Showing papers by "Masaki Takata published in 1998"
TL;DR: The cage structure of Sc@C82 (isomer I) has been determined by a synchrotron radiation powder structural analysis using the maximum entropy method (MEM).
135 citations
TL;DR: In this paper, the authors investigated the third-order nonlinearity on resonance with the confined exciton level in CuBr nanocrystals with radii in the range $2.7 -42$ nm embedded in glass by means of degenerate four-wave mixing, time-resolved luminescence, and resonant luminecence measurements.
Abstract: We have investigated the third-order nonlinearity on resonance with the confined exciton level in CuBr nanocrystals with radii in the range $2.7--42$ nm embedded in glass by means of degenerate four-wave mixing, time-resolved luminescence, and resonant luminescence measurements. The third-order optical susceptibility ${\ensuremath{\chi}}^{(3)}$ exhibits resonant behaviors at ${Z}_{12}$ and ${Z}_{3}$ excitons, which are weakly confined in nanocrystals. The figure of merit $|{\ensuremath{\chi}}^{(3)}|/\ensuremath{\alpha}$ increases with increasing radius $R$ in the whole range studied here. We have measured homogeneous widths and lifetimes of ${Z}_{12}$ excitons and obtained size dependences of those relaxation parameters. Assuming a two-level atomic model for the confined exciton, we have deduced the size dependence of the oscillator strength of ${Z}_{12}$ excitons from the measured lifetimes, homogeneous widths, and ${\ensuremath{\chi}}^{(3)}$. The oscillator strength exhibits the ${R}^{2.0}$ dependence in the whole range studied here, which reveals the giant oscillator strength effect on the confined exciton that is coherently generated in the nanocrystal. The oscillator strength per nanocrystal is enhanced by a factor $1.7\ifmmode\times\else\texttimes\fi{}{10}^{3}$ for $R=42$ nm compared to that of bulk excitons. We also discuss the derivation of ${\ensuremath{\chi}}^{(3)}$ in the stationary regime from the ${\ensuremath{\chi}}^{(3)}$ in the transient regime where the nonlinear time response shows a multiexponential decay.
50 citations
TL;DR: The crystal structure of MoSi2 was investigated using a synchrotron X-ray powder diffraction method as discussed by the authors, and it was shown that there was a strong covalent bond between Si atoms aligned along the c axis.
32 citations
TL;DR: In this article, the charge density distributions of Strontium titanate (SrTiO 3 ) were obtained by applying the Maximum Entropy Method (MEM) to X-ray diffraction data at Room Temperature (R.T.) and 70 K.
22 citations
TL;DR: In this article, the electron density distribution of wurtzite-type gallium nitride (w-GaN) was obtained by the Maximum Entropy Method (MEM) using the Synchrotron Radiation powder data.
Abstract: The electron density distribution of wurtzite-type gallium nitride (w-GaN) was obtained by the Maximum Entropy Method (MEM) using the Synchrotron Radiation powder data. Contribution of the very minor zinc blende-type phase (z-GaN) to the observed powder data was eliminated by the modified Rietveld method. In the obtained MEM electron density distribution map, there are two kinds of Ga–N covalent bonds. The electron density at the saddle point of Ga–N bond parallel to [001] axis is 0.5 [e/A 3 ]. On the other hand, that of the other three equivalent Ga–N bonds are 0.8 [e/A 3 ]. Furthermore, it is found that the electron distribution of N atom shows asymmetric distortion. These features suggest asymmetric thermal vibrations of N atom which are restricted by Ga–N bonds.
17 citations
TL;DR: In order to explore the nature of the hydrogen bond, the electron density distribution of KH 2 PO 4 (KDP) at room temperature was obtained by the maximum entropy method (MEM) using synchrotron radiation X-ray powder diffraction data as discussed by the authors.
Abstract: In order to explore the nature of the hydrogen bond, the electron density distribution of KH 2 PO 4 (KDP) at room temperature was obtained by the maximum entropy method (MEM) using synchrotron radiation X-ray powder diffraction data. In the obtained electron density distribution maps, the contour lines linked the two oxygen atoms of neighboring PO 4 groups and a hydrogen-bonding network was clearly seen. However, no local maxima of electron density due to hydrogen atoms were recognized. This figure of hydrogen bonding in KDP is similar to that in ice (I h ), which is the only hydrogen-bonded compound previously analyzed by MEM. In both cases, the hydrogen atoms are believed to be disordered between the two oxygen atoms. The present study shows that the character of the hydrogen bond in crystalline materials can be described clearly by MEM, even in the case where heavier atoms such as K and P exist besides O atoms.
15 citations
TL;DR: A combination of the maximum-entropy method (MEM) and Rietveld refinement was employed to analyse the complicated powder pattern and the obtained MEM charge densities show a clear distinction of the endohedral natures of the mono- and dimetallofullerenes.
Abstract: The endohedral natures of the metallofullerenes Y@C82 and Sc2@C84 are described based on synchrotron radiation powder diffraction experiments. For structural analysis, a combination of the maximum-entropy method (MEM) and Rietveld refinement was employed to analyse the complicated powder pattern. The obtained MEM charge densities show a clear distinction of the endohedral natures of the mono- and dimetallofullerenes.
2 citations