7,528 research outputs found
"Exact" Algorithm for Random-Bond Ising Models in 2D
We present an efficient algorithm for calculating the properties of Ising
models in two dimensions, directly in the spin basis, without the need for
mapping to fermion or dimer models. The algorithm gives numerically exact
results for the partition function and correlation functions at a single
temperature on any planar network of N Ising spins in O(N^{3/2}) time or less.
The method can handle continuous or discrete bond disorder and is especially
efficient in the case of bond or site dilution, where it executes in O(L^2 ln
L) time near the percolation threshold. We demonstrate its feasibility on the
ferromagnetic Ising model and the +/- J random-bond Ising model (RBIM) and
discuss the regime of applicability in cases of full frustration such as the
Ising antiferromagnet on a triangular lattice.Comment: 4.2 pages, 5 figures, accepted for publication in Phys. Rev. Let
Solar X-rays scattered by Venus, Mars and the Moon
Scattering process of solar X rays with photoionization fluorescence by planetary atmosphere
Noise Predictions for STM in Systems with Local Electron Nematic Order
We propose that thermal noise in local stripe orientation should be readily
detectable via STM on systems in which local stripe orientations are strongly
affected by quenched disorder. Stripes, a unidirectional, nanoscale modulation
of electronic charge, are strongly affected by quenched disorder in
two-dimensional and quasi-two-dimensional systems. While stripe orientations
tend to lock to major lattice directions, dopant disorder locally breaks
rotational symmetry. In a host crystal with otherwise rotational
symmetry, stripe orientations in the presence of quenched disorder map to the
random field Ising model. While the low temperature state of such a system is
generally a stripe glass in two dimensional or strongly layered systems, as the
temperature is raised, stripe orientational fluctuations become more prevalent.
We propose that these thermally excited fluctuations should be readily
detectable in scanning tunneling spectroscopy as {\em telegraph noise} in the
high voltage part of the local curves. We predict the spatial, temporal,
and thermal evolution of such noise, including the circumstances under which
such noise is most likely to be observed. In addition, we propose an in-situ
test, amenable to any local scanning probe, for assessing whether such noise is
due to correlated fluctuations rather than independent switchers.Comment: 8 pages, 8 figure
Assessment of synchrony in multiple neural spike trains using loglinear point process models
Neural spike trains, which are sequences of very brief jumps in voltage
across the cell membrane, were one of the motivating applications for the
development of point process methodology. Early work required the assumption of
stationarity, but contemporary experiments often use time-varying stimuli and
produce time-varying neural responses. More recently, many statistical methods
have been developed for nonstationary neural point process data. There has also
been much interest in identifying synchrony, meaning events across two or more
neurons that are nearly simultaneous at the time scale of the recordings. A
natural statistical approach is to discretize time, using short time bins, and
to introduce loglinear models for dependency among neurons, but previous use of
loglinear modeling technology has assumed stationarity. We introduce a succinct
yet powerful class of time-varying loglinear models by (a) allowing
individual-neuron effects (main effects) to involve time-varying intensities;
(b) also allowing the individual-neuron effects to involve autocovariation
effects (history effects) due to past spiking, (c) assuming excess synchrony
effects (interaction effects) do not depend on history, and (d) assuming all
effects vary smoothly across time.Comment: Published in at http://dx.doi.org/10.1214/10-AOAS429 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Estimating Third-Order Moments for an Absorber Catalog
Thanks to the recent availability of large surveys, there has been renewed
interest in third-order correlation statistics. Measures of third-order
clustering are sensitive to the structure of filaments and voids in the
universe and are useful for studying large-scale structure. Thus, statistics of
these third-order measures can be used to test and constrain parameters in
cosmological models. Third-order measures such as the three-point correlation
function are now commonly estimated for galaxy surveys. Studies of third-order
clustering of absorption systems will complement these analyses. We define a
statistic, which we denote K, that measures third-order clustering of a data
set of point observations and focus on estimating this statistic for an
absorber catalog. The statistic K can be considered a third-order version of
the second-order Ripley K-function and allows one to study the abundance of
various configurations of point triplets. In particular, configurations
consisting of point triplets that lie close to a straight line can be examined.
Studying third-order clustering of absorbers requires consideration of the
absorbers as a three-dimensional process, observed on QSO lines of sight that
extend radially in three-dimensional space from Earth. Since most of this
three-dimensional space is not probed by the lines of sight, edge corrections
become important. We use an analytical form of edge correction weights and
construct an estimator of the statistic K for use with an absorber catalog. We
show that with these weights, ratio-unbiased estimates of K can be obtained.
Results from a simulation study also verify unbiasedness and provide
information on the decrease of standard errors with increasing number of lines
of sight.Comment: 19 pages, 4 figure
Vlasov Description Of Dense Quark Matter
We discuss properties of quark matter at finite baryon densities and zero
temperature in a Vlasov approach. We use a screened interquark Richardson's
potential consistent with the indications of Lattice QCD calculations.
We analyze the choices of the quark masses and the parameters entering the
potential which reproduce the binding energy (B.E.) of infinite nuclear matter.
There is a transition from nuclear to quark matter at densities 5 times above
normal nuclear matter density. The transition could be revealed from the
determination of the position of the shifted meson masses in dense baryonic
matter. A scaling form of the meson masses in dense matter is given.Comment: 15 pages 4 figure
Short-Range Correlations and Cooling of Ultracold Fermions in the Honeycomb Lattice
We use determinantal quantum Monte Carlo simulations and numerical
linked-cluster expansions to study thermodynamic properties and short-range
spin correlations of fermions in the honeycomb lattice. We find that, at half
filling and finite temperatures, nearest-neighbor spin correlations can be
stronger in this lattice than in the square lattice, even in regimes where the
ground state in the former is a semimetal or a spin liquid. The honeycomb
lattice also exhibits a more pronounced anomalous region in the double
occupancy that leads to stronger adiabatic cooling than in the square lattice.
We discuss the implications of these findings for optical lattice experiments.Comment: 5 pages, 4 figure
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