699 research outputs found
Ground-state properties and superfluidity of two- and quasi two-dimensional solid 4He
In a recent study we have reported a new type of trial wave function
symmetric under the exchange of particles and which is able to describe a
supersolid phase. In this work, we use the diffusion Monte Carlo method and
this model wave function to study the properties of solid 4He in two- and quasi
two-dimensional geometries. In the purely two-dimensional case, we obtain
results for the total ground-state energy and freezing and melting densities
which are in good agreement with previous exact Monte Carlo calculations
performed with a slightly different interatomic potential model. We calculate
the value of the zero-temperature superfluid fraction \rho_{s} / \rho of 2D
solid 4He and find that it is negligible in all the considered cases, similarly
to what is obtained in the perfect (free of defects) three-dimensional crystal
using the same computational approach. Interestingly, by allowing the atoms to
move locally in the perpendicular direction to the plane where they are
confined to zero-point oscillations (quasi two-dimensional crystal) we observe
the emergence of a finite superfluid density that coexists with the periodicity
of the system.Comment: 16 pages, 8 figure
The biting and predaceous midges of Guadeloupe (Diptera: Ceratopogonidae). I. Species of the subfamily Ceratopogoninae
We provide new records of biting and predaceous midges (Diptera: Ceratopogonidae) from Guadeloupe in the subfamily Ceratopogoninae, including descriptions and illustrations of three new predaceous species in the genera, Parabezzia Malloch, Stilobezzia Kieffer and Palpomyia Meigen, respectively, and the first records of the New World predaceous genus, Amerohelea Grogan and Wirth, from the Caribbean region. We also provide the first Guadeloupe records of the biting midges, Culicoides (Anilomyia) decor (Williston), C. (Avaritia) pusillus Lutz, C. (Drymodesmyia) bredini Wirth and Blanton, C. (D.) poikilonotus Macfie, C. (Haematomyidium) hoffmani Fox, C. (Hoffmania) insignis Lutz, C. rangeli Ortiz and Mirsa and C. trilineatus Fox, and the predaceous midges, Brachypogon (Brachypogon) bifidus Spinelli and Grogan, B. (B.) telesfordi Spinelli and Grogan, B. (B.) woodruffi Spinelli and Grogan, Monohelea maya Felippe-Bauer, Huerta and Ibáñez-Bernal, Stilobezzia (Stilobezzia) diminuta Lane and Forattini, S. (S.) thomsenae Wirth, Amerohelea galindoi Grogan and Wirth, Bezzia (Bezzia) flinti Spinelli and Wirth, B. (Homobezzia) venustula (Williston) and Palpomyia insularis Spinelli and Grogan
Quantum Phase Transition with a Simple Variational Ansatz
We study the zero-temperature quantum phase transition between liquid and hcp
solid helium-4. We use the variational method with a simple yet
exchange-symmetric and fully explicit wavefunction. It is found that the
optimized wavefunction undergoes spontaneous symmetry breaking and describes
the quantum solidification of helium at 22 atm. The explicit form of the
wavefunction allows to consider various contributions to the phase transition.
