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At least 469 records · Page 26Linked to original sources
PubMed2000

Cu NQR study of the stripe phase local structure in the lanthanum cuprates

Using Cu nuclear quadrupole resonance (NQR) in Eu-doped La2-xSrxCuO4 we find the evidence of the pinned stripe phase at 1.3 K for 0. 08</=x</=0.18. The pinned fraction increases by 1 order of magnitude near hole doping x = 1/8. The NQR line shape reveals three inequivalent Cu positions: (i) sites in the charged stripe, (ii) nonmagnetic sites outside the stripes, and (iii) sites with a magnetic moment of 0.29&mgr;(B) in the antiferromagnetically correlated regions. A dramatic change of the NQR signal for x>0.18 correlating with the onset of bulk superconductivity corresponds to the depinning of the stripe phase.

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PubMed2000

Dynamics of localized structures in vectorial waves

Dynamical properties of topological defects in a two dimensional complex vector field are considered. These objects naturally arise in the study of polarized transverse light waves. Dynamics is modeled by a vector complex Ginzburg-Landau equation with parameter values appropriate for linearly polarized laser emission. Creation and annihilation processes, and self-organization of defects in lattice structures, are described. We find "glassy" configurations dominated by vectorial defects and a melting process associated with topological-charge unbinding.

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PubMed2003

Field cooled and zero field cooled 207Pb NMR and the local structure of relaxor PbMg1/3Nb2/3O3.

An isotropic 207Pb NMR spectrum corresponding to the glassy matrix with spherical shell type Pb shifts from the cubic sites, as well as an anisotropic spectral component corresponding to polar nanoclusters with a Pb shift parallel to the [111] direction, have been observed in a PbMg(1/3)Nb(2/3)O3 (PMN) single crystal. This represents a microscopic confirmation of the model of relaxors first proposed by Burns and Dacol. A sudden increase in the intensity of the anisotropic cluster line is seen for electric fields larger than the critical field around 210 K. This demonstrates the occurrence of an orientational percolation type transition to the field-induced ferroelectric phase with about 50% of the Pb nuclei still remaining in the spherical glass matrix. A similar though smaller increase of the intensity of this line is also seen in the zero field cooled data, demonstrating that relaxor PMN is an incipient ferroelectric.

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PubMed2005

In situ high-energy x-ray diffraction study of the local structure of supercooled liquid Si.

Employing the technique of electrostatic levitation, coupled with high-energy x-ray diffraction and rapid data acquisition methods, we have obtained high quality structural data more deeply into the supercooled regime of liquid silicon than has been possible before. No change in coordination number is observed in this temperature region, calling into question previous experimental claims of structural evidence for the existence of a liquid-liquid phase transition.

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PubMed1979

beta-1,3-Glucan in Developing Cotton Fibers: Structure, Localization, and Relationship of Synthesis to That of Secondary Wall Cellulose.

Evidence is presented for the existence of a noncellulosic beta-1,3-glucan in cotton fibers. The glucan can be isolated as distinct fractions of varying solubility. When fibers are homogenized rigorously in aqueous buffer, part of the total beta-1,3-glucan is found as a soluble polymer in homogenates freed of cell walls. The proportion of total beta-1,3-glucan which is found as the soluble polymer varies somewhat as a function of fiber age. The insoluble fraction of the beta-1,3-glucan remains associated with the cell wall fraction. Of this cell wall beta-1,3-glucan, a variable portion can be solubilized by treatment of walls with hot water, a further portion can be solubilized by alkaline extraction of the walls, and 17 to 29% of the glucan remains associated with cellulose even after alkaline extraction. A portion of this glucan can also be removed from the cell walls of intact cotton fibers by digestion with an endo-beta-1,3-glucanase. The glucan fraction which can be isolated as a soluble polymer in homogenates freed of cell walls is not associated with membranous material, and we propose that it represents glucan which is also extracellular but not tightly associated with the cell wall. Enzyme digestion studies indicate that all of the cotton fiber glucan is beta-linked, and methylation analyses and enzyme studies both show that the predominant linkage in the glucan is 1 --> 3. The possibility of some minor branching at C-6 can also be deduced from the methylation analyses. The timing of deposition of the beta-1,3-glucan during fiber development coincides closely with the onset of secondary wall cellulose synthesis. Kinetic studies performed with ovules and fibers cultured in vitro show that incorporation of radioactivity from [(14)C]glucose into beta-1,3-glucan is linear with respect to time almost from the start of the labeling period; however, a lag is observed before incorporation into cellulose becomes linear with time, suggesting that these two different glucans are not polymerized directly from the same substrate pool. Pulse-chase experiments indicate that neither the beta-1,3-glucan nor cellulose exhibits significant turnover after synthesis.

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PubMed2001

Local structure of silver clusters in the channels of zeolite 4A.

The Ag K-edge EXAFS measurements have been carried out at 50 K for fully Ag+-exchanged zeolite 4A (Ag-4A) dehydrated at different temperatures under vacuum. In the Fourier transforms of the EXAFS oscillation the two distinct peaks appear around 2.2 A and 3.0 A, and a shoulder around 2.8 A. The structural parameters (distance, coordination number and root mean square displacement) were derived by a three-shell (two Ag-O contacts and Ag-Ag contact) curve-fitting analysis. It is found that a silver cluster is formed with Ag-Ag distance 2.82+/-0.02 A and coordination number 4.9+/-0.2. When the silver cluster is formed 6 Ag0 atoms and 8 Ag+ ions are arranged in a cubic closed-packed manner in the cage of the Ag-4A zeolite. The 8 Ag+ ions and 6 Ag0 atoms are confined in the framework of oxygens at distances of 2.28+/-0.02 and 2.88+/-0.02 A, respectively.

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PubMed2001

Three-dimensional analysis of the local structure of Cu on TiO2(110) by in situ polarization-dependent total-reflection fluorescence XAFS.

Cu K-edge XAFS of Cu/TiO2(110) was measured by polarization-dependent total-reflection fluorescence XAFS technique. XAFS of [001], [110], and [110] directions were measured to elucidate the three dimensional structure of Cu species on the TiO2(110) surface prepared by the deposition of Cu(DPM)2 followed by reduction with H2. Simulation of the EXAFS functions as well as conventional curve fitting analysis revealed that plane Cu3,4 small clusters with similar structure to Cu(111) plane were formed by the reduction at 363 K. The small clusters converted into spherical metallic Cu particles by the reduction at 473 K.

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