15,281 research outputs found
Bose Einstein Condensation of incommensurate solid 4He
It is pointed out that simulation computation of energy performed so far
cannot be used to decide if the ground state of solid 4He has the number of
lattice sites equal to the number of atoms (commensurate state) or if it is
different (incommensurate state). The best variational wave function, a shadow
wave function, gives an incommensurate state but the equilibrium concentration
of vacancies remains to be determined. In order to investigate the presence of
a supersolid phase we have computed the one--body density matrix in solid 4He
for the incommensurate state by means of the exact Shadow Path Integral Ground
State projector method. We find a vacancy induced Bose Einstein condensation of
about 0.23 atoms per vacancy at a pressure of 54 bar. This means that bulk
solid 4He is supersolid at low enough temperature if the exact ground state is
incommensurate.Comment: 5 pages, 2 figure
The electronic structure of liquid water within density functional theory
In the last decade, computational studies of liquid water have mostly
concentrated on ground state properties. However recent spectroscopic
measurements have been used to infer the structure of water, and the
interpretation of optical and x-ray spectra requires accurate theoretical
models of excited electronic states, not only of the ground state. To this end,
we investigate the electronic properties of water at ambient conditions using
ab initio density functional theory within the generalized gradient
approximation (DFT/GGA), focussing on the unoccupied subspace of Kohn-Sham
eigenstates. We generate long (250 ps) classical trajectories for large
supercells, up to 256 molecules, from which uncorrelated configurations of
water molecules are extracted for use in DFT/GGA calculations of the electronic
structure. We find that the density of occupied states of this molecular liquid
is well described with 32 molecule supercells using a single k-point (k = 0) to
approximate integration over the first Brillouin zone. However, the description
of the density of unoccupied states (u-EDOS) is sensitive to finite size
effects. Small, 32 molecule supercell calculations, using Gamma-the point
approximation, yield a spuriously isolated state above the Fermi level.
Nevertheless, the more accurate u-EDOS of large, 256 molecule supercells may be
reproduced using smaller supercells and increased k-point sampling. This
indicates that the electronic structure of molecular liquids like water is
relatively insensitive to the long-range disorder in the molecular structure.
These results have important implications for efficiently increasing the
accuracy of spectral calculations for water and other molecular liquids.Comment: 12 pages, 11 figures (low quality) Submitted to JChemPhy
Band structure and optical properties of opal photonic crystals
A theoretical approach for the interpretation of reflectance spectra of opal
photonic crystals with fcc structure and (111) surface orientation is
presented. It is based on the calculation of photonic bands and density of
states corresponding to a specified angle of incidence in air. The results
yield a clear distinction between diffraction in the direction of light
propagation by (111) family planes (leading to the formation of a stop band)
and diffraction in other directions by higher-order planes (corresponding to
the excitation of photonic modes in the crystal). Reflectance measurements on
artificial opals made of self-assembled polystyrene spheres are analyzed
according to the theoretical scheme and give evidence of diffraction by
higher-order crystalline planes in the photonic structure.Comment: to appear in PR
Sub-structure formation in starless cores
Motivated by recent observational searches of sub-structure in starless
molecular cloud cores, we investigate the evolution of density perturbations on
scales smaller than the Jeans length embedded in contracting isothermal clouds,
adopting the same formalism developed for the expanding Universe and the solar
wind. We find that initially small amplitude, Jeans-stable perturbations
(propagating as sound waves in the absence of a magnetic field), are amplified
adiabatically during the contraction, approximately conserving the wave action
density, until they either become nonlinear and steepen into shocks at a time
, or become gravitationally unstable when the Jeans length
decreases below the scale of the perturbations at a time . We
evaluate analytically the time at which the perturbations enter
the non-linear stage using a Burgers' equation approach, and we verify
numerically that this time marks the beginning of the phase of rapid
dissipation of the kinetic energy of the perturbations. We then show that for
typical values of the rms Mach number in molecular cloud cores, is
smaller than , and therefore density perturbations likely dissipate
before becoming gravitational unstable. Solenoidal modes grow at a faster rate
than compressible modes, and may eventually promote fragmentation through the
formation of vortical structures.Comment: 8 pages, 4 figure
Change in format, register and narration style in the biomedical literature: A 1948 example
Scientific communication has evolved over time and the formats of scientific writing, including its stylistic modules, have changed accordingly. Research articles from the past fit a research world that had not been taken over by the internet, electronic searches, the new media and even the science mass production of today and reflect a reality where scientific publications were designed to be read and appreciated by actual readers. It is therefore useful to have a look back to what science looked like in the past and examine the biomedical literature from older archives because several features of those publications may actually harbor vital insights for today's communication. Maintaining a vivid awareness of the evolution of science language and modalities of communication may ensure a better and steadfast progression and ameliorate academic writing in the years to come. With this goal in mind, the present commentary set out to review a 1948 scientific report by I.L. Bennett Jr, entitled "A study on the relationship between the fevers caused by bacterial pyrogens and by the intravenous injection of the sterile exudates of acute inflammation", which appeared in the Journal of Experimental Medicine in September 1948
- …