We find that the employed wavefunction is an excellent candidate for describing
both a first-order quantum phase transition and the ground state of a Bose
solid
Effect of organic loading rate on the production of Polyhydroxyalkanoates from sewage sludge
The aim of this work was to study the effect of organic loading rate on the production of Polyhydroxyalkanoates (PHA) from sewage sludge. Synthesis of PHA using sewage sludge as platform was achieved in this work. Three pilot-scale selection-sequencing batch reactors (S-SBR) were used for obtaining a culture able to accumulate PHA following a strategy of aerobic dynamic feeding (ADF) at different volumetric organic-loading-rate (vOLR): 1.3, 1.8 and 0.8 g COD L-1 d-1 for S-SBR 1, S-SBR 2 and S-SBR 3, respectively. Decreasing the vOLR enhanced the general performance of the process as for organic matter removal (from 99.2% ± 0.3% in S-SBR-3 to 92 ± 2 in S-SBR-2) while the opposite trend was recorded for PHA production (6.0 PHA % w/w in S-SBR-3 vs 13.7 PHA % w/w in S-SBR-2 at the end of the feast phase). Furthermore, indirect and direct emissions, as N2O, were evaluated during the process for the first time. Finally, three accumulation tests were performed achieving 24% w/w
Zero-temperature generalized phase diagram of the 4d transition metals under pressure
We use an accurate implementation of density functional theory (DFT) to
calculate the zero-temperature generalized phase diagram of the 4 series of
transition metals from Y to Pd as a function of pressure and atomic number
. The implementation used is full-potential linearized augmented plane waves
(FP-LAPW), and we employ the exchange-correlation functional recently developed
by Wu and Cohen. For each element, we obtain the ground-state energy for
several crystal structures over a range of volumes, the energy being converged
with respect to all technical parameters to within meV/atom. The
calculated transition pressures for all the elements and all transitions we
have found are compared with experiment wherever possible, and we discuss the
origin of the significant discrepancies. Agreement with experiment for the
zero-temperature equation of state is generally excellent. The generalized
phase diagram of the 4 series shows that the major boundaries slope towards
lower with increasing for the early elements, as expected from the
pressure induced transfer of electrons from states to states, but are
almost independent of for the later elements. Our results for Mo indicate a
transition from bcc to fcc, rather than the bcc-hcp transition expected from
- transfer.Comment: 28 pages and 10 figures. Submitted to Phys. Rev.
Caracterización de la ictiofauna en la Laguna del Monte, provincia de Buenos Aires
The purpose of this study was to describe as well as to characterize the fish populations living in Del Monte lagoon, which belongs to the Encadenadas del Oeste system in the province of Buenos Aires, Argentina. This research was carried out using trammel nets with meshes of six different sizes at three sampling stations during October 2003. The species captured in the area of study were firstly identified. The specific diversity of each fishing site was subsequently estimated. In addition, sex ratio, size structure, and the relative abundance both in number and weight were also determined for each species. Total capture included 647 individuals, which corresponded to the following six species: mojarra Astyanax eigenmanniorum, dientudo Oligosarcus jenynsii, pejerrey Odontesthes bonariensis, carp Cyprinus carpio, sabalito Cyphocharax voga, and catfish Rhamdia quelen. The specific diversity was very similar at the three sampling stations. The mojarra Astyanax eigenmanniorum evidenced the highest relative abundance in number, it was followed by the dientudo Oligosarcus jenynsii and the pejerrey Odontesthes bonariensis. The highest ichthyomass corresponded to the carp Cyprinus carpio. The entrance of the latter to the Del Monte lagoon was corroborated, the highest number of samples was recorded near the outlet of Guaminí and Malleo-Leufú brooks. A marked decrease in the abundance of the pejerrey Odontesthes bonariensis as well as an increase in the number of the dientudo Oligosarcus jenynsii and the mojarra Astyanax eigenmanniorum were also observed.The purpose of this study was to describe as well as to characterize the fish populations living in Del Monte lagoon, which belongs to the Encadenadas del Oeste system in the province of Buenos Aires, Argentina. This research was carried out using trammel nets with meshes of six different sizes at three sampling stations during October 2003. The species captured in the area of study were firstly identified. The specific diversity of each fishing site was subsequently estimated. In addition, sex ratio, size structure, and the relative abundance both in number and weight were also determined for each species. Total capture included 647 individuals, which corresponded to the following six species: mojarra Astyanax eigenmanniorum, dientudo Oligosarcus jenynsii, pejerrey Odontesthes bonariensis, carp Cyprinus carpio, sabalito Cyphocharax voga, and catfish Rhamdia quelen. The specific diversity was very similar at the three sampling stations. The mojarra Astyanax eigenmanniorum evidenced the highest relative abundance in number, it was followed by the dientudo Oligosarcus jenynsii and the pejerrey Odontesthes bonariensis. The highest ichthyomass corresponded to the carp Cyprinus carpio. The entrance of the latter to the Del Monte lagoon was corroborated, the highest number of samples was recorded near the outlet of Guaminí and Malleo-Leufú brooks. A marked decrease in the abundance of the pejerrey Odontesthes bonariensis as well as an increase in the number of the dientudo Oligosarcus jenynsii and the mojarra Astyanax eigenmanniorum were also observed
