24,981 research outputs found
Exact renormalization-group analysis of first order phase transitions in clock models
We analyze the exact behavior of the renormalization group flow in
one-dimensional clock-models which undergo first order phase transitions by the
presence of complex interactions. The flow, defined by decimation, is shown to
be single-valued and continuous throughout its domain of definition, which
contains the transition points. This fact is in disagreement with a recently
proposed scenario for first order phase transitions claiming the existence of
discontinuities of the renormalization group. The results are in partial
agreement with the standard scenario. However in the vicinity of some fixed
points of the critical surface the renormalized measure does not correspond to
a renormalized Hamiltonian for some choices of renormalization blocks. These
pathologies although similar to Griffiths-Pearce pathologies have a different
physical origin: the complex character of the interactions. We elucidate the
dynamical reason for such a pathological behavior: entire regions of coupling
constants blow up under the renormalization group transformation. The flows
provide non-perturbative patterns for the renormalization group behavior of
electric conductivities in the quantum Hall effect.Comment: 13 pages + 3 ps figures not included, TeX, DFTUZ 91.3
Estimation of Kalman filter model parameters from an ensemble of tests
A methodology for estimating initial mean and covariance parameters in a Kalman filter model from an ensemble of nonidentical tests is presented. In addition, the problem of estimating time constants and process noise levels is addressed. Practical problems such as developing and validating inertial instrument error models from laboratory test data or developing error models of individual phases of a test are generally considered
Filamin cross-linked semiflexible networks: Fragility under strain
The semiflexible F-actin network of the cytoskeleton is cross-linked by a
variety of proteins including filamin, which contain Ig-domains that unfold
under applied tension. We examine a simple semiflexible network model
cross-linked by such unfolding linkers that captures the main mechanical
features of F-actin networks cross-linked by filamin proteins and show that
under sufficiently high strain the network spontaneously self-organizes so that
an appreciable fraction of the filamin cross-linkers are at the threshold of
domain unfolding. We propose an explanation of this organization based on a
mean-field model and suggest a qualitative experimental signature of this type
of network reorganization under applied strain that may be observable in
intracellular microrheology experiments of Crocker et al.Comment: 4 Pages, 3 figures, Revtex4, submitted to PR
An exactly solvable dissipative transport model
We introduce a class of one-dimensional lattice models in which a quantity,
that may be thought of as an energy, is either transported from one site to a
neighbouring one, or locally dissipated. Transport is controlled by a
continuous bias parameter q, which allows us to study symmetric as well as
asymmetric cases. We derive sufficient conditions for the factorization of the
N-body stationary distribution and give an explicit solution for the latter,
before briefly discussing physically relevant situations.Comment: 7 pages, 1 figure, submitted to J. Phys.
The approach to criticality in sandpiles
A popular theory of self-organized criticality relates the critical behavior
of driven dissipative systems to that of systems with conservation. In
particular, this theory predicts that the stationary density of the abelian
sandpile model should be equal to the threshold density of the corresponding
fixed-energy sandpile. This "density conjecture" has been proved for the
underlying graph Z. We show (by simulation or by proof) that the density
conjecture is false when the underlying graph is any of Z^2, the complete graph
K_n, the Cayley tree, the ladder graph, the bracelet graph, or the flower
graph. Driven dissipative sandpiles continue to evolve even after a constant
fraction of the sand has been lost at the sink. These results cast doubt on the
validity of using fixed-energy sandpiles to explore the critical behavior of
the abelian sandpile model at stationarity.Comment: 30 pages, 8 figures, long version of arXiv:0912.320
Dynamic instabilities of fracture under biaxial strain using a phase field model
We present a phase field model of the propagation of fracture under plane
strain. This model, based on simple physical considerations, is able to
accurately reproduce the different behavior of cracks (the principle of local
symmetry, the Griffith and Irwin criteria, and mode-I branching). In addition,
we test our model against recent experimental findings showing the presence of
oscillating cracks under bi-axial load. Our model again reproduces well
observed supercritical Hopf bifurcation, and is therefore the first simulation
which does so
Geologic application of thermal inertia imaging using HCMM data
Three test sites in the western US were selected to discriminate among surface geologic materials on the basis of their thermal properties as determined from HCMM data. Attempts to determine quantitatively accurate thermal inertia values from HCMM digital data met with only partial success due to the effects of sensor miscalibrations, radiative transfer in the atmosphere, and varying meteorology and elevation across a scene. In most instances, apparent thermal inertia was found to be an excellent qualitative representation of true thermal inertia. Computer processing of digital day and night HCMM data allowed construction of geologically useful images. At some test sites, more information was provided by data than LANDSAT data. Soil moisture effects and differences in spectrally dark materials were more effectively displayed using the thermal data
Critical phase in non-conserving zero-range processes and equilibrium networks
Zero-range processes, in which particles hop between sites on a lattice, are
closely related to equilibrium networks, in which rewiring of links take place.
