1,047 research outputs found
Local properties in the two-dimensional t-t'-U model
We have studied the t-t'-U model by means of the composite operator method.
The effect of the bare diagonal hopping term t' that appears to be material
dependent for high-Tc cuprate superconductors is analyzed in detail. In
particular, some local quantities are computed and a comprehensive comparison
with the data by numerical simulations on finite size lattices is presented.
The result show a good agreement with those obtained by Monte Carlo methods.Comment: 3 pages, 3 figures in one Postscript file, RevTeX, to be published in
Physica
The two-dimensional t-t'-U model as a minimal model for cuprate materials
The addition to the Hubbard Hamiltonian of a t' diagonal hopping term, which
is considered to be material dependent for high-Tc cuprate superconductors, is
generally suggested to obtain a model capable to describe the physics of
high-Tc cuprate materials. In this line of thinking, the two-dimensional t-t'-U
model has been studied by means of the Composite Operator Method, which allows
to determine the dynamics in a fully self-consistent way by use of symmetry
requirements, as the ones coming from the Pauli principle. At first, some local
quantities have been calculated to be compared with quantum Monte Carlo data.
Then, the structure of the energy bands, the shape of the Fermi surface and the
position of the van Hove singularity have been computed as functions of the
model parameters and studied by the light of the available experimental data.
The results of our study show that there exists two sets of parameters that
allows the model to describe the relevant features of 1-layer compounds NCCO
and LSCO. On the other hand, for the 2-layer compound YBCO is not possible to
find a reasonable set of parameters which could reproduce the position of the
van Hove singularity as predicted by ARPES experiments. Hence, it results
questionable the existence of an unique model that could properly describe the
variety of cuprate superconductors, as the t-t'-U model was thought to be.Comment: 8 pages, RevTex, 10 Postscript figures, submitted to Phys. Rev. B.
This paper, as many others from the same authors, can be downloaded by
anonymous FTP at ftp://pcalea.csied.unisa.i
Global Distribution of Fe K alpha Lines in the Galactic Center Region Observed with the Suzaku Satellite
We have surveyed spatial profiles of the Fe K lines in the Galactic
center diffuse X-rays (GCDX), including the transient region from the GCDX to
the Galactic ridge X-ray emission (GRXE), with the Suzaku satellite. We
resolved Fe K line complex into three lines of Fe \emissiontype{I}, Fe
\emissiontype{XXV} and Fe \emissiontype{XXVI} K, and obtained their
spatial intensity profiles with the resolution of \sim \timeform{0D.1}. We
compared the Fe \emissiontype{XXV} K profile with a stellar mass
distribution (SMD) model made from near infrared observations. The intensity
profile of Fe \emissiontype{XXV} K is nicely fitted with the SMD model
in the GRXE region, while that in the GCDX region shows
(\timeform{0D.2}<|l|<\timeform{1D.5}) or (|l|<\timeform{0D.2})
times excess over the best-fit SMD model in the GRXE region. Thus Fe
\emissiontype{XXV} K in the GCDX is hardly explained by the same origin
of the GRXE. In the case of point source origin, a new population with the
extremely strong Fe \emissiontype{XXV} K line is required. An
alternative possibility is that the majority of the GCDX is truly diffuse
optically thin thermal plasma.Comment: Accepted by PAS
No X-Ray Excess from the HESS J1741-302 Region except a New Intermediate Polar Candidate
With the Suzaku satellite, we observed an unidentified TeV gamma-ray source
HESS J1741302 and its surroundings. No diffuse or point-like X-ray sources
are detected from the bright southern emission peak of HESS J1741302. From
its neighborhood, we found a new intermediate polar candidate at the position
of (\alpha, \delta)_{\rm J2000.0} = (\timeform{17h40m35.6s},
\timeform{-30D14m16s}), which is designated as Suzaku J174035.6301416. The
spectrum of Suzaku J174035.6301416 exhibits emission lines at the energy of
6.4, 6.7 and 7.0 keV, which can be assigned as the K lines from
neutral, He-like and H-like iron, respectively. A coherent pulsation is found
at a period of 432.1 0.1 s. The pulse profile is quasi-sinusoidal in the
hard X-ray band (48 keV), but is more complicated in the soft X-ray band
(13 keV). The moderate period of pulsation, the energy flux, and the
presence of the iron K lines indicate that Suzaku J174035.6301416 is
likely an intermediate polar, a subclass of magnetized white dwarf binaries
(cataclysmic variables).
