7,933 research outputs found
Spin Gap of Two-Dimensional Antiferromagnet Representing CaVO
We examined a two-dimensional Heisenberg model with two kinds of exchange
energies, and . This model describes localized spins at vanadium
ions in a layer of CaVO, for which a spin gap is found by a recent
experiment. Comparing the high temperature expansion of the magnetic
susceptibility to experimental data, we determined the exchange energies as
610 K and 150 K. By the numerical diagonalization we
estimated the spin gap as 120 K, which consists
with the experimental value 107 K. Frustration by finite enhances the
spin gap.Comment: 12 pages of LaTex, 4 figures availavule upon reques
Superconductivity in S-substituted FeTe
We have successfully synthesized a new superconducting phase of FeTe1-xSx
with a PbO-type structure. It has the simplest crystal structure in iron-based
superconductors. Superconducting transition temperature is about 10 K at x =
0.2. The upper critical field Hc2 was estimated to be ~70 T. The coherent
length was calculated to be ~2.2 nm. Because FeTe1-xSx is composed of nontoxic
elements, this material is a candidate for applications and will activate more
and more research on iron-based superconductor.Comment: 13 pages, 10 figure
Prebiotic Organic Microstructures
Micro- and sub-micrometer spheres, tubules and fiber-filament soft structures have been synthesized in our experiments conducted with 3 MeV proton irradiations of a mixture of simple inorganic constituents, CO, N2 and H2O. We analysed the irradiation products, with scanning electron microscopy (SEM) and atomic force microscopy (AFM). These laboratory organic structures produced wide variety of proteinous and non-proteinous amino acids after HCl hydrolysis. The enantiomer analysis for D-, L- alanine confirmed that the amino acids were abiotically synthesized during the laboratory experiment. Considering hydrothermal activity, the presence of CO2 and H2, of a ferromagnesian silicate mineral environment, of an Earth magnetic field which was much less intense during Archean times than nowadays and consequently of a proton excitation source which was much more abundant, we propose that our laboratory organic microstructures might be synthesized during Archean times. We show similarities in morphology and in formation with some terrestrial Archean microstructures and we suggest that some of the observed Archean carbon spherical and filamentous microstructures might be composed of abiogenic organic molecules. We further propose a search for such prebiotic organic signatures on Mars. This article has been posted on Nature precedings on 21 July 2010 [1]. Extinct radionuclides as source of excitation have been replaced by cosmic radiations which were much more intense 3.5 Ga ago because of a much less intense Earth magnetic field. The new version of the article has been presented at the ORIGINS conference in Montpellier in july 2011 [2] and has since been published in Origins of Life and Evolution of Biospheres 42 (4) 307-316, 2012. 
DOI: 10.1007/s11084-012-9290-5 


Residue network in protein native structure belongs to the universality class of three dimensional critical percolation cluster
A single protein molecule is regarded as a contact network of amino-acid
residues. Some studies have indicated that this network is a small world
network (SWN), while other results have implied that this is a fractal network
(FN). However, SWN and FN are essentially different in the dependence of the
shortest path length on the number of nodes. In this paper, we investigate this
dependence in the residue contact networks of proteins in native structures,
and show that the networks are not SWN but FN. FN is generally characterized by
several dimensions. Among them, we focus on three dimensions; the network
topological dimension , the fractal dimension , and the spectral
dimension . We find that proteins universally yield ,
and . These values are in surprisingly good
coincidence with those in three dimensional critical percolation cluster. Hence
the residue contact networks in the protein native structures belong to the
universality class of three dimensional percolation cluster. The criticality is
relevant to the ambivalent nature of the protein native structures, i.e., the
coexistence of stability and instability, both of which are necessary for a
protein to function as a molecular machine or an allosteric enzyme.Comment: 4 pages, 3 figure
Study of the Bipolar Nebula IRAS 19312+1950. II. Circumstellar Chemistry
The bipolar nebula, IRAS 19312+1950, is a unique SiO maser source exhibiting
both properties as young and evolved objects. To clarify the nature of this
object, we made molecular line observations with the Nobeyama 45-m radio
telescope. We detected emission from O-bearing (HCO^+, SiO, SO, and SO_2), C-
and N-bearing molecules (CN, CS, HCN, HNC, NH_3, N_2H^+, HC_3N, H_2CS, and
CH_3OH), and their isotopic species (C^17O, ^13C^18O, and C^34S). Line profiles
consist of a weak broad (Dv 30 km/s) and/or a strong narrow (DV< 5 km/s)
components, depending on molecular species. Strong time variations of H_2O and
SiO masers were also observed. Numerical modeling of the envelope with the
LVG-code results in a good fit of the model with a mass loss rate of 2.6 10^-4
M_sun/yr to the observed intensities for the broad-component lines. On the
other hand, non-O-bearing molecules, which have only the narrow profiles, were
found to have abundances typical to those in cool dust clouds. No isotopic
enrichment was found, indicating little evidence of the narrow cool component
being an ejecta of the central AGB star or a possible companion. These facts
compelled us to conclude that IRAS 19312+1950 is an exotic mass-losing evolved
star embedded in a low-mass (~20 M_sun) dark cloud.Comment: 13 pages, 7 figures, high res. figure available at
http://www.nro.nao.ac.jp/library/report/list.html (No. 614). PASJ Dec. 25
issue in pres
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