places demanding requirements on the catalyst's heat resistance. Conventional catalyst systems thus incorporate a sufficient excess of precious metal to guarantee continuous catalytic activity for vehicle use over 50,000 miles (80,000 km). Here we use X-ray diffraction and absorption to show that LaFe(0.57)Co(0.38)Pd(0.05)O(3), one of the perovskite-based catalysts investigated for catalytic converter applications since the early 1970s, retains its high metal dispersion owing to structural responses to the fluctuations in exhaust-gas composition that occur in state-of-the-art petrol engines. We find that as the catalyst is cycled between oxidative and reductive atmospheres typically encountered in exhaust gas, palladium (Pd) reversibly moves into and out of the perovskite lattice. This movement appears to suppress the growth of metallic Pd particles, and hence explains the retention of high catalyst activity during long-term use and ageing.

EXAFS spectra above Pb and Pt K edges observed at low temperature.

X-ray absorption spectra near K edges of Pb (88.0keV) and Pt (78.4keV) for the foils were measured in transmission mode at SPring-8. We succeeded for the first time in observing the EXAFS oscillation of the Pb foil at 12K, although it was failed at room temperature in a previous experiment. The amplitude of the EXAFS oscillation of the Pt foil at 12K considerably increased in comparison with that at room temperature, indicating that the amplitude is very sensitive to the Debye-Waller factor at high-k values.

Rab3B is essential for GnRH-induced gonadotrophin release from anterior pituitary cells.

Gonadotrophin-releasing hormone (GnRH) induces the release of gonadotrophins via an increase in cytosolic Ca2+ concentration ([Ca2+]). Rab3B, a member of the small GTP-binding protein Rab family, is known to be involved in Ca(2+)-regulated exocytosis in pituitary cells. However, it is not known whether Rab3B functions in the physiological process regulated by GnRH in gonadotrophs. In this study using antisense oligonucleotide against Rab3B (AS-Rab3B) we determined that Rab3B is involved in GnRH-induced gonadotrophin release. Rab3B immunopositive cells were reduced in 24% of pituitary cells by AS-Rab3B. This treatment did not affect the population of gonadotrophs or the intracellular contents of gonadotrophins. However, AS-Rab3B significantly inhibited the total amount of basal and GnRH-induced gonadotrophin released from pituitary cells. These results show that Rab3B is involved in basal and GnRH-induced gonadotrophins release but not the storage of gonadotrophins. Next, the changes in [Ca2+] and exocytosis in gonadotrophs treated with AS-Rab3B were compared among Rab3B-positive and -negative cells. The change in [Ca2+] was not different in the two groups, but exocytosis was significantly inhibited in Rab3B-negative cells. These results suggest that Rab3B is essential for GnRH-regulated exocytosis downstream of cytosolic Ca2+ in gonadotrophs.

An additional axis for the surface X-ray diffractometer.

A new surface X-ray diffractometer based on a kappa-type diffractometer will be installed in BL14B1, SPring-8. This diffractometer has an additional axis on its detector arm for rotating the receiving slit about the normal of the slit plane, in addition to two axes for positioning the detector. This additional axis is founded on the consideration of the correction factor which has been derived so as to be valid for the z-axis mode measurement using any incoming and outgoing angles of the X-ray beam. The rotational slit allows accurate measurement of the surface structure factor up to large perpendicular momentum transfer.

Bihormonal cells secreting both prolactin and gonadotropins in normal rat pituitary cells.

The present study was designed to characterize bihormonal cells in rat pituitary cells which secrete PRL and gonadotropins. This was done by using sandwich cell immunoblot assay (CIBA) and reverse hemolytic plaque assay (RHPA) in combination with immunocytochemistry (ICC) and by measuring the intracellular free calcium concentration ([Ca2+]i). The result of the experiment with sandwich CIBA revealed that the populations of LH- and FSH-secreting cells in the PRL-secreting cells were 6.23% and 5.91%, respectively, and the populations of the PRL secreting cells in the LH- and FSH-secreting cells were 18.4% and 15.5%, respectively. Additional studies by the combined techniques of RHPA with ICC revealed that the populations of LH- and FSH-containing cells in the PRL secreting cells were 4.43% and 2.40%, respectively, which were consistent with the results of Sandwich CIBA. Some of the PRL-secreting cells determined by RHPA showed responsiveness to TRH and GnRH in [Ca2+]i. These results suggest that bihormonal cells which secrete both PRL and LH, or both PRL and FSH are present in the normal rat pituitaries.

Simultaneous measurements of exocytosis and intracellular calcium concentration with fluorescent indicators in single pituitary gonadotropes.

Previously, we established a method for the estimation of exocytosis in single gonadotropes using an impermeable fluorescent membrane probe, TMA-DPH. In this study, we have developed a method for the simultaneous measurement of exocytosis and intracellular free Ca2+ concentration ([Ca2+]i) by double-labeling with TMA-DPH and the intracellular Ca2+ probe, Fura-2/AM, using a fluorescence microscope with a 3-wavelength excitation and 2-wavelength emission system. We, therefore, clarified the relationship between spontaneous [Ca2+]i oscillation or gonadotropin releasing hormone (GnRH)-induced intracellular Ca2+ mobilization and exocytosis in gonadotropes. Under resting conditions, some gonadotropes showed various types of spontaneous [Ca2+]i oscillations, while others did not, but all showed basal exocytosis. Each [Ca2+]i peak oscillation did not cause Ca(2+)-regulated exocytosis, and even complete blockage of the [Ca2+]i increase by the intracellular Ca2+ chelator BAPTA/AM had no effect on basal exocytosis. Both GnRH-induced intracellular Ca2+ mobilization and regulated exocytosis showed a similar pattern of peaks and plateaus. Blockage of the [Ca2+]i increase by BAPTA/AM almost completely inhibited the GnRH-stimulated exocytosis. These results show that spontaneous [Ca2+]i oscillations under resting conditions are not linked to regulated or basal exocytosis, and that intracellular Ca2+ mobilization is essential for GnRH-stimulated exocytosis.

Activin inhibits but inhibin activates mouse placental lactogen-II secretion.

The regulation of mouse placental lactogen (mPL)-I and mPL-II secretion by activin and inhibin and the expression of activin and inhibin subunit mRNAs in the mouse decidua were examined. Activin-A at a concentration of 10 nM/l significantly inhibited mPL-II secretion by placental cells from days 9 and 12 of pregnancy. However, activin-A did not affect mPL-I secretion by cells from days 7 and 9 of pregnancy nor mPL-II secretion by cells from day 7 of pregnancy. By contrast, 10 nM/l inhibin activated mPL-II secretion by cells from day 12 of pregnancy. These effects of activin and inhibin on mPL-II secretion were dose-dependent. Follistatin, which binds to activin and blocks its bioactivity, completely eliminated the inhibitory effect of activin on mPL-II secretion. Incubation of placental cells from day 12 of pregnancy with activin-A resulted in a significant reduction of the mPL-II mRNA level assessed by Northern blot analysis. Northern blot analysis using poly(A)+RNA extracted from the decidua indicated that mouse decidua, as well as the placenta, express all activin and inhibin subunits and that their gene expressions increased during gestation. The expression of these mRNAs in the decidua was much higher than those in the placenta. These findings suggest that activin and inhibin regulate mPL-II secretion and suggest the presence of an autocrine or paracrine regulation of mPL-II secretion in mouse placenta by activin and inhibin after mid-pregnancy in vivo.