Thallium under extreme compression
We present a combined theoretical and experimental study of the high-pressure
behavior of thallium. X-ray diffraction experiments have been carried out at
room temperature up to 125 GPa using diamond-anvil cells, nearly doubling the
pressure range of previous experiments. We have confirmed the hcp-fcc
transition at 3.5 GPa and determined that the fcc structure remains stable up
to the highest pressure attained in the experiments. In addition, HP-HT
experiments have been performed up to 8 GPa and 700 K by using a combination of
x-ray diffraction and a resistively heated diamond-anvil cell. Information on
the phase boundaries is obtained, as well as crystallographic information on
the HT bcc phase. The equation of state for different phases is reported. Ab
initio calculations have also been carried out considering several potential
high-pressure structures. They are consistent with the experimental results and
predict that, among the structures considered in the calculations, the fcc
structure of thallium is stable up to 4.3 TPa. Calculations also predict the
post-fcc phase to have a close-packed orthorhombic structure above 4.3 TPa.Comment: 29 pages, 14 figure
Caracterización de la ictiofauna en la Laguna del Monte, provincia de Buenos Aires
The purpose of this study was to describe as well as to characterize the fish populations living in Del Monte lagoon, which belongs to the Encadenadas del Oeste system in the province of Buenos Aires, Argentina. This research was carried out using trammel nets with meshes of six different sizes at three sampling stations during October 2003. The species captured in the area of study were firstly identified. The specific diversity of each fishing site was subsequently estimated. In addition, sex ratio, size structure, and the relative abundance both in number and weight were also determined for each species. Total capture included 647 individuals, which corresponded to the following six species: mojarra Astyanax eigenmanniorum, dientudo Oligosarcus jenynsii, pejerrey Odontesthes bonariensis, carp Cyprinus carpio, sabalito Cyphocharax voga, and catfish Rhamdia quelen. The specific diversity was very similar at the three sampling stations. The mojarra Astyanax eigenmanniorum evidenced the highest relative abundance in number, it was followed by the dientudo Oligosarcus jenynsii and the pejerrey Odontesthes bonariensis. The highest ichthyomass corresponded to the carp Cyprinus carpio. The entrance of the latter to the Del Monte lagoon was corroborated, the highest number of samples was recorded near the outlet of Guaminí and Malleo-Leufú brooks. A marked decrease in the abundance of the pejerrey Odontesthes bonariensis as well as an increase in the number of the dientudo Oligosarcus jenynsii and the mojarra Astyanax eigenmanniorum were also observed.Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) - Instituto de Limnología "Dr. Raul A. Ringuelet" (ILPLA
Caracterización de la ictiofauna en la Laguna del Monte, provincia de Buenos Aires
The purpose of this study was to describe as well as to characterize the fish populations living in Del Monte lagoon, which belongs to the Encadenadas del Oeste system in the province of Buenos Aires, Argentina. This research was carried out using trammel nets with meshes of six different sizes at three sampling stations during October 2003. The species captured in the area of study were firstly identified. The specific diversity of each fishing site was subsequently estimated. In addition, sex ratio, size structure, and the relative abundance both in number and weight were also determined for each species. Total capture included 647 individuals, which corresponded to the following six species: mojarra Astyanax eigenmanniorum, dientudo Oligosarcus jenynsii, pejerrey Odontesthes bonariensis, carp Cyprinus carpio, sabalito Cyphocharax voga, and catfish Rhamdia quelen. The specific diversity was very similar at the three sampling stations. The mojarra Astyanax eigenmanniorum evidenced the highest relative abundance in number, it was followed by the dientudo Oligosarcus jenynsii and the pejerrey Odontesthes bonariensis. The highest ichthyomass corresponded to the carp Cyprinus carpio. The entrance of the latter to the Del Monte lagoon was corroborated, the highest number of samples was recorded near the outlet of Guaminí and Malleo-Leufú brooks. A marked decrease in the abundance of the pejerrey Odontesthes bonariensis as well as an increase in the number of the dientudo Oligosarcus jenynsii and the mojarra Astyanax eigenmanniorum were also observed.Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET) - Instituto de Limnología "Dr. Raul A. Ringuelet" (ILPLA
- …