10,116 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
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
Novel substrates for Helium adsorption: Graphane and Graphene-Fluoride
The discovery of fullerenes has stimulated extensive exploration of the
resulting behavior of adsorbed films. Our study addresses the planar substrates
graphene-fluoride (GF) and graphane (GH) in comparison to graphene. We present
initial results concerning the potential energy, energy bands and low density
behavior of 4He and 3He films on such different surfaces. For example, while
graphene presents an adsorption potential that is qualitatively similar to that
on graphite, GF and GH yield potentials with different symmetry, a number of
adsorption sites double that on graphene/graphite and a larger corrugation for
the adatom. In the case of GF, the lowest energy band width is similar to that
on graphite but the He atom has a significantly larger effective mass and the
adsorption energy is about three time that on graphite. Implications concerning
the monolayer phase diagram of 4He are explored with the exact path integral
ground state method. A commensurate ordered state similar to the sqrt{3} x
sqrt{3} R30^o state on graphite is found the be unstable both on GF and on GH.
The ground states of submonolayer 4He on both GF and GH are superfluids with a
Bose Einstein condensate fraction of about 10%.Comment: 6 pages, 3 figures, LT26 proceedings, accepted for publication in
Journal of Physics: Conference Serie
Exciton polaritons in two-dimensional photonic crystals
Experimental evidence of strong coupling between excitons confined in a
quantum well and the photonic modes of a two-dimensional dielectric lattice is
reported. Both resonant scattering and photoluminescence spectra at low
temperature show the anticrossing of the polariton branches, fingerprint of
strong coupling regime. The experiments are successfully interpreted in terms
of a quantum theory of exciton-photon coupling in the investigated structure.
These results show that the polariton dispersion can be tailored by properly
varying the photonic crystal lattice parameter, which opens the possibility to
obtain the generation of entangled photon pairs through polariton stimulated
scattering.Comment: 5 pages, 4 figure
Total energy global optimizations using non orthogonal localized orbitals
An energy functional for orbital based calculations is proposed, which
depends on a number of non orthogonal, localized orbitals larger than the
number of occupied states in the system, and on a parameter, the electronic
chemical potential, determining the number of electrons. We show that the
minimization of the functional with respect to overlapping localized orbitals
can be performed so as to attain directly the ground state energy, without
being trapped at local minima. The present approach overcomes the multiple
minima problem present within the original formulation of orbital based
methods; it therefore makes it possible to perform calculations for an
arbitrary system, without including any information about the system bonding
properties in the construction of the input wavefunctions. Furthermore, while
retaining the same computational cost as the original approach, our formulation
allows one to improve the variational estimate of the ground state energy, and
the energy conservation during a molecular dynamics run. Several numerical
examples for surfaces, bulk systems and clusters are presented and discussed.Comment: 24 pages, RevTex file, 5 figures available upon reques
In vitro assessment of proinflammatory and genotoxicological effects of wood combustion-generated ultrafine particles
Wavelet decomposition and advanced denoising techniquesn for analysis and classification of seismic signals
This work describes an automatic classification procedure for seismic signals
suitable for the analysis of complex, broad-band waveforms commonly
associated with fluid-rock interaction in volcanic and hydrothermal systems.
Based on Discrete Wavelet Transform, a set of significant seismic signal
features that characterize the type of event is identified (e.g. noise, volcano
tectonic, long period). These features are initially assessed for events whose
category (class) can be previously determined by an expert analyst. A Bayesian
Pattern Recognition supervised technique based on these features is adopted
to classify a new ‘unlabelled pattern’, whose class is unknown. In this way
values computed for known events are used to classify events of unknown
identity ('supervised classification'). A test was performed on seismological data
recorded at Campi Flegrei (Italy), which was divided into three classes.
Automatic classification accuracy ranges from 82% to 100% over a broad range
of datasets
ELECTRICAL STIMULATION OF THE TRICEPS SURAE FOR MUSCULAR STRENGTH IMPROVEMENT IN VOLLEYBALL PLAYERS.
INTRODUCTION
Beside the studies carried out at "CentrO Scoliosi" of Istituto Ortopedico "Gaetano Pini" about idiopathic scoliosis pathogenesis (Dacquino et al., 1970) and its treatment using the electrical stimulation (Crivellini and Divieti, 1972, 1975; Crivellini et al., 1976, 1979; Divieti, 1979), an experiment was carried out to analyze the muscular strength increase, obtained by means of the same stimulation procedure for the scoliotic patients. The triceps surae muscle was chosen because of its size and accessibility. The electrical stimulation was performed on a group of subjects not involved in any sporting activity by Caiulo et al., 1980b. This study has shown that electrical stimulation increased the maximum spontaneous force (J 1.9%), the jumping index (23.7%) measured with the Abalakov method (Kots and Xvilon, 1971; Kots et al., 1971), and the maximum limb circumference (6%) measured near the belly of the stimulated muscle.
Moreover this experience has shown that the stimulation of the whole gastrocnemius muscle, with well tolerable current values, causes not only the contraction of the triceps surae muscle but also the contraction of the posterior tibialis muscle, normally quite difficult to stimulate because it is deeply positioned under the soleus muscle. In this work the force of the triceps surae was transduced by means of a particular experimental device. The force of this muscle is measurable considering the action of the foot during the flexion movement. The same experiment was repeated on a group of athletes of a volleyball team in order to compare the results. It is important to note that the electrical stimulation first recruits the big superficial motor units that are the last activated in maximal voluntary contractions
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