1,547 research outputs found
A lattice model of hydrophobic interactions
Hydrogen bonding is modeled in terms of virtual exchange of protons between
water molecules. A simple lattice model is analyzed, using ideas and techniques
from the theory of correlated electrons in metals. Reasonable parameters
reproduce observed magnitudes and temperature dependence of the hydrophobic
interaction between substitutional impurities and water within this lattice.Comment: 7 pages, 3 figures. To appear in Europhysics Letter
Effects of counterion fluctuations in a polyelectrolyte brush
We investigate the effect of counterion fluctuations in a single
polyelectrolyte brush in the absence of added salt by systematically expanding
the counterion free energy about Poisson-Boltzmann mean field theory. We find
that for strongly charged brushes, there is a collapse regime in which the
brush height decreases with increasing charge on the polyelectrolyte chains.
The transition to this collapsed regime is similar to the liquid-gas
transition, which has a first-order line terminating at a critical point. We
find that for monovalent counterions the transition is discontinuous in theta
solvent, while for multivalent counterions the transition is generally
continuous. For collapsed brushes, the brush height is not independent of
grafting density as it is for osmotic brushes, but scales linear with it.Comment: 9 pages, 9 figure
Charge-Fluctuation-Induced Non-analytic Bending Rigidity
In this Letter, we consider a neutral system of mobile positive and negative
charges confined on the surface of curved films. This may be an appropriate
model for: i) a highly charged membrane whose counterions are confined to a
sheath near its surface; ii) a membrane composed of an equimolar mixture of
anionic and cationic surfactants in aqueous solution. We find that the charge
fluctuations contribute a non-analytic term to the bending rigidity that varies
logarithmically with the radius of curvature. This may lead to spontaneous
vesicle formation, which is indeed observed in similar systems.Comment: Revtex, 9 pages, no figures, submitted to PR
Universal reduction of pressure between charged surfaces by long-wavelength surface charge modulation
We predict theoretically that long-wavelength surface charge modulations
universally reduce the pressure between the charged surfaces with counterions
compared with the case of uniformly charged surfaces with the same average
surface charge density. The physical origin of this effect is the fact that
surface charge modulations always lead to enhanced counterion localization near
the surfaces, and hence, fewer charges at the midplane. We confirm the last
prediction with Monte Carlo simulations.Comment: 8 pages 1 figure, Europhys. Lett., in pres
Fractal Dimensions of Confined Clusters in Two-Dimensional Directed Percolation
The fractal structure of directed percolation clusters, grown at the
percolation threshold inside parabolic-like systems, is studied in two
dimensions via Monte Carlo simulations. With a free surface at y=\pm Cx^k and a
dynamical exponent z, the surface shape is a relevant perturbation when k<1/z
and the fractal dimensions of the anisotropic clusters vary continuously with
k. Analytic expressions for these variations are obtained using a blob picture
approach.Comment: 6 pages, Plain TeX file, epsf, 3 postscript-figure
On the nature of continuous physical quantities in classical and quantum mechanics
Within the traditional Hilbert space formalism of quantum mechanics, it is
not possible to describe a particle as possessing, simultaneously, a sharp
position value and a sharp momentum value. Is it possible, though, to describe
a particle as possessing just a sharp position value (or just a sharp momentum
value)? Some, such as Teller (Journal of Philosophy, 1979), have thought that
the answer to this question is No -- that the status of individual continuous
quantities is very different in quantum mechanics than in classical mechanics.
On the contrary, I shall show that the same subtle issues arise with respect to
continuous quantities in classical and quantum mechanics; and that it is, after
all, possible to describe a particle as possessing a sharp position value
without altering the standard formalism of quantum mechanics.Comment: 26 pages, LaTe
Coiling Instabilities in Multilamellar Tubes
Myelin figures are densely packed stacks of coaxial cylindrical bilayers that
are unstable to the formation of coils or double helices. These myelin figures
appear to have no intrinsic chirality. We show that such cylindrical membrane
stacks can develop an instability when they acquire a spontaneous curvature or
when the equilibrium distance between membranes is decreased. This instability
breaks the chiral symmetry of the stack and may result in coiling. A
unilamellar cylindrical vesicle, on the other hand, will develop an
axisymmetric instability, possibly related to the pearling instability.Comment: 6 pages, 2 figure
Adiabatic-antiadiabatic crossover in a spin-Peierls chain
We consider an XXZ spin-1/2 chain coupled to optical phonons with non-zero
frequency . In the adiabatic limit (small ), the chain is
expected to spontaneously dimerize and open a spin gap, while the phonons
become static. In the antiadiabatic limit (large ), phonons are
expected to give rise to frustration, so that dimerization and formation of
spin-gap are obtained only when the spin-phonon interaction is large enough. We
study this crossover using bosonization technique. The effective action is
solved both by the Self Consistent Harmonic Approximation (SCHA)and by
Renormalization Group (RG) approach starting from a bosonized description. The
SCHA allows to analyze the lowfrequency regime and determine the coupling
constant associated with the spin-Peierls transition. However, it fails to
describe the SU(2) invariant limit. This limit is tackled by the RG. Three
regimes are found. For , where is the gap in
the static limit , the system is in the adiabatic regime, and
the gap remains of order . For , the system enters
the antiadiabatic regime, and the gap decreases rapidly as
increases. Finally, for , where is an
increasing function of the spin phonon coupling, the spin gap vanishes via a
Berezinskii-Kosterlitz-Thouless transition. Our results are discussed in
relation with numerical and experimental studies of spin-Peierls systems.Comment: Revtex, 21 pages, 5 EPS figures (v1); 23 pages, 6 EPS figures, more
detailed comparison with ED results, referenes added (v2
Surface Shape and Local Critical Behaviour in Two-Dimensional Directed Percolation
Two-dimensional directed site percolation is studied in systems directed
along the x-axis and limited by a free surface at y=\pm Cx^k. Scaling
considerations show that the surface is a relevant perturbation to the local
critical behaviour when k<1/z where z=\nu_\parallel/\nu is the dynamical
exponent. The tip-to-bulk order parameter correlation function is calculated in
the mean-field approximation. The tip percolation probability and the fractal
dimensions of critical clusters are obtained through Monte-Carlo simulations.
The tip order parameter has a nonuniversal, C-dependent, scaling dimension in
the marginal case, k=1/z, and displays a stretched exponential behaviour when
the perturbation is relevant. The k-dependence of the fractal dimensions in the
relevant case is in agreement with the results of a blob picture approach.Comment: 13 pages, Plain TeX file, epsf, 6 postscript-figures, minor
correction
Penetration Depth Measurements in MgB_2: Evidence for Unconventional Superconductivity
We have measured the magnetic penetration depth of the recently discovered
binary superconductor MgB_2 using muon spin rotation and low field
-susceptibility. From the damping of the muon precession signal we find the
penetration depth at zero temperature is about 85nm. The low temperature
penetration depth shows a quadratic temperature dependence, indicating the
presence of nodes in the superconducting energy gap.Comment: 4 pages 3 figure
- …