2,452 research outputs found
A finite-temperature liquid-quasicrystal transition in a lattice model
We consider a tiling model of the two-dimensional square-lattice, where each
site is tiled with one of the sixteen Wang tiles. The ground states of this
model are all quasi-periodic. The systems undergoes a disorder to
quasi-periodicity phase transition at finite temperature. Introducing a proper
order-parameter, we study the system at criticality, and extract the critical
exponents characterizing the transition. The exponents obtained are consistent
with hyper-scaling
Ions in Fluctuating Channels: Transistors Alive
Ion channels are proteins with a hole down the middle embedded in cell
membranes. Membranes form insulating structures and the channels through them
allow and control the movement of charged particles, spherical ions, mostly
Na+, K+, Ca++, and Cl-. Membranes contain hundreds or thousands of types of
channels, fluctuating between open conducting, and closed insulating states.
Channels control an enormous range of biological function by opening and
closing in response to specific stimuli using mechanisms that are not yet
understood in physical language. Open channels conduct current of charged
particles following laws of Brownian movement of charged spheres rather like
the laws of electrodiffusion of quasi-particles in semiconductors. Open
channels select between similar ions using a combination of electrostatic and
'crowded charge' (Lennard-Jones) forces. The specific location of atoms and the
exact atomic structure of the channel protein seems much less important than
certain properties of the structure, namely the volume accessible to ions and
the effective density of fixed and polarization charge. There is no sign of
other chemical effects like delocalization of electron orbitals between ions
and the channel protein. Channels play a role in biology as important as
transistors in computers, and they use rather similar physics to perform part
of that role. Understanding their fluctuations awaits physical insight into the
source of the variance and mathematical analysis of the coupling of the
fluctuations to the other components and forces of the system.Comment: Revised version of earlier submission, as invited, refereed, and
published by journa
Glassy behavior induced by geometrical frustration in a hard-core lattice gas model
We introduce a hard-core lattice-gas model on generalized Bethe lattices and
investigate analytically and numerically its compaction behavior. If
compactified slowly, the system undergoes a first-order crystallization
transition. If compactified much faster, the system stays in a meta-stable
liquid state and undergoes a glass transition under further compaction. We show
that this behavior is induced by geometrical frustration which appears due to
the existence of short loops in the generalized Bethe lattices. We also compare
our results to numerical simulations of a three-dimensional analog of the
model.Comment: 7 pages, 4 figures, revised versio
The nuclear AC-Stark shift in super-intense laser fields
The direct interaction of super-intense laser fields in the optical frequency
domain with nuclei is studied. As main observable, we consider the nuclear
AC-Stark shift of low-lying nuclear states due to the off-resonant excitation
by the laser field. We include the case of accelerated nuclei to be able to
control the frequency and the intensity of the laser field in the nuclear rest
frame over a wide range of parameters. We find that AC-Stark shifts of the same
order as in typical quantum optical systems relative to the respective
transition frequencies are feasible with state-of-the-art or near-future laser
field intensities and moderate acceleration of the target nuclei. Along with
this shift, we find laser-induced modifications to the proton root-mean-square
radii and to the proton density distribution. We thus expect direct
laser-nucleus interaction to become of relevance together with other
super-intense light-matter interaction processes such as pair creation.Comment: 10 pages, 2 eps figure
Systematic Analysis Method for Color Transparency Experiments
We introduce a data analysis procedure for color transparency experiments
which is considerably less model dependent than the transparency ratio method.
The new method is based on fitting the shape of the A dependence of the nuclear
cross section at fixed momentum transfer to determine the effective attenuation
cross section for hadrons propagating through the nucleus. The procedure does
not require assumptions about the hard scattering rate inside the nuclear
medium. Instead, the hard scattering rate is deduced directly from the data.
