50 research outputs found
Quasi one dimensional He inside carbon nanotubes
We report results of diffusion Monte Carlo calculations for both He
absorbed in a narrow single walled carbon nanotube (R = 3.42 \AA) and strictly
one dimensional He. Inside the tube, the binding energy of liquid He is
approximately three times larger than on planar graphite. At low linear
densities, He in a nanotube is an experimental realization of a
one-dimensional quantum fluid. However, when the density increases the
structural and energetic properties of both systems differ. At high density, a
quasi-continuous liquid-solid phase transition is observed in both cases.Comment: 11 pages, 3ps figures, to appear in Phys. Rev. B (RC
Preliminary Heat Capacity and Vapor Pressure Measurements of 2D 4He on ZYX Graphite
We report preliminary heat capacity and vapor pressure measurements of the
first and second layers of 4He adsorbed on ZYX graphite. ZYX is known to have
much better crystallinity than Grafoil, the most commonly-used exfoliated
graphite substrate, such as a ten-times larger platelet size. This allows us to
distinguish different phases in 2D helium-4 much more clearly and may provide
qualitatively different insights into this system. We found a significantly
asymmetric density-dependence of the heat-capacity peak associated with the 1/3
phase formation comparing with that obtained with Grafoil. The 2nd-layer
promotion density is determined as 11.8+-0.3 nm-2 from the heat-capacity
measurement of low density samples in the 2nd layer and vapor pressure
measurement.Comment: 7 pages, 7 figures, accepted for publication in JLTP - QFS201
Beyond the binary collision approximation for the large- response of liquid He
We discuss corrections to the linear response of a many-body system beyond
the binary collision approximation. We first derive for smooth pair
interactions an exact expression of the response , considerably
simplifying existing forms and present also the generalization for interactions
with a strong, short-range repulsion. We then apply the latter to the case of
liquid He. We display the numerical influence of the correction
around the quasi-elastic peak and in the low-intensity wings of the response,
far from that peak. Finally we resolve an apparent contradiction in previous
discussions around the fourth order cumulant expansion coefficient. Our results
prove that the large- response of liquid He can be accurately understood
on the basis of a dynamical theory.Comment: 19 p. Figs. available on reques
Two-dimensional clusters of liquid 4-He
The binding energies of two-dimensional clusters (puddles) of He are
calculated in the framework of the diffusion Monte Carlo method. The results
are very well fitted by a mass formula in powers of , where is
the number of particles. The analysis of the mass formula allows for the
extraction of the line tension, which turns out to be 0.121 K/A. Sizes and
density profiles of the puddles are also reported.Comment: 19 pages including 2 figs; revtex4. Matches published version: major
changes in section V: profiles are now calculated with an unbiased estimator.
Conclusions remain unchange
Path Integral Monte Carlo Approach to the U(1) Lattice Gauge Theory in (2+1) Dimensions
Path Integral Monte Carlo simulations have been performed for U(1) lattice
gauge theory in (2+1) dimensions on anisotropic lattices. We extractthe static
quark potential, the string tension and the low-lying "glueball" spectrum.The
Euclidean string tension and mass gap decrease exponentially at weakcoupling in
excellent agreement with the predictions of Polyakov and G{\" o}pfert and Mack,
but their magnitudes are five times bigger than predicted. Extrapolations are
made to the extreme anisotropic or Hamiltonian limit, and comparisons are made
with previous estimates obtained in the Hamiltonian formulation.Comment: 12 pages, 16 figure
Structural and dynamical properties of superfluid helium: a density functional approach
We present a novel density functional for liquid 4He, properly accounting for
the static response function and the phonon-roton dispersion in the uniform
liquid. The functional is used to study both structural and dynamical
properties of superfluid helium in various geometries. The equilibrium
properties of the free surface, droplets and films at zero temperature are
calculated. Our predictions agree closely to the results of ab initio Monte
Carlo calculations, when available. The introduction of a phenomenological
velocity dependent interaction, which accounts for backflow effects, is
