273 research outputs found
Three-dimensional Binary Superlattices of Oppositely-charged Colloids
We report the equilibrium self-assembly of binary crystals of
oppositely-charged colloidal microspheres at high density. By varying the
magnitude of the charge on near equal-sized spheres we show that the structure
of the binary crystal may be switched between face-centered cubic, cesium
chloride and sodium chloride. We interpret these transformations in terms of a
competition between entropic and Coulombic forces
Equilibrium phase behavior of polydisperse hard spheres
We calculate the phase behavior of hard spheres with size polydispersity,
using accurate free energy expressions for the fluid and solid phases. Cloud
and shadow curves, which determine the onset of phase coexistence, are found
exactly by the moment free energy method, but we also compute the complete
phase diagram, taking full account of fractionation effects. In contrast to
earlier, simplified treatments we find no point of equal concentration between
fluid and solid or re-entrant melting at higher densities. Rather, the fluid
cloud curve continues to the largest polydispersity that we study (14%); from
the equilibrium phase behavior a terminal polydispersity can thus only be
defined for the solid, where we find it to be around 7%. At sufficiently large
polydispersity, fractionation into several solid phases can occur, consistent
with previous approximate calculations; we find in addition that coexistence of
several solids with a fluid phase is also possible
On chemiluminescent emission from an infiltrated chiral sculptured thin film
The theory describing the far-field emission from a dipole source embedded
inside a chiral sculptured thin film (CSTF), based on a spectral Green function
formalism, was further developed to allow for infiltration of the void regions
of the CSTF by a fluid. In doing so, the extended Bruggeman homogenization
formalism--which accommodates constituent particles that are small compared to
wavelength but not vanishingly small--was used to estimate the relative
permittivity parameters of the infiltrated CSTF. For a numerical example, we
found that left circularly polarized (LCP) light was preferentially emitted
through one face of the CSTF while right circularly polarized (RCP) light was
preferentially emitted through the opposite face, at wavelengths within the
Bragg regime. The centre wavelength for the preferential emission of LCP/RCP
light was red shifted as the refractive index of the infiltrating fluid
increased from unity, and this red shift was accentuated when the size of the
constituent particles in our homogenization model was increased. Also, the
bandwidth of the preferential LCP/RCP emission regime decreased as the
refractive index of the infiltrating fluid increased from unity
Weighted-density approximation for general nonuniform fluid mixtures
In order to construct a general density-functional theory for nonuniform
fluid mixtures, we propose an extension to multicomponent systems of the
weighted-density approximation (WDA) of Curtin and Ashcroft [Phys. Rev. A 32,
2909 (1985)]. This extension corrects a deficiency in a similar extension
proposed earlier by Denton and Ashcroft [Phys. Rev. A 42, 7312 (1990)], in that
that functional cannot be applied to the multi-component nonuniform fluid
systems with spatially varying composition, such as solid-fluid interfaces. As
a test of the accuracy of our new functional, we apply it to the calculation of
the freezing phase diagram of a binary hard-sphere fluid, and compare the
results to simulation and the Denton-Ashcroft extension.Comment: 4 pages, 4 figures, to appear in Phys. Rev. E as Brief Repor
First-Principle Homogenization Theory for Periodic Metamaterials
We derive from first principles an accurate homogenized description of
periodic metamaterials made of magnetodielectric inclusions, highlighting and
overcoming relevant limitations of standard homogenization methods. We obtain
closed-form expressions for the effective constitutive parameters, pointing out
the relevance of inherent spatial dispersion effects, present even in the
long-wavelength limit. Our results clarify the limitations of quasi-static
homogenization models, restore the physical meaning of homogenized metamaterial
parameters and outline the reasons behind magnetoelectric coupling effects that
may arise also in the case of center-symmetric inclusions.Comment: 58 pages, 10 figures Phys. Rev. B, in press (2011
Structure of Colloid-Polymer Suspensions
We discuss structural correlations in mixtures of free polymer and colloidal
particles based on a microscopic, 2-component liquid state integral equation
theory. Whereas in the case of polymers much smaller than the spherical
particles the relevant polymer degree of freedom is the center of mass, for
polymers larger than the (nano-) particles conformational rearrangements need
to be considered. They have the important consequence that the polymer
depletion layer exhibits two widely different length scales, one of the order
of the particle radius, the other of the order of the polymer radius or the
polymer density screening length in dilute or semidilute concentrations,
respectively. Their consequences on phase stability and structural correlations
are discussed extensively.Comment: 37 pages, 17 figures; topical feature articl
Spin-wave Scattering in the Effective Lagrangian Perspective
Nonrelativistic systems exhibiting collective magnetic behavior are analyzed
in the framework of effective Lagrangians. The method, formulating the dynamics
in terms of Goldstone bosons, allows to investigate the consequences of
spontaneous symmetry breaking from a unified point of view. Low energy theorems
concerning spin-wave scattering in ferro- and antiferromagnets are established,
emphasizing the simplicity of actual calculations. The present work includes
approximate symmetries and discusses the modification of the low energy
structure imposed by an external magnetic and an anisotropy field,
respectively. Throughout the paper, analogies between condensed matter physics
and Lorentz-invariant theories are pointed out, demonstrating the universal
feature of the effective Lagrangian technique.Comment: Published versio
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