3,988 research outputs found
Domain formation in membranes with quenched protein obstacles: Lateral heterogeneity and the connection to universality classes
We show that lateral fluidity in membranes containing quenched protein
obstacles belongs to the universality class of the two-dimensional random-field
Ising model. The main feature of this class is the absence of a phase
transition: there is no critical point, and macroscopic domain formation does
not occur. Instead, there is only one phase. This phase is highly
heterogeneous, with a structure consisting of micro-domains. The presence of
quenched protein obstacles thus provides a mechanism to stabilize lipid rafts
in equilibrium. Crucial for two-dimensional random-field Ising universality is
that the obstacles are randomly distributed, and have a preferred affinity to
one of the lipid species. When these conditions are not met, standard Ising or
diluted Ising universality apply. In these cases, a critical point does exist,
marking the onset toward macroscopic demixing.Comment: 10 pages, 10 figure
Fluids with quenched disorder: Scaling of the free energy barrier near critical points
In the context of Monte Carlo simulations, the analysis of the probability
distribution of the order parameter , as obtained in simulation
boxes of finite linear extension , allows for an easy estimation of the
location of the critical point and the critical exponents. For Ising-like
systems without quenched disorder, becomes scale invariant at the
critical point, where it assumes a characteristic bimodal shape featuring two
overlapping peaks. In particular, the ratio between the value of at
the peaks () and the value at the minimum in-between ()
becomes -independent at criticality. However, for Ising-like systems with
quenched random fields, we argue that instead should be observed, where is the
"violation of hyperscaling" exponent. Since is substantially non-zero,
the scaling of with system size should be easily detectable in
simulations. For two fluid models with quenched disorder, versus
was measured, and the expected scaling was confirmed. This provides further
evidence that fluids with quenched disorder belong to the universality class of
the random-field Ising model.Comment: sent to J. Phys. Cond. Mat
An explanation for the curious mass loss history of massive stars: from OB stars, through Luminous Blue Variables to Wolf-Rayet stars
The stellar winds of massive stars show large changes in mass-loss rates and
terminal velocities during their evolution from O-star through the Luminous
Blue Variable phase to the Wolf-Rayet phase. The luminosity remains
approximately unchanged during these phases. These large changes in wind
properties are explained in the context of the radiation driven wind theory, of
which we consider four different models. They are due to the evolutionary
changes in radius, gravity and surface composition and to the change from
optically thin (in continuum) line driven winds to optically thick radiation
driven winds.Comment: Accepted for publication in Astronomy and Astrophysics (Letter to the
Editor
Small-Scale X-ray Variability in the Cassiopeia A Supernova Remnant
A comparison of X-ray observations of the Cassiopeia A supernova remnant
taken in 2000, 2002, and 2004 with the Chandra ACIS-S3 reveals the presence of
several small scale features (<= 10 arcsec) which exhibit significant intensity
changes over a 4 year time frame. Here we report on the variability of six
features, four of which show count rate increases from ~ 10% to over 90%, and
two which show decreases of ~ 30% -- 40%. While extracted 1-4.5 keV X-ray
spectra do not reveal gross changes in emission line strengths, spectral fits
using non-equilibrium ionization, metal-rich plasma models indicate increased
or decreased electron temperatures for features showing increasing or
decreasing count rates, respectively. Based on the observed count rate changes
and the assumption that the freely expanding ejecta has a velocity of ~ 5000
km/s at the reverse shock front, we estimate the unshocked ejecta to have
spatial scale variations of 0.02 - 0.03 pc, which is consistent with the X-ray
emitting ejecta belonging to a more diffuse component of the supernova ejecta
than that seen in the optically emitting ejecta, which have spatial scales ~
0.001 pc.Comment: 9 pages, 8 figures, to be published in Astronomical Journa
A numerical test of the continuum index theorem on the lattice
The overlap formalism of chiral fermions provides a tool to measure the
index, Q, of the chiral Dirac operator in a fixed gauge field background on the
lattice. This enables a numerical measurement of the probability distribution,
p(Q), in Yang-Mills theories. We have obtained an estimate for p(Q) in pure
SU(2) gauge theory by measuring Q on 140 independent gauge field configurations
generated on a 12^4 lattice using the standard single plaquette Wilson action
at a coupling of beta=2.4. This distribution is in good agreement with a recent
measurement [8] of the distribution of the topological charge on the same
lattice using the same coupling and the same lattice gauge action. In
particular we find =3.3(4) to be compared with = 3.9(5) found in
[8]. The good agreement between the two distributions is an indication that the
continuum index theorem can be carried over in a probabilistic sense on to the
lattice.Comment: 17 pages, 5 figures, plain TeX, uses eps
A cryogenic amplifier for fast real-time detection of single-electron tunneling
We employ a cryogenic High Electron Mobility Transistor (HEMT) amplifier to
increase the bandwidth of a charge detection setup with a quantum point contact
(QPC) charge sensor. The HEMT is operating at 1K and the circuit has a
bandwidth of 1 MHz. The noise contribution of the HEMT at high frequencies is
only a few times higher than that of the QPC shot noise. We use this setup to
monitor single-electron tunneling to and from an adjacent quantum dot and we
measure fluctuations in the dot occupation as short as 400 nanoseconds, 20
times faster than in previous work.Comment: 4 pages, 3 figure
Laplacian gauge and instantons
We exhibit the connection between local gauge singularities in the Laplacian
gauge and topological charge, which opens the possibility of studying instanton
excitations without cooling. We describe our version of Laplacian gauge-fixing
for SU(N).Comment: Lattice 2000 (Topology and Vacuum), 4 pages, 3 figures -- cosmetic
change
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