9,277 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
Bose Einstein Condensation in solid 4He
We have computed the one--body density matrix rho_1 in solid 4He at T=0 K
using the Shadow Wave Function (SWF) variational technique. The accuracy of the
SWF has been tested with an exact projector method. We find that off-diagonal
long range order is present in rho_1 for a perfect hcp and bcc solid 4He for a
range of densities above the melting one, at least up to 54 bars. This is the
first microscopic indication that Bose Einstein Condensation (BEC) is present
in perfect solid 4He. At melting the condensate fraction in the hcp solid is
5*10^{-6} and it decreases by increasing the density. The key process giving
rise to BEC is the formation of vacancy--interstitial pairs. We also present
values for Leggett's upper bound on the superfluid fraction deduced from the
exact local density.Comment: 4 pages, 3 figures, accepted for publication as a Rapid Communication
in Physical Review
BeppoSAX observations of the black hole candidates LMC X-1 and LMC X-3
We describe BeppoSAX observations of the black hole candidates LMC X--1 and
LMC X--3 performed in Oct. 1997. Both sources can be modelled by a multicolor
accretion disk spectrum, with temperature keV. However, there is some
evidence that a thin emitting component coexists with the thick disk at these
temperatures. In the direction of LMC X--1, we detected a significant emission
above 10 keV, which we suspect originates from the nearby source PSR 0540-69.
For LMC X--1, we estimate an absorbing column density of cm, which is almost ten times larger than that found for LMC
X--3. In both sources, we find no indication of emission or absorption features
whatsoever.Comment: 4 pages, 2 figures. Accepted for pubblication in the Proc. of 32nd
Cospar scientific assembly, Nagoya, 13-15 July 199
Quantum dislocations: the fate of multiple vacancies in two dimensional solid 4He
Defects are believed to play a fundamental role in the supersolid state of
4He. We have studied solid 4He in two dimensions (2D) as function of the number
of vacancies n_v, up to 30, inserted in the initial configuration at rho =
0.0765 A^-2, close to the melting density, with the exact zero temperature
Shadow Path Integral Ground State method. The crystalline order is found to be
stable also in presence of many vacancies and we observe two completely
different regimes. For small n_v, up to about 6, vacancies form a bound state
and cause a decrease of the crystalline order. At larger n_v, the formation
energy of an extra vacancy at fixed density decreases by one order of magnitude
to about 0.6 K. In the equilibrated state it is no more possible to recognize
vacancies because they mainly transform into quantum dislocations and
crystalline order is found almost independent on how many vacancies have been
inserted in the initial configuration. The one--body density matrix in this
latter regime shows a non decaying large distance tail: dislocations, that in
2D are point defects, turn out to be mobile, their number is fluctuating, and
they are able to induce exchanges of particles across the system mainly
triggered by the dislocation cores. These results indicate that the notion of
incommensurate versus commensurate state loses meaning for solid 4He in 2D,
because the number of lattice sites becomes ill defined when the system is not
commensurate. Crystalline order is found to be stable also in 3D in presence of
up to 100 vacancies
Excitation spectrum in two-dimensional superfluid ⁴He
In this work we perform an ab-initio study of an ideal two-dimensional sample of
⁴He atoms, a model for
⁴He
films adsorbed on several kinds of substrates. Starting from a realistic hamiltonian we face the microscopic study
of the excitation phonon–roton spectrum of the system at zero temperature. Our approach relies on path integral
ground state Monte Carlo projection methods, allowing to evaluate exactly the dynamical density correlation
functions in imaginary time, and this gives access to the dynamical structure factor of the system S(q, ), containing
information about the excitation spectrum E(q), resulting in sharp peaks in S(q, ). The actual evaluation of
S(q, ) requires the inversion of the Laplace transform in ill-posed conditions, which we face via the genetic inversion
via falsification of theories technique. We explore the full density range from the region of spinodal decomposition
to the freezing density, i.e., 0.0321 Å⁻²
– 0.0658 Å⁻². In particular we follow the density dependence
of the excitation spectrum, focusing on the low-wave vector behavior of E(q), the roton dispersion, the strength
of single quasiparticle peak, Z(q), and the static density response function, (q). As the density increases, the
