1,077 research outputs found
Gas phase elemental abundances in molecular cloudS (GEMS): III. Unlocking the CS chemistry: The CS+O reaction
ArtÃculo escrito por un elevado número de autores, solo se referencia el que aparece en primer lugar, el nombre del grupo de colaboración, si lo hubiere, y los autores pertenecientes a la UAMContext. Carbon monosulphide (CS) is among the most abundant gas-phase S-bearing molecules in cold dark molecular clouds. It is easily observable with several transitions in the millimeter wavelength range, and has been widely used as a tracer of the gas density in the interstellar medium in our Galaxy and external galaxies. However, chemical models fail to account for the observed CS abundances when assuming the cosmic value for the elemental abundance of sulfur. Aims. The CS+O → CO + S reaction has been proposed as a relevant CS destruction mechanism at low temperatures, and could explain the discrepancy between models and observations. Its reaction rate has been experimentally measured at temperatures of 150-400 K, but the extrapolation to lower temperatures is doubtful. Our goal is to calculate the CS+O reaction rate at temperatures <150 K which are prevailing in the interstellar medium. Methods. We performed ab initio calculations to obtain the three lowest potential energy surfaces (PES) of the CS+O system. These PESs are used to study the reaction dynamics, using several methods (classical, quantum, and semiclassical) to eventually calculate the CS + O thermal reaction rates. In order to check the accuracy of our calculations, we compare the results of our theoretical calculations for T ~ 150-400 K with those obtained in the laboratory. Results. Our detailed theoretical study on the CS+O reaction, which is in agreement with the experimental data obtained at 150-400 K, demonstrates the reliability of our approach. After a careful analysis at lower temperatures, we find that the rate constant at 10 K is negligible, below 10-15 cm3 s-1, which is consistent with the extrapolation of experimental data using the Arrhenius expression. Conclusions. We use the updated chemical network to model the sulfur chemistry in Taurus Molecular Cloud 1 (TMC 1) based on molecular abundances determined from Gas phase Elemental abundances in Molecular CloudS (GEMS) project observations. In our model, we take into account the expected decrease of the cosmic ray ionization rate, ζH2, along the cloud. The abundance of CS is still overestimated when assuming the cosmic value for the sulfur abundanceThe research leading to these results has received funding from MICIU (Spain) under grants FIS2017-83473-C2, AYA2016-75066- C2-2-P, ESP2017-86582-C4-1-R, AYA2017-85111-P, PID2019-105552RB-C41 and PID2019-106235GB-I00. N.B. acknowledges the computing facilities by TUBITAK-TRUBA, and O.R. and A.A. acknowledge computing time at Finisterre (CESGA) and Marenostrum (BSC) under RES computational grants ACCT-2019-3-0004 and AECT-2020-1-0003. SPTM acknowledges the European Union’s Horizon 2020 research and innovation program for funding support under agreement No 639459 (PROMISE
d_{x^2-y^2} Symmetry and the Pairing Mechanism
An important question is if the gap in the high temperature cuprates has
d_{x^2-y^2} symmetry, what does that tell us about the underlying interaction
responsible for pairing. Here we explore this by determining how three
different types of electron-phonon interactions affect the d_{x^2-y^2} pairing
found within an RPA treatment of the 2D Hubbard model. These results imply that
interactions which become more positive as the momentum transfer increases
favor d_{x^2-y^2} pairing in a nearly half-filled band.Comment: 9 pages and 2 eps figs, uses revtex with epsf, in press, PR
Theory of spin and charge fluctuations in the Hubbard model
A self-consistent theory of both spin and charge fluctuations in the Hubbard
model is presented. It is in quantitative agreement with Monte Carlo data at
least up to intermediate coupling . It includes both
short-wavelength quantum renormalization effects, and long-wavelength thermal
fluctuations which can destroy long-range order in two dimensions. This last
effect leads to a small energy scale, as often observed in high temperature
superconductors. The theory is conserving, satisfies the Pauli principle and
includes three-particle correlations necessary to account for the incipient
Mott transition.Comment: J1K 2R1 10 pages, Revtex 3.0, 4 uuencoded postscript figures, report#
CRPS-93-4
Anisotropy on the Fermi Surface of the Two-Dimensional Hubbard Model
We investigate anisotropic charge fluctuations in the two-dimensional Hubbard
model at half filling. By the quantum Monte Carlo method, we calculate a
momentum-resolved charge compressibility , which shows effects of an infinitesimal doping. At the temperature
, shows peak structure at the points along the line. A similar peak
structure is reproduced in the mean-filed calculation for the d-wave pairing
state or the staggered flux state.Comment: 5 pages, 3 figures, figures and presentation are modifie
Nonlinear Meissner Effect in CuO Superconductors
Recent theories of the NMR in the CuO superconductors are based on a
spin-singlet order parameter. Since this state has nodal lines on
the Fermi surface, nonlinear effects associated with low-energy quasiparticles
become important, particularly at low temperatures. We show that the
field-dependence of the supercurrent, below the nucleation field for vortices,
can be used to locate the positions of the nodal lines of an unconventional gap
in momentum space, and hence test the proposed state.Comment: 5 pages (RevTex), 1 figure (postscript file incl.