discussed. The spectrum of the elementary excitations of the free surface and
films is studied.Comment: 37 pages, REVTeX 3.0, figures on request at [email protected]
Atom lasers: production, properties and prospects for precision inertial measurement
We review experimental progress on atom lasers out-coupled from Bose-Einstein
condensates, and consider the properties of such beams in the context of
precision inertial sensing. The atom laser is the matter-wave analog of the
optical laser. Both devices rely on Bose-enhanced scattering to produce a
macroscopically populated trapped mode that is output-coupled to produce an
intense beam. In both cases, the beams often display highly desirable
properties such as low divergence, high spectral flux and a simple spatial mode
that make them useful in practical applications, as well as the potential to
perform measurements at or below the quantum projection noise limit. Both
devices display similar second-order correlations that differ from thermal
sources. Because of these properties, atom lasers are a promising source for
application to precision inertial measurements.Comment: This is a review paper. It contains 40 pages, including references
and figure
To wet or not to wet: that is the question
Wetting transitions have been predicted and observed to occur for various
combinations of fluids and surfaces. This paper describes the origin of such
transitions, for liquid films on solid surfaces, in terms of the gas-surface
interaction potentials V(r), which depend on the specific adsorption system.
The transitions of light inert gases and H2 molecules on alkali metal surfaces
have been explored extensively and are relatively well understood in terms of
the least attractive adsorption interactions in nature. Much less thoroughly
investigated are wetting transitions of Hg, water, heavy inert gases and other
molecular films. The basic idea is that nonwetting occurs, for energetic
reasons, if the adsorption potential's well-depth D is smaller than, or
comparable to, the well-depth of the adsorbate-adsorbate mutual interaction. At
the wetting temperature, Tw, the transition to wetting occurs, for entropic
reasons, when the liquid's surface tension is sufficiently small that the free
energy cost in forming a thick film is sufficiently compensated by the fluid-
surface interaction energy. Guidelines useful for exploring wetting transitions
of other systems are analyzed, in terms of generic criteria involving the
"simple model", which yields results in terms of gas-surface interaction
parameters and thermodynamic properties of the bulk adsorbate.Comment: Article accepted for publication in J. Low Temp. Phy
Molecular genetic variation of boll weevil populations in North America estimated with microsatellites: Implications for patterns of dispersal
Quantum Monte Carlo Simulations Of Solid4he
Recent experimental investigations [20] of solid 4He have been interpreted as showing possible superfluidity in the solid at low temperatures, below 0.2 K. A solid behaving this way, exhibiting both long range translational order and superfluidity, has been called a supersolid phase. The existence of a supersolid phase was proposed many years ago [1], and has been discussed theoretically. In this paper we review simulations of the solid state of bulk 4He at or near absolute zero temperature by quantum Monte Carlo techniques. The techniques considered are variational calculations at zero temperature which use traditional Bijl-Dingle-Jastrow wavefunctions or more recently, shadow wavefunctions; Green's function Monte Carlo calculations at zero temperature; diffusion Monte Carlo, and finally, the finite temperature path integral Monte Carlo method. A brief introduction to the technique will be given followed by a discussion of the simulation results with respect to solid helium. © Springer-Verlag Berlin Heidelberg 2006.3743 LNCS4052Andreev, A.F., Lifshitz, L.M., (1969) Soviet Phys. JETP, 29, p. 1107Aziz, R.A., Nain, V.P.S., Carley, J.S., Taylor, W.L., McConville, G.T., An accurate intermolecular potential for helium (1979) J. Chem. Phys., 70, pp. 4330-4342(1976) The Physics of Liquid and Solid Helium, , K.H. Bennemann and J.B. Ketterson, eds.: Wiley, New YorkDe Boer, J., Michels, A., Contribution to the quantum mechanical theory of the equation of state and the law of corresponding states. Determination of the law of force of Helium (1938) Physica (Utrecht), 5, pp. 945-957Ceperley, D.M., Bernu, B., Ring exchanges and the supersolid phase of4He (2004) Phys. Rev. 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