3,765 research outputs found
Dynamic structure factor for 3He in two-dimensions
Recent neutron scattering experiments on 3He films have observed a zero-sound
mode, its dispersion relation and its merging with -and possibly emerging from-
the particle-hole continuum. Here we address the study of the excitations in
the system via quantum Monte Carlo methods: we suggest a practical scheme to
calculate imaginary time correlation functions for moderate-size fermionic
systems. Combined with an efficient method for analytic continuation, this
scheme affords an extremely convincing description of the experimental
findings.Comment: 5 pages, 5 figure
Controle biológico de insetos-praga em frutÃferas de Clima Temperado: uma opção viável, mas desafiadora.
bitstream/item/33948/1/documento-208.pd
Exact ground state Monte Carlo method for Bosons without importance sampling
Generally ``exact'' Quantum Monte Carlo computations for the ground state of
many Bosons make use of importance sampling. The importance sampling is based,
either on a guiding function or on an initial variational wave function. Here
we investigate the need of importance sampling in the case of Path Integral
Ground State (PIGS) Monte Carlo. PIGS is based on a discrete imaginary time
evolution of an initial wave function with a non zero overlap with the ground
state, that gives rise to a discrete path which is sampled via a Metropolis
like algorithm. In principle the exact ground state is reached in the limit of
an infinite imaginary time evolution, but actual computations are based on
finite time evolutions and the question is whether such computations give
unbiased exact results. We have studied bulk liquid and solid 4He with PIGS by
considering as initial wave function a constant, i.e. the ground state of an
ideal Bose gas. This implies that the evolution toward the ground state is
driven only by the imaginary time propagator, i.e. there is no importance
sampling. For both the phases we obtain results converging to those obtained by
considering the best available variational wave function (the Shadow wave
function) as initial wave function. Moreover we obtain the same results even by
considering wave functions with the wrong correlations, for instance a wave
function of a strongly localized Einstein crystal for the liquid phase. This
convergence is true not only for diagonal properties such as the energy, the
radial distribution function and the static structure factor, but also for
off-diagonal ones, such as the one--body density matrix. From this analysis we
conclude that zero temperature PIGS calculations can be as unbiased as those of
finite temperature Path Integral Monte Carlo.Comment: 11 pages, 10 figure
The reaction NH2 + PH3 yields NH3 + PH2: Absolute rate constant measurement and implication for NH3 and PH3 photochemistry in the atmosphere of Jupiter
The rate constant is measured over the temperature interval 218-456 K using the technique of flash photolysis-laser-induced fluorescence. NH2 radicals are produced by the flash photolysis of ammonia highly diluted in argon, and the decay of fluorescent NH2 photons is measured by multiscaling techniques. For each of the five temperatures employed in the study, the results are shown to be independent of variations in PH3 concentration, total pressure (argon), and flash intensity. It is found that the rate constant results are best represented for T between 218 and 456 K by the expression k = (1.52 + or - 0.16) x 10 to the -12th exp(-928 + or - 56/T) cu cm per molecule per sec; the error quoted is 1 standard deviation. This is the first determination of the rate constant for the reaction NH2 + PH3. The data are compared with an estimate made in order to explain results of the radiolysis of NH3-PH3 mixtures. The Arrhenius parameters determined here for NH2 + PH3 are then constrasted with those for the corresponding reactions of H and OH with PH3
Cosmological constraints with GRBs: homogeneous medium vs wind density profile
We present the constraints on the cosmological parameters obtained with the
-- correlation found with the most recent sample of
19 GRBs with spectroscopically measured redshift and well determined prompt
emission spectral and afterglow parameters. We compare our results obtained in
the two possible uniform jet scenarios, i.e. assuming a homogeneous density
profile (HM) or a wind density profile (WM) for the circumburst medium. Better
constraints on and are obtained with the
(tighter) -- correlation derived in the wind density
scenario. We explore the improvements to the constraints of the cosmological
parameters that could be reached with a large sample, 150 GRBs, in the
future. We study the possibility to calibrate the slope of these correlations.
Our optimization analysis suggests that GRBs with redshift
can be used to calibrate the -- with
a precision better than 1%. The same precision is expected for the same number
of bursts with . This result suggests that we do not
necessarily need a large sample of low z GRBs for calibrating the slope of
these correlations.Comment: 7 pages, 7 figures, submitted to A&
Afterglows from precursors in Gamma Ray Bursts. Application to the optical afterglow of GRB 091024
About 15% of Gamma Ray Bursts have precursors, i.e. emission episodes
preceding the main event, whose spectral and temporal properties are similar to
the main emission. We propose that precursors have their own fireball,
producing afterglow emission due to the dissipation of the kinetic energy via
external shock. In the time lapse between the precursor and the main event, we
assume that the central engine is not completely turned off, but it continues
to eject relativistic material at a smaller rate, whose emission is below the
background level. The precursor fireball generates a first afterglow by the
interaction with the external circumburst medium. Matter injected by the
central engine during the "quasi-quiescent" phase replenishes the external
medium with material in relativistic motion. The fireball corresponding to the
main prompt emission episode crashes with this moving material, producing a
second afterglow, and finally catches up and merges with the first precursor
fireball. We apply this new model to GRB 091024, an event with a precursor in
the prompt light curve and two well defined bumps in the optical afterglow,
obtaining an excellent agreement with the existing data.Comment: 11 pages, 6 figures, 3 tables. Accepted for publication in MNRAS,
Main Journa
Ocorrência de Diaphorina citri Kuwayama (Hemiptera: Liviidae) no Estado do Rio Grande do Sul.
O objetivo do trabalho foi determinar os locais de ocorrência de D. citri no Estado do RS e estudar a flutuação populacional da praga no municÃpio de Rosário do Sul
Bioecologia e controle de Anastrepha fraterculus e Ceratitis capitata em pessegueiro.
bitstream/item/34758/1/documento-315.pd
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