d_{x^2-y^2}-wave superconductivity and the Hubbard model
The numerical studies of d_{x^2-y^2}-wave pairing in the two-dimensional (2D)
and the 2-leg Hubbard models are reviewed. For this purpose, the results
obtained from the determinantal Quantum Monte Carlo and the density-matrix
renormalization-group calculations are presented. These are calculations which
were motivated by the discovery of the high-T_c cuprates. In this review, the
emphasis is placed on the microscopic many-body processes which are responsible
for the d_{x^2-y^2}-wave pairing correlations observed in the 2D and the 2-leg
Hubbard models. In order to gain insight into these processes, the results on
the effective pairing interaction as well as the magnetic, density and the
single-particle excitations will be reviewed. In addition, comparisons will be
made with the other numerical approaches to the Hubbard model and the numerical
results on the t-J model. The results reviewed here indicate that an effective
pairing interaction which is repulsive at (pi,pi) momentum transfer and
enhanced single-particle spectral weight near the (pi,0) and (0,pi) points of
the Brillouin zone create optimum conditions for d_{x^2-y^2}-wave pairing.
These are two effects which act to enhance the d_{x^2-y^2}-wave pairing
correlations in the Hubbard model. Finding additional ways is an active
research problem.Comment: 85 pages, 63 figures, to appear in Advances in Physics, vol. 51, no.
6 (2002
Quasiparticle Dispersion of the 2D Hubbard Model: From an Insulator to a Metal
On the basis of Quantum-Monte-Carlo results the evolution of the spectral
weight of the two-dimensional Hubbard model is studied from
insulating to metallic behavior. As observed in recent photoemission
experiments for cuprates, the electronic excitations display essentially
doping-independent features: a quasiparticle-like dispersive narrow band of
width of the order of the exchange interaction and a broad valence- and
conduction-band background. The continuous evolution is traced back to one and
the same many-body origin: the doping-dependent antiferromagnetic spin-spin
correlation.Comment: 11 pages, REVtex, 4 figures (in uuencoded postscript format
One particle spectral weight of the three dimensional single band Hubbard model
Dynamic properties of the three-dimensional single-band Hubbard model are
studied using Quantum Monte Carlo combined with the maximum entropy technique.
At half-filling, there is a clear gap in the density of states and well-defined
quasiparticle peaks at the top (bottom) of the lower (upper) Hubbard band. We
find an antiferromagnetically induced weight above the naive Fermi momentum.
Upon hole doping, the chemical potential moves to the top of the lower band
where a robust peak is observed. Results are compared with spin-density-wave
(SDW) mean-field and self consistent Born approximation results, and also with
the infinite dimensional Hubbard model, and experimental photoemission (PES)
for three dimensional transition-metal oxides.Comment: 11 pages, REVTeX, 16 figures included using psfig.sty. Ref.30
correcte
Randomised field trial to evaluate serological response after foot-and-mouth disease vaccination in Turkey
AbstractDespite years of biannual mass vaccination of cattle, foot-and-mouth disease (FMD) remains uncontrolled in Anatolian Turkey. To evaluate protection after mass vaccination we measured post-vaccination antibodies in a cohort of cattle (serotypes O, A and Asia-1). To obtain results reflecting typical field protection, participants were randomly sampled from across Central and Western Turkey after routine vaccination. Giving two-doses one month apart is recommended when cattle are first vaccinated against FMD. However, due to cost and logistics, this is not routinely performed in Turkey, and elsewhere. Nested within the cohort, we conducted a randomised trial comparing post-vaccination antibodies after a single-dose versus a two-dose primary vaccination course.Four to five months after vaccination, only a third of single-vaccinated cattle had antibody levels above a threshold associated with protection. A third never reached this threshold, even at peak response one month after vaccination. It was not until animals had received three vaccine doses in their lifetime, vaccinating every six months, that most (64% to 86% depending on serotype) maintained antibody levels above this threshold. By this time cattle would be >20 months old with almost half the population below this age. Consequently, many vaccinated animals will be unprotected for much of the year. Compared to a single-dose, a primary vaccination course of two-doses greatly improved the level and duration of immunity. We concluded that the FMD vaccination programme in Anatolian Turkey did not produce the high levels of immunity required. Higher potency vaccines are now used throughout Turkey, with a two-dose primary course in certain areas.Monitoring post-vaccination serology is an important component of evaluation for FMD vaccination programmes. However, consideration must be given to which antigens are present in the test, the vaccine and the field virus. Differences between these antigens affect the relationship between antibody titre and protection
Quasi-particle Lifetimes in a d_{x^2-y^2} Superconductor
We consider the lifetime of quasi-particles in a d-wave superconductor due to
scattering from antiferromagnetic spin-fluctuations, and explicitly separate
the contribution from Umklapp processes which determines the electrical
conductivity. Results for the temperature dependence of the total scattering
rate and the Umklapp scattering rate are compared with relaxation rates
obtained from thermal and microwave conductivity measurements, respectively.Comment: 14 pages, 4 figure
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