dispersion E(q) at low-wave vector changes from a superlinear (anomalous dispersion) trend to a sublinear (normal
dispersion) one, anticipating the crystallization of the system; at the same time the maxon–roton structure,
which is barely visible at low density, becomes well developed at high densities and the roton wave vector has a
strong density dependence. Connection is made with recent inelastic neutron scattering results from highly ordered
silica nanopores partially filled with
⁴He
Hydrogen induced optically-active defects in silicon photonic nanocavities
This work was supported by Era-NET NanoSci LECSIN project coordinated by F. Priolo, by the Italian Ministry of University and Research, FIRB contract No. RBAP06L4S5 and by the EPSRC UKSp project. Partial financial support by the Norwegian Research Council is also acknowledged.We demonstrate intense room temperature photoluminescence (PL) from optically active hydrogen- related defects incorporated into crystalline silicon. Hydrogen was incorporated into the device layer of a silicon on insulator (SOI) wafer by two methods: hydrogen plasma treatment and ion implantation. The room temperature PL spectra show two broad PL bands centered at 1300 and 1500 nm wavelengths: the first one relates to implanted defects while the other band mainly relates to the plasma treatment. Structural characterization reveals the presence of nanometric platelets and bubbles and we attribute different features of the emission spectrum to the presence of these different kind of defects. The emission is further enhanced by introducing defects into photonic crystal (PhC) nanocavities. Transmission electron microscopy analyses revealed that the isotropicity of plasma treatment causes the formation of a higher defects density around the whole cavity compared to the ion implantation technique, while ion implantation creates a lower density of defects embedded in the Si layer, resulting in a higher PL enhancement. These results further increase the understanding of the nature of optically active hydrogen defects and their relation with the observed photoluminescence, which will ultimately lead to the development of intense and tunable crystalline silicon light sources at room temperature.Publisher PDFPeer reviewe
Study of solid 4He in two dimensions. The issue of zero-point defects and study of confined crystal
Defects are believed to play a fundamental role in the supersolid state of
4He. We report on studies by exact Quantum Monte Carlo (QMC) simulations at
zero temperature of the properties of solid 4He in presence of many vacancies,
up to 30 in two dimensions (2D). In all studied cases the crystalline order is
stable at least as long as the concentration of vacancies is below 2.5%. In the
2D system for a small number, n_v, of vacancies such defects can be identified
in the crystalline lattice and are strongly correlated with an attractive
interaction. On the contrary when n_v~10 vacancies in the relaxed system
disappear and in their place one finds dislocations and a revival of the
Bose-Einstein condensation. Thus, should zero-point motion defects be present
in solid 4He, such defects would be dislocations and not vacancies, at least in
2D. In order to avoid using periodic boundary conditions we have studied the
exact ground state of solid 4He confined in a circular region by an external
potential. We find that defects tend to be localized in an interfacial region
of width of about 15 A. Our computation allows to put as upper bound limit to
zero--point defects the concentration 0.003 in the 2D system close to melting
density.Comment: 17 pages, accepted for publication in J. Low Temp. Phys., Special
Issue on Supersolid
Pycnogonids (Arthropoda: Pycnogonida) of Portofino, Ligurian Sea (North-Western Mediterranean Sea)
Pycnogonida collected monthly from September 2017 to August 2018 in the Portofino Marine Protected Area at
0\u20135 m depth were studied. A total of 499 specimens were collected, 457 of which were identified to species level. These
were classified as belonging to 10 species: Achelia echinata*, Ascorhynchus castelli, Neotrygaeus communis*, Tanystylum
conirostre*, Anoplodactylus angulatus, A. petiolatus, A. pygmaeus*, A. virescens, Callipallene phantoma and C. tiberi*. For
five dominant species (those marked with an asterisk) the annual phenology was outlined. Four hundred and seventeen
additional specimens, collected from the same area and depth range mainly during the 1970s and 1980s were identified to
species level for completeness of information, leading to the addition of Pycnogonum pusillum and Endeis spinosa
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