199 research outputs found
Band structures of P-, D-, and G-surfaces
We present a theoretical study on the band structures of the electron
constrained to move along triply-periodic minimal surfaces. Three well known
surfaces connected via Bonnet transformations, namely P-, D-, and G-surfaces,
are considered. The six-dimensional algebra of the Bonnet transformations [C.
Oguey and J.-F. Sadoc, J. Phys. I France 3, 839 (1993)] is used to prove that
the eigenstates for these surfaces are interrelated at a set of special points
in the Brillouin zones. The global connectivity of the band structures is,
however, different due to the topological differences of the surfaces. A
numerical investigation of the band structures as well as a detailed analysis
on their symmetry properties is presented. It is shown that the presence of
nodal lines are closely related to the symmetry properties. The present study
will provide a basis for understanding further the connection between the
topology and the band structures.Comment: 21 pages, 8 figures, 3 tables, submitted to Phys. Rev.
Phyllotaxis: a non-conventional crystalline solution to packing efficiency in situations with radial symmetry:
Phyllotaxis, the search for the most homogeneous and dense organizations of small discs inside a large circular domain, was first developed to analyse arrangements of leaves or florets in plants. It has since become an object of study not only in botany, but also in mathematics, computer simulations and physics. Although the mathematical solution is now well known, an algorithm setting out the centres of the small discs on a Fermat spiral, the very nature of this organization and its properties of symmetry remain to be examined. The purpose of this paper is to describe a phyllotactic organization of points through its Voronoi cells and Delaunay triangulation and to refer to the concept of defects developed in condensed matter physics. The topological constraint of circular symmetry introduces an original inflationdeflation symmetry taking the place of the translational and rotational symmetries of classical crystallography
Does Young's equation hold on the nanoscale? A Monte Carlo test for the binary Lennard-Jones fluid
When a phase-separated binary () mixture is exposed to a wall, that
preferentially attracts one of the components, interfaces between A-rich and
B-rich domains in general meet the wall making a contact angle .
Young's equation describes this angle in terms of a balance between the
interfacial tension and the surface tensions ,
between, respectively, the - and -rich phases and the wall,
. By Monte Carlo simulations
of bridges, formed by one of the components in a binary Lennard-Jones liquid,
connecting the two walls of a nanoscopic slit pore, is estimated from
the inclination of the interfaces, as a function of the wall-fluid interaction
strength. The information on the surface tensions ,
are obtained independently from a new thermodynamic integration method, while
is found from the finite-size scaling analysis of the
concentration distribution function. We show that Young's equation describes
the contact angles of the actual nanoscale interfaces for this model rather
accurately and location of the (first order) wetting transition is estimated.Comment: 6 pages, 6 figure
Pressure Induced Hydration Dynamics of Membranes
Pressure-jump initiated time-resolved x-ray diffraction studies of dynamics
of the hydration of the hexagonal phase in biological membranes show that (i)
the relaxation of the unit cell spacing is non-exponential in time; (ii) the
Bragg peaks shift smoothly to their final positions without significant
broadening or loss in crystalline order. This suggests that the hydration is
not diffusion limited but occurs via a rather homogeneous swelling of the whole
lattice, described by power law kinetics with an exponent .Comment: REVTEX 3, 10 pages,3 figures(available on request),#
Soap Froths and Crystal Structures
We propose a physical mechanism to explain the crystal symmetries found in
macromolecular and supramolecular micellar materials. We argue that the packing
entropy of the hard micellar cores is frustrated by the entropic interaction of
their brush-like coronas. The latter interaction is treated as a surface effect
between neighboring Voronoi cells. The observed crystal structures correspond
to the Kelvin and Weaire-Phelan minimal foams. We show that these structures
are stable for reasonable areal entropy densities.Comment: 4 pages, RevTeX, 2 included eps figure
From Capillary Condensation to Interface Localization Transitions in Colloid Polymer Mixtures Confined in Thin Film Geometry