Both systems exhibit a condensation transition for appropriate choices of the
dynamical rules. The transition results in a macroscopically occupied site for
zero-range processes and a macroscopically connected node for networks.
Criticality, characterized by a scale-free distribution, is obtained only at
the transition point. This is in contrast with the widespread scale-free
real-life networks. Here we propose a generalization of these models whereby
criticality is obtained throughout an entire phase, and the scale-free
distribution does not depend on any fine-tuned parameter.Comment: 4 pages, 4 figure
Triple-Star Candidates Among the Kepler Binaries
We present the results of a search through the photometric database of
eclipsing Kepler binaries (Prsa et al. 2011; Slawson et al. 2011) looking for
evidence of hierarchical triple star systems. The presence of a third star
orbiting the binary can be inferred from eclipse timing variations. We apply a
simple algorithm in an automated determination of the eclipse times for all
2157 binaries. The "calculated" eclipse times, based on a constant period
model, are subtracted from those observed. The resulting O-C (observed minus
calculated times) curves are then visually inspected for periodicities in order
to find triple-star candidates. After eliminating false positives due to the
beat frequency between the ~1/2-hour Kepler cadence and the binary period, 39
candidate triple systems were identified. The periodic O-C curves for these
candidates were then fit for contributions from both the classical Roemer delay
and so-called "physical" delay, in an attempt to extract a number of the system
parameters of the triple. We discuss the limitations of the information that
can be inferred from these O-C curves without further supplemental input, e.g.,
ground-based spectroscopy. Based on the limited range of orbital periods for
the triple star systems to which this search is sensitive, we can extrapolate
to estimate that at least 20% of all close binaries have tertiary companions.Comment: 19 pages, 13 figures, 3 tables; ApJ, 2013, 768, 33; corrected Fig. 7,
updated references, minor fixes to tex
Cold collisions of OH and Rb. I: the free collision
We have calculated elastic and state-resolved inelastic cross sections for
cold and ultracold collisions in the Rb() + OH() system,
including fine-structure and hyperfine effects. We have developed a new set of
five potential energy surfaces for Rb-OH() from high-level {\em ab
initio} electronic structure calculations, which exhibit conical intersections
between covalent and ion-pair states. The surfaces are transformed to a
quasidiabatic representation. The collision problem is expanded in a set of
channels suitable for handling the system in the presence of electric and/or
magnetic fields, although we consider the zero-field limit in this work.
Because of the large number of scattering channels involved, we propose and
make use of suitable approximations. To account for the hyperfine structure of
both collision partners in the short-range region we develop a
frame-transformation procedure which includes most of the hyperfine
Hamiltonian. Scattering cross sections on the order of cm are
predicted for temperatures typical of Stark decelerators. We also conclude that
spin orientation of the partners is completely disrupted during the collision.
Implications for both sympathetic cooling of OH molecules in an environment of
ultracold Rb atoms and experimental observability of the collisions are
discussed.Comment: 20 pages, 16 figure
- …