The only theoretical input necessary is in modelling the attenuation due to the
nuclear medium, for which we use a simple exponential law. We apply this
procedure to the Brookhaven experiment of Carroll et al and find that it
clearly shows color transparency: the effective attenuation cross section in
events with momentum transfer is approximately $40\ mb\ (2.2\
GeV^2/Q^2)$. The fit to the data also supports the idea that the hard
scattering inside the nuclear medium is closer to perturbative QCD predictions
than is the scattering of isolated protons in free space. We also discuss the
application of our approach to electroproduction experiments.Comment: 11 pages, 11 figures (figures not included, available upon request),
report # KU-HEP-92-2
Stability of vortex solitons in a photorefractive optical lattice
Stability of off-site vortex solitons in a photorefractive optical lattice is
analyzed. It is shown that such solitons are linearly unstable in both the high
and low intensity limits. In the high-intensity limit, the vortex looks like a
familiar ring vortex, and it suffers oscillatory instabilities. In the
low-intensity limit, the vortex suffers both oscillatory and Vakhitov-Kolokolov
instabilities. However, in the moderate-intensity regime, the vortex becomes
stable if the lattice intensity or the applied voltage is above a certain
threshold value. Stability regions of vortices are also determined at typical
experimental parameters.Comment: 3 pages, 5 figure
Metropolis simulations of Met-Enkephalin with solvent-accessible area parameterizations
We investigate the solvent-accessible area method by means of Metropolis
simulations of the brain peptide Met-Enkephalin at 300. For the energy
function ECEPP/2 nine atomic solvation parameter (ASP) sets are studied. The
simulations are compared with one another, with simulations with a distance
dependent electrostatic permittivity , and with vacuum
simulations (). Parallel tempering and the biased Metropolis
techniques RM are employed and their performance is evaluated. The measured
observables include energy and dihedral probability densities (pds), integrated
autocorrelation times, and acceptance rates. Two of the ASP sets turn out to be
unsuitable for these simulations. For all other systems selected configurations
are minimized in search of the global energy minima, which are found for the
vacuum and the system, but for none of the ASP models. Other
observables show a remarkable dependence on the ASPs. In particular, we find
three ASP sets for which the autocorrelations at 300K are considerably
smaller than for vacuum simulations.Comment: 10 pages and 8 figure
Beyond capitalism and liberal democracy: on the relevance of GDH Cole’s sociological critique and alternative
This article argues for a return to the social thought of the often ignored early 20th-century English thinker GDH Cole. The authors contend that Cole combined a sociological critique of capitalism and liberal democracy with a well-developed alternative in his work on guild socialism bearing particular relevance to advanced capitalist societies. Both of these, with their focus on the limitations on ‘free communal service’ in associations and the inability of capitalism to yield emancipation in either production or consumption, are relevant to social theorists looking to understand, critique and contribute to the subversion of neoliberalism. Therefore, the authors suggest that Cole’s associational sociology, and the invitation it provides to think of formations beyond capitalism and liberal democracy, is a timely and valuable resource which should be returned to
Nucleonic resonance excitations with linearly polarized photon in
In this work, an improved quark model approach to the meson
photo-production with an effective Lagrangian is presented. The {\it t}-channel
{\it natural}-parity exchange is taken into account through the Pomeron
exchange, while the {\it unnatural}-parity exchange is described by the
exchange. With a very limited number of parameters, the available experimental
data in the low energy regime can be consistently accounted for. We find that
the beam polarization observables show sensitivities to some {\it s}-channel
individual resonances in the quark model symmetry limit.
Especially, the two resonances and , which belong
to the representation , have dominant contributions
over other excited states. Concerning the essential motivation of searching for
"missing resonances" in meson photo-production, this approach provides a
feasible framework, on which systematic investigations can be done.Comment: 16 pages, Revtex, 9 eps figures, to appear in PR
Pairing Properties In Relativistic Mean Field Models Obtained From Effective Field Theory
We apply recently developed effective field theory nuclear models in mean
field approximation (parameter sets G1 and G2) to describe ground-state
properties of nuclei from the valley of -stability up to the drip lines.
For faster calculations of open-shell nuclei we employ a modified BCS approach
which takes into account quasi-bound levels owing to their centrifugal barrier,
with a constant pairing strength. We test this simple prescription by comparing
with available Hartree-plus-Bogoliubov results. Using the new effective
parameter sets we then compute separation energies, density distributions and
spin--orbit potentials in isotopic (isotonic) chains of nuclei with magic
neutron (proton) numbers. The new forces describe the experimental systematics
similarly to conventional non-linear relativistic force
parameters like NL3.Comment: 29 pages, 17 figures, accepted for publication in PR
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