Showing papers by "Masaki Takata published in 1996"

The Seismic Structure of the Sun

Abstract: Global Oscillation Network Group data reveal that the internal structure of the sun can be well represented by a calibrated standard model. However, immediately beneath the convection zone and at the edge of the energy-generating core, the sound-speed variation is somewhat smoother in the sun than it is in the model. This could be a consequence of chemical inhomogeneity that is too severe in the model, perhaps owing to inaccurate modeling of gravitational settling or to neglected macroscopic motion that may be present in the sun. Accurate knowledge of the sun's structure enables inferences to be made about the physics that controls the sun; for example, through the opacity, the equation of state, or wave motion. Those inferences can then be used elsewhere in astrophysics.

The influence of the completeness of the data set on the charge density obtained with the maximum-entropy method. A re-examination of the electron-density distribution in Si

Abstract: The charge densities derived with the maximum-entropy method (MEM) may be influenced to some extent by the completeness of the data set. In order to examine the effects of the incompleteness, structure-factor data of Si measured by the Pendellosung method [Saka & Kato (1986). Acta Cryst. A42, 469–478] were re-analysed by the MEM. This data set is incomplete: it contains all space-group-allowed reflections with sinθ/λ = 0.86 A−1, and in addition 844 and 880 with sinθ/λ = 1.04 A−1. Results of a MEM analysis of the complete subset of data are compared with those from the full but incomplete set published previously [Sakata & Sato (1990). Acta Cryst. A46, 263–270]. The smaller but complete set was found to give a smooth charge-density distribution that is consistent with previous theoretical work. It is found that the sharp peak maximum at the bond midpoint reported previously is exaggerated owing to the highest-order reflection 880. The completeness of the data set appears to be one of the key factors for obtaining reliable charge densities with MEM. The incompleteness of the data set may cause non-physical fine features of the MEM density distribution.

Direct observation of PrCuO2 hybridization in PrBa2Cu3O7−δ by the MEM charge density study

Abstract: In order to prove the existence of hybridization between the Pr and CuO2 conduction planes, a charge density study of PrBa2Cu3O6.87(PrBCO) and YBa2Cu3O6.97(YBCO) has been carried out by the maximum entropy method (MEM) using synchrotron radiation (SR) powder data. In the charge density of PrBCO obtained by MEM, there can be found directional robes of the charge density from the Pr atom toward the O atoms in the CuO2-planes. For YBCO there have not been observed appreciable charge densities in the interatomic region around the Y atom. The distinct difference of the charge densities between PrBCO and YBCO presents clear experimental evidence of the hybridization between Pr 4f-O 2pπ orbitals which supports the idea that the hole trapping by the hybridized states suppresses the superconductivity in PrBCO.

The principle of the maximum entropy method

Abstract: The theoretical background of the maximum entropy method (MEM) when it is applied to restore the electron or nuclear densities from diffraction data is described. In MEM, the concept of “entropy” is introduced to deal with any incompleteness in an observation in a proper way. An incompleteness causes some ambiguities in the results to some extent. The essence of the method is to find a solution which necessarily agrees with the observation, leaving the measure of ambiguities (entropy) maximum. A few results for simple structures with typical types of chemical bonding are also presented.

The nuclear density of YBCO by the maximum entropy method using neutron powder data

Abstract: In order to examine the nature of thermal motions of the YBCO superconductor, particularly in the CuO 2 and CuO planes, the nuclear density distribution was observed by the maximum entropy method (MEM) using neutron powder diffraction data measured at 15 K. The characteristic squarish deformation of the nuclear density due to anharmonic thermal motion was found even at such a low temperature for the Cu atom in the CuO 2 plane but not in the CuO plane. The obtained MEM nuclear density is consistent with the existence of strong CuO bonds in the CuO 2 plane and not in the CuO plane.