Based on these discoveries, we give some implications on the origin of GCDX
and brief comments on HESS J1741302 and PSR B173730.Comment: Accepted by PAS
Chiral Ordering in Supercooled Liquid Water and Amorphous Ice
The emergence of homochiral domains in supercooled liquid water is presented using molecular dynamics simulations. An individual water molecule possesses neither a chiral center nor a twisted conformation that can cause spontaneous chiral resolution. However, an aggregation of water molecules will naturally give rise to a collective chirality. Such homochiral domains possess obvious topological and geometrical orders and are energetically more stable than the average. However, homochiral domains cannot grow into macroscopic homogeneous structures due to geometrical frustrations arising from their icosahedral local order. Homochiral domains are the major constituent of supercooled liquid water and the origin of heterogeneity in that substance, and are expected to be enhanced in low-density amorphous ice at lower temperatures
On the Occurrence of Clathrate Hydrates in Extreme Conditions: Dissociation Pressures and Occupancies at Cryogenic Temperatures with Application to Planetary Systems
We investigate the thermodynamic stability of clathrate hydrates at cryogenic temperatures from the 0 K limit to 200 K in a wide range of pressures, covering the thermodynamic conditions of interstellar space and the surface of the hydrosphere in satellites. Our evaluation of the phase behaviors is performed by setting up quantum partition functions with variable pressures on the basis of a rigorous statistical mechanics theory that requires only the intermolecular interactions as input. Noble gases, hydrocarbons, nitrogen, and oxygen are chosen as the guest species, which are key components of the volatiles in such satellites. We explore the hydrate/water two-phase boundary of those clathrate hydrates in water-rich conditions and the hydrate/guest two-phase boundary in guest-rich conditions, either of which occurs on the surface or subsurface of icy satellites. The obtained phase diagrams indicate that clathrate hydrates can be in equilibrium with either water or the guest species over a wide range far distant from the three-phase coexistence condition and that the stable pressure zone of each clathrate hydrate expands significantly on intense cooling. The implication of our findings for the stable form of water in Titan is that water on the surface exists only as clathrate hydrate with the atmosphere down to a shallow region of the crust, but clathrate hydrate in the remaining part of the crust can coexist with water ice. This is in sharp contrast to the surfaces of Europa and Ganymede, where the thin oxygen air coexists exclusively with pure ice
Lennard-Jones Parameters Determined to Reproduce the Solubility of NaCl and KCl in SPC/E, TIP3P, and TIP4P/2005 Water
Most classical nonpolarizable ion potential models underestimate the solubility values of NaCl and KCl in water significantly. We determine Lennard-Jones parameters of Na+, K+, and Cl– that reproduce the solubility as well as the hydration free energy in dilute aqueous solutions for three water potential models, SPC/E, TIP3P, and TIP4P/2005. The ion–oxygen distance in the solution and the cation–anion distance in salt are also considered in the parametrization. In addition to the target properties, the hydration enthalpy, hydration entropy, self-diffusion coefficient, coordination number, lattice energy, enthalpy of solution, density, viscosity, and number of contact ion pairs are calculated for comparison with 17 frequently used or recently developed ion potential models. The overall performance of each ion model is represented by a global score using a scheme that was originally developed for comparison of water potential models. The global score is better for our models than for the other 17 models not only because of the quite good prediction for the solubility but also because of the relatively small deviation from the experimental value for many of the other properties
{μ-Bis(1-methylÂimidazol-2-yl)methane-κ2 N 3:N 3′}bisÂ{[(1-methylÂimidazol-2-yl)methane-κ2 N 3,N 3′]copper(I)} bisÂ(triÂfluoroÂmethaneÂsulfonate)
The title compound, [Cu2(C9H12N4)3](CF3SO3)2, contains two CuI ions, three bisÂ(1-methylÂimidazol-2-yl)methane (Me2BIM) ligands, and two trifluoromethanesulfonate anions in the asymmetric unit. Each CuI ion has a distorted trigonal-planar geometry and is coordinated by two N atoms from the Me2BIM ligand and another N atom from the Me2BIM that acts as a bridging ligand, another N atom of the bridging Me2BIM being linked to the second CuI ion. The imidazole rings of Me2BIM form intraÂmolecular π–π stacking interÂactions [centroid–centroid distances = 3.445 (2) and 3.547 (2) Å]
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