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 $Q$ and $Q_{\cal S}$ - for an ensemble of trajectories and for a single realization of noises of duration ${\cal S}$, respectively. Via numerical simulations we show that both approaches become identical for long observation times ${\cal S}$. This reveals an ergodic behavior and implies that a single-realization order parameter $Q_{\cal S}$ 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&rsquo;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-Schr&ouml;der 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 of&eacute;lidos objeto del presente estudio corresponden al material recolectado por el Programa de Investigaci&oacute;n Ant&aacute;rtico de los Estados Unidos as&iacute; como material adicional procedente de Sudam&eacute;rica. Este material ha sido puestos a disposici&oacute;n de los autores por la Instituci&oacute;n Smithsoniana, Washington, D.C., Estados Unidos de Am&eacute;rica. Los Opheliidae de los mares ant&aacute;rticos fueron previamente estudiados por Hartman (1966, 1967, 1978), quien reuni&oacute; lo publicado hasta el momento y describi&oacute; nuevas especies. De los cinco g&eacute;neros registrados por ella, dos (Travisia y Kesun) han sido desde entonces sinonimizados y uno, Ammotrypane, ha sido incluido en Ophelina. As&iacute;, los registros de Hartman incluyen cuatro g&eacute;neros, tres de los cuales est&aacute;n representados en las colecciones estudiadas. No se han encontrados representantes de Euzonus (Thoracophelia). En el presente estudio se registran las siguientes especies de la Ant&aacute;rtida: Ophelia algida n. sp., Ophelina breviata (Ehlers, 1913), Ophelina cf. cylindricaudata (sensu Hartmann-Schr&ouml;der 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 br&aacute;nquias a partir del chaet&iacute;gero 2 (y no a partir del chaetigero 8-11 como ocurre en la mayoria de las especies de este g&eacute;nero). Travisia palmeri posee 22 chaetigeros y se caracteriza por sus largas br&aacute;nquias. Travisia kerguelensis gravieri se eleva a rango de especie y se renombra como T. monroi. Travisia tincta n. sp., caracterizada por sus largas br&aacute;nquias y un peculiar patr&oacute;n al ser te&ntilde;ida, se describe de las costas de Per&uacute;. &nbsp

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