Monte Carlo simulations of the Asakura-Oosawa (AO) model for colloid-polymer
mixtures confined between two parallel repulsive structureless walls are
presented and analyzed in the light of current theories on capillary
condensation and interface localization transitions. Choosing a polymer to
colloid size ratio of q=0.8 and studying ultrathin films in the range of D=3 to
D=10 colloid diameters thickness, grand canonical Monte Carlo methods are used;
phase transitions are analyzed via finite size scaling, as in previous work on
bulk systems and under confinement between identical types of walls. Unlike the
latter work, inequivalent walls are used here: while the left wall has a
hard-core repulsion for both polymers and colloids, at the right wall an
additional square-well repulsion of variable strength acting only on the
colloids is present. We study how the phase separation into colloid-rich and
colloid-poor phases occurring already in the bulk is modified by such a
confinement. When the asymmetry of the wall-colloid interaction increases, the
character of the transition smoothly changes from capillary condensation-type
to interface localization-type. The critical behavior of these transitions is
discussed, as well as the colloid and polymer density profiles across the film
in the various phases, and the correlation of interfacial fluctuations in the
direction parallel to the confining walls. The experimental observability of
these phenomena also is briefly discussed.Comment: 36 pages, 15 figure
Pressure Induced Topological Phase Transitions in Membranes
Some highly unusual features of a lipid-water liquid crystal are revealed by
high pressure x-ray diffraction, light scattering and dilatometric studies of
the lamellar (bilayer ) to nonlamellar inverse hexagonal ()
phase transition. (i) The size of the unit cell of the phase increases
with increasing pressure. (ii) The transition volume, ,
decreases and appears to vanish as the pressure is increased. (iii) The
intensity of scattered light increases as decreases. Data are
presented which suggest that this increase is due to the formation of an
intermediate cubic phase, as predicted by recent theoretical suggestions of the
underlying universal phase sequence.Comment: 12 pages, typed using REVTEX 2.
Valency of rare earths in RIn3 and RSn3: Ab initio analysis of electric-field gradients
In RIn3 and RSn3 the rare earth (R) is trivalent, except for Eu and Yb, which
are divalent. This was experimentally determined in 1977 by perturbed angular
correlation measurements of the electric-field gradient on a 111Cd impurity. At
that time, the data were interpreted using a point charge model, which is now
known to be unphysical and unreliable. This makes the valency determination
potentially questionable. We revisit these data, and analyze them using ab
initio calculations of the electric-field gradient. From these calculations,
the physical mechanism that is responsible for the influence of the valency on
the electric-field gradient is derived. A generally applicable scheme to
interpret electric-field gradients is used, which in a transparent way
correlates the size of the field gradient with chemical properties of the
system.Comment: 10 page
Identification of a Protein with Antioxidant Activity that is Important for the Protection against Beer Ageing
This study was carried out with fresh Australian lager beer which was sampled directly off the production line, the same samples aged for 12 weeks at 30 °C, and the vintage beer which was kept at 20 °C for 5 years. Characteristic Australian lager flavour was maintained in the fresh and vintage beers but was lost in the aged beer. Sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and free thiol group labelling analyses of beer proteins found that this flavour stability correlated with the presence of an unknown 10 kilodaltons (kDa) protein with a higher level of free thiols. The protein was purified by size-exclusion chromatography, then peptide sequencing and database matching identified it as the barley lipid transfer protein (LTP1). Further characterisation using diphenylpicrylhydrazyl (DPPH) free radical scavenging and a Saccharomyces cerevisiae-based antioxidant screening assay demonstrated that the LTP1 protein was active in DPPH reduction and antioxidant activity. The absence of free thiol in the aged beer indicates that the thiol functional groups within the LTP1 protein were saturated and suggests that it is important in the flavour stability of beer by maintaining reduction capacity during the ageing process
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