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Horibe, Yoichi,Yang, Junjie,Cho, Yong-Heum,Luo, Xuan,Kim, Sung Baek,Oh, Yoon Seok,Huang, Fei-Ting,Asada, Toshihiro,Tanimura, Makoto,Jeong, Dalyoung,Cheong, Sang-Wook American Chemical Society 2014 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.136 No.23
<P>Common mathematical theories can have profound applications in understanding real materials. The intrinsic connection between aperiodic orders observed in the Fibonacci sequence, Penrose tiling, and quasicrystals is a well-known example. Another example is the self-similarity in fractals and dendrites. From transmission electron microscopy experiments, we found that FexTaS2 crystals with x = 1/4 and 1/3 exhibit complicated antiphase and chiral domain structures related to ordering of intercalated Fe ions with 2a x 2a and root 3a x root 3a superstructures, respectively. These complex domain patterns are found to be deeply related with the four color theorem, stating that four colors are sufficient to identify the countries on a planar map with proper coloring and its variations for two-step proper coloring. Furthermore, the domain topology is closely relevant to their magnetic properties. Our discovery unveils the importance of understanding the global topology of domain configurations in functional materials.</P>
Structural phase transitions and dielectric anomaly in YFe2O4-δ
Y. Horibe,K. Kishimoto,N. Ikeda,S. Mori 한국물리학회 2005 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.46 No.1
We examined structural changes related to charge ordering (CO) in the temperature window between 100 K and room temperature (RT) in YFe2O4 by transmission electron microscopy. Characteristic diuse scattering elongated along the [001] direction through the (1/3 1/3 0)-type reciprocal positions was found at RT, which is related to the microdomains due to the CO with a size of 10 { 20 nm. In addition, it was also found that the superlattice structures characterized by the (1/3 { 1/3 + 0)-type modulation vector, which should be due to the vacancy ordering,coexist with the microdomains due to the CO. On the other hand, in stoichimetric YFe2O4, we found temperature variations of the superlattice structures on warming from 100 K, which should be strongly related to the dielectric anomalies in YFe2O4.
우현주,Yoichi Horibe,Kaushal Jha,장기완,정중현 한국물리학회 2018 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.72 No.11
In this work, a silicate based phosphor doped with divalent and trivalent europium (Eu(2+/3+)) ions have been synthesized by two different synthesis processes, PGMS (propylene glycol modified silane) assisted sol-gel and solid state reaction (SSR) method. The structural, morphological and optical properties were studied in detail. The emission intensity of Li2SrSiO4:Eu3+ phosphors synthesized through the PGMS assisted sol-gel increased up to 10 times more than that prepared by SSR method. However, Li2SrSiO4:Eu2+ phosphor prepared by PGMS exhibited lower emission characteristics and decreased to one-third of that prepared by SSR. The Eu3+ doped Li2SrSiO4 synthesized by SSR method using different silicate (Si) sources revealed peculiar optical properties and this is the first kind of its report. The obtained results suggest that the optical properties of Eu(3+/2+) doped in silicate host can be regulated by the alteration of Si source besides sample preparation method. These results open a way to understand the mechanisms responsible for such optical properties and to develop new materials for white LEDs.
Statistics of velocity centroids: effects of density–velocity correlations and non-Gaussianity
Esquivel, A.,Lazarian, A.,Horibe, S.,Cho, J.,Ossenkopf, V.,Stutzki, J. Blackwell Publishing Ltd 2007 MONTHLY NOTICES- ROYAL ASTRONOMICAL SOCIETY Vol.381 No.4
<P>ABSTRACT</P><P>We continue with our previous work on statistics of velocity centroids, to retrieve information about the scaling properties of an underlying turbulent velocity field from spectroscopic observations. We use synthetic data sets with extreme effects of velocity–density correlations that we create artificially, which also have a non-Gaussian distribution of fluctuations. We confirm that centroids can be used to obtain the scaling properties of the turbulent velocity when the ratio of the density dispersion to the mean density is less than unity, regardless of velocity–density correlations and non-Gaussianity. It was found that extreme velocity–density correlations can distort the statistics of velocity centroids, impeding the recovery of the turbulent velocity spectral index from centroids. We show that such correlations introduce high-order moments to the maps of centroids, which we disregarded in previous work, but that they are only important when the density dispersion is large in comparison with the mean density. It was also found that non-Gaussian velocity and/or density distort the statistics of centroids too, but to a lower degree than extreme cross-correlations.</P>
Ferroelectric Domains in Hexagonal YMnO3
S Mori,T Katsufuji,Y Horibe 한국물리학회 2005 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.46 No.1
In order to clarify the microstructure related to the ferroelectric properties in multi-ferroic YMnO3, we carefully examined the ferroelectric-domain conguration by transmission electron microscopy (TEM). It was found that ferroelectric domains in YMnO3 consist of six anti-phase domains at room temperature. On warming from room temperature, the ferroelectric-domain con-guration changes into one consisting of three anti-phase domains around 738 K. In addition, the ferroelectric domains in YMnO3 change drastically into microdomains with a size of 10 20 nmby substitution of Ti ions for some of the Mn ones.