210 research outputs found
Phase diagram of a model for 3He-4He mixtures in three dimensions
A lattice model of 3He - 4He mixtures which takes into account the continuous
rotational symmetry O(2) of the superfluid degrees of freedom of 4He is studied
in the molecular-field approximation and by Monte Carlo simulations in three
dimensions. In contrast to its two-dimensional version, for reasonable values
of the interaction parameters the resulting phase diagram resembles that
observed experimentally for 3He - 4He mixtures, for which phase separation
occurs as a consequence of the superfluid transition. The corresponding
continuum Ginzburg-Landau model with two order parameters describing 3He- 4He
mixtures near tricriticality is derived from the considered lattice model. All
coupling constants appearing in the continuum model are explicitly expressed in
terms of the mean concentration of 4He, the temperature, and the microscopic
interaction parameters characterizing the lattice system.Comment: 32 pages, 12 figures, submitted to the Phys. Rev.
Effects of Confinement on Critical Adsorption: Absence of Critical Depletion for Fluids in Slit Pores
The adsorption of a near-critical fluid confined in a slit pore is
investigated by means of density functional theory and by Monte Carlo
simulation for a Lennard-Jones fluid. Our work was stimulated by recent
experiments for SF_6 adsorbed in a mesoporous glass which showed the striking
phenomenon of critical depletion, i.e. the adsorption excess "Gamma" first
increases but then decreases very rapidly to negative values as the bulk
critical temperature T_c is approached from above along near-critical
isochores. By contrast, our density functional and simulation results, for a
range of strongly attractive wall-fluid potentials, show Gamma monotonically
increasing and eventually saturating as the temperature is lowered towards T_c
along both the critical (rho=rho_c) and sub-critical isochores (rho<\rho_c).
Such behaviour results from the increasingly slow decay of the density profile
away from the walls, into the middle of the slit, as T->T_c. For rho < rho_c we
find that in the fluid the effective bulk field, which is negative and which
favours desorption, is insufficient to dominate the effects of the surface
fields which favour adsorption. We compare this situation with earlier results
for the lattice gas model with a constant (negative) bulk field where critical
depletion was found. Qualitatively different behaviour of the density profiles
and adsorption is found in simulations for intermediate and weakly attractive
wall-fluid potentials but in no case do we observe the critical depletion found
in experiments. We conclude that the latter cannot be accounted for by a single
pore model.Comment: 21 pages Revtex. Submitted to Phys. Rev.
Critical Casimir interactions around the consolute point of a binary solvent
Spatial confinement of a near-critical medium changes its fluctuation
spectrum and modifies the corresponding order parameter distribution. These
effects result in effective, so-called critical Casimir forces (CCFs) acting on
the confining surfaces. These forces are attractive for like boundary
conditions of the order parameter at the opposing surfaces of the confinement.
For colloidal particles dissolved in a binary liquid mixture acting as a
solvent close to its critical point of demixing, one thus expects the emergence
of phase segregation into equilibrium colloidal liquid and gas phases. We
analyze how such phenomena occur asymmetrically in the whole thermodynamic
neighborhood of the consolute point of the binary solvent. By applying
field-theoretical methods within mean-field approximation and the
semi-empirical de Gennes-Fisher functional, we study the CCFs acting between
planar parallel walls as well as between two spherical colloids and their
dependence on temperature and on the composition of the near-critical binary
mixture. We find that for compositions slightly poor in the molecules
preferentially adsorbed at the surfaces, the CCFs are significantly stronger
than at the critical composition, thus leading to pronounced colloidal
segregation. The segregation phase diagram of the colloid solution following
from the calculated effective pair potential between the colloids agrees
surprisingly well with experiments and simulations
Current-mediated synchronization of a pair of beating non-identical flagella
The basic phenomenology of experimentally observed synchronization (i.e., a
stochastic phase locking) of identical, beating flagella of a biflagellate alga
is known to be captured well by a minimal model describing the dynamics of
coupled, limit-cycle, noisy oscillators (known as the noisy Kuramoto model). As
demonstrated experimentally, the amplitudes of the noise terms therein, which
stem from fluctuations of the rotary motors, depend on the flagella length.
