8,420 research outputs found
Pattern Formation as a Signature of Quantum Degeneracy in a Cold Exciton System
The development of a Turing instability to a spatially modulated state in a
photoexcited electron-hole system is proposed as a novel signature of exciton
Bose statistics. We show that such an instability, which is driven by kinetics
of exciton formation, can result from stimulated processes that build up near
quantum degeneracy. In the spatially uniform 2d electron-hole system, the
instability leads to a triangular lattice pattern while, at an electron-hole
interface, a periodic 1d pattern develops. We analyze the mechanism of
wavelength selection, and show that the transition is abrupt (type I) for the
uniform 2d system, and continuous (type II) for the electron-hole interface.Comment: 5 pages, 3 figure
Distribution of the spacing between two adjacent avoided crossings
We consider the frequency at which avoided crossings appear in an energy
level structure when an external field is applied to a quantum chaotic system.
The distribution of the spacing in the parameter between two adjacent avoided
crossings is investigated. Using a random matrix model, we find that the
distribution of these spacings is well fitted by a power-law distribution for
small spacings. The powers are 2 and 3 for the Gaussian orthogonal ensemble and
Gaussian unitary ensemble, respectively. We also find that the distributions
decay exponentially for large spacings. The distributions in concrete quantum
chaotic systems agree with those of the random matrix model.Comment: 11 page
Early Cenozoic mammalian faunas, Fayum Province, Egypt: Part I. African Oligocene mammals: Introduction, history of study, and faunal succession. Part II. The African Oligocene Rodentia. Part II. The African Oligocene Rodentia.
Part I. The vertebrate microfaunas described in this paper have been recovered as a result of six seasons of paleontological exploration in the upper Eocene and Oligocene badlands exposures of the Fayum Province, U.A.R., a project initiated and directed by E. L. Simons.... Part II. The large Yale collections of rodents from the Early Oligocene Jebel el Qatrani Formation of the Fayum, Egypt, permit a thorough study of this, the earliest rodent faunule from Africa….https://elischolar.library.yale.edu/peabody_museum_natural_history_bulletin/1027/thumbnail.jp
Bayesian inversion for finite fault earthquake source models I—theory and algorithm
The estimation of finite fault earthquake source models is an inherently underdetermined
problem: there is no unique solution to the inverse problem of determining the rupture history
at depth as a function of time and space when our data are limited to observations at
the Earth’s surface. Bayesian methods allow us to determine the set of all plausible source
model parameters that are consistent with the observations, our a priori assumptions about the
physics of the earthquake source and wave propagation, and models for the observation errors
and the errors due to the limitations in our forward model. Because our inversion approach
does not require inverting any matrices other than covariance matrices, we can restrict our
ensemble of solutions to only those models that are physically defensible while avoiding the
need to restrict our class of models based on considerations of numerical invertibility. We
only use prior information that is consistent with the physics of the problem rather than some
artefice (such as smoothing) needed to produce a unique optimal model estimate. Bayesian inference
can also be used to estimate model-dependent and internally consistent effective errors
due to shortcomings in the forward model or data interpretation, such as poor Green’s functions
or extraneous signals recorded by our instruments. Until recently, Bayesian techniques
have been of limited utility for earthquake source inversions because they are computationally
intractable for problems with as many free parameters as typically used in kinematic
finite fault models. Our algorithm, called cascading adaptive transitional metropolis in parallel
(CATMIP), allows sampling of high-dimensional problems in a parallel computing framework.
CATMIP combines the Metropolis algorithm with elements of simulated annealing and
genetic algorithms to dynamically optimize the algorithm’s efficiency as it runs. The algorithm
is a generic Bayesian Markov Chain Monte Carlo sampler; it works independently of the
model design, a priori constraints and data under consideration, and so can be used for a wide
variety of scientific problems. We compare CATMIP’s efficiency relative to several existing
sampling algorithms and then present synthetic performance tests of finite fault earthquake
rupture models computed using CATMIP
Events, processes, and the time of a killing
The paper proposes a novel solution to the problem of the time of a killing (ToK), which persistently besets theories of act-individuation. The solution proposed claims to expose a crucial wrong-headed assumption in the debate, according to which ToK is essentially a problem of locating some event that corresponds to the killing. The alternative proposal put forward here turns on recognizing a separate category of dynamic occurents, viz. processes. The paper does not aim to mount a comprehensive defense of process ontology, relying instead on extant defenses. The primary aim is rather to put process ontology to work in diagnosing the current state of play over ToK, and indeed in solving it
Correlation Functions in Disordered Systems
{Recently, we found that the correlation between the eigenvalues of random
hermitean matrices exhibits universal behavior. Here we study this universal
behavior and develop a diagrammatic approach which enables us to extend our
previous work to the case in which the random matrix evolves in time or varies
as some external parameters vary. We compute the current-current correlation
function, discuss various generalizations, and compare our work with the work
of other authors. We study the distribution of eigenvalues of Hamiltonians
consisting of a sum of a deterministic term and a random term. The correlation
between the eigenvalues when the deterministic term is varied is calculated.}Comment: 19 pages, figures not included (available on request), Tex,
NSF-ITP-93-12
Tail States in Disordered Superconductors with Magnetic Impurities: the Unitarity Limit
When subject to a weak magnetic impurity distribution, the order parameter
and quasi-particle energy gap of a weakly disordered bulk s-wave superconductor
are suppressed. In the Born scattering limit, recent investigations have shown
that `optimal fluctuations' of the random impurity potential can lead to the
nucleation of `domains' of localised states within the gap region predicted by
the conventional Abrikosov-Gor'kov mean-field theory, rendering the
superconducting system gapless at any finite impurity concentration. By
implementing a field theoretic scheme tailored to the weakly disordered system,
the aim of the present paper is to extend this analysis to the consideration of
magnetic impurities in the unitarity scattering limit. This investigation
reveals that the qualitative behaviour is maintained while the density of
states exhibits a rich structure.Comment: 18 pages AMSLaTeX (with LaTeX2e), 6 eps figure
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