Here we address the conceptually important question which kind of synchrony
occurs if the two flagella have different lengths such that the noises acting
on each of them have different amplitudes. On the basis of a minimal model,
too, we show that a different kind of synchrony emerges, and here it is
mediated by a current carrying, steady-state; it manifests itself via
correlated "drifts" of phases. We quantify such a synchronization mechanism in
terms of appropriate order parameters and - for an ensemble of
trajectories and for a single realization of noises of duration ,
respectively. Via numerical simulations we show that both approaches become
identical for long observation times . This reveals an ergodic
behavior and implies that a single-realization order parameter is
suitable for experimental analysis for which ensemble averaging is not always
possible.Comment: 10 pages, 2 figure
Opheliidae (Polychaeta) recolectados por el BIO Hero y las campañas del USNS Eltanin en los océanos meridionales y sudamérica
Opheliid polychaetes collected by the United States Antarctic Research Program and additional material from South America were made available for study through the Smithsonian Institution, Washington, D.C., USA. The Opheliidae from Antarctic seas were treated by Hartman (1966, 1967, 1978), who summarized earlier work and also described several new species. Of the five genera she recorded, two (Travisia and Kesun) have since been synonymized and one, Ammotrypane, has been referred to Ophelina; Hartman’s records therefore encompass four genera, three of which are represented in the present collections. No representatives of Euzonus (Thoracophelia) were found. The following species were recorded from Antarctica in the present study: Ophelia algida n. sp., Ophelina breviata (Ehlers, 1913), Ophelina cf. cylindricaudata (sensu Hartmann-Schröder and Rosenfeldt, 1989), Ophelina nematoides (Ehlers, 1913), Ophelina scaphigera (Ehlers, 1900), Ophelina cf. setigera (Hartman, 1978), Ophelina syringopyge (Ehlers, 1901), Travisia antarctica Hartman, 1967 (emended 1978), Travisia kerguelensis McIntosh, 1885, Travisia kerguelensis gravieri Monro, 1930, Travisia palmeri n. sp. and Travisia tincta n. sp. The new species of Ophelia is distinguished by having branchiae starting on chaetiger 2 (rather than on or after chaetiger 8 as in most species of this genus). Travisia palmeri has 22 chaetigers and is distinguished by long branchiae. Travisia kerguelensis gravieri is raised to full species status and renamed T. monroi. Travisia tincta n. sp., distinguished by a distinctive Methyl Green staining pattern and long branchiae, is described from off Peru.Los poliquetos ofélidos objeto del presente estudio corresponden al material recolectado por el Programa de Investigación Antártico de los Estados Unidos así como material adicional procedente de Sudamérica. Este material ha sido puestos a disposición de los autores por la Institución Smithsoniana, Washington, D.C., Estados Unidos de América. Los Opheliidae de los mares antárticos fueron previamente estudiados por Hartman (1966, 1967, 1978), quien reunió lo publicado hasta el momento y describió nuevas especies. De los cinco géneros registrados por ella, dos (Travisia y Kesun) han sido desde entonces sinonimizados y uno, Ammotrypane, ha sido incluido en Ophelina. Así, los registros de Hartman incluyen cuatro géneros, tres de los cuales están representados en las colecciones estudiadas. No se han encontrados representantes de Euzonus (Thoracophelia). En el presente estudio se registran las siguientes especies de la Antártida: Ophelia algida n. sp., Ophelina breviata (Ehlers, 1913), Ophelina cf. cylindricaudata (sensu Hartmann-Schröder and Rosenfeldt, 1989), Ophelina nematoides (Ehlers, 1913), Ophelina scaphigera (Ehlers, 1900), Ophelina cf. setigera (Hartman, 1978), Ophelina syringopyge (Ehlers, 1901), Travisia antarctia Hartman, 1967 (enmendada 1978), Travisia kerguelensis McIntosh, 1885, Travisia kerguelensis gravieri Monro, 1930, Travisia palmeri n. sp. y Travisia tincta n. sp. La nueva especie de Ophelia se caracteriza por presentar bránquias a partir del chaetígero 2 (y no a partir del chaetigero 8-11 como ocurre en la mayoria de las especies de este género). Travisia palmeri posee 22 chaetigeros y se caracteriza por sus largas bránquias. Travisia kerguelensis gravieri se eleva a rango de especie y se renombra como T. monroi. Travisia tincta n. sp., caracterizada por sus largas bránquias y un peculiar patrón al ser teñida, se describe de las costas de Perú.  
Sensitivity of the thermodynamics of two-dimensional systems towards the topological classes of their surfaces
Using Monte Carlo simulations we study the two-dimensional Ising model on
triangular, square, and hexagonal lattices with various topologies. We focus on
the behavior of the magnetic susceptibility and of the specific heat near the
critical point of the planar bulk system. We find that scaling functions of
these quantities on the spherical surface (Euler characteristic K = 2) differ
from the scaling functions on the projective plane (K = 1) which, in turn,
differ from the scaling functions on the torus and on the Klein bottle (both K
= 0). This provides strong evidence that phase transitions of the Ising model
on two-dimensional surfaces depend on their topologies.Comment: 25 pages, 10 figure
Dip coating process: Silicon sheet growth development for the large-area silicon sheet task of the low-cost silicon solar array project
The research program to investigate the technical and economic feasibility of producing solar-cell-quality sheet silicon by dip-coating one surface of carbonized ceramic substrates with a thin layer of large-grain polycrystalline silicon is reported. The initial effort concentrated on the design and construction of the experimental dip-coating facility. The design was completed and its experimental features are discussed. Current status of the program is reported, including progress toward solar cell junction diffusion and miscellaneous ceramic substrate procurement
Improved detectivity of pyroelectric detectors
High detectivity single-element SBN pyroelectric detectors were fabricated. The theory and technology developments related to improved detector performance were identified and formulated. Improved methods of material characterization, thinning, mounting, blackening and amplifier matching are discussed. Detectors with detectivities of 1.3 x 10 to the 9th power square root of Hz/watt at 1 Hz are reported. Factors limiting performance and recommendations for future work are discussed
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