7,305 research outputs found
Entropic Effects in the Very Low Temperature Regime of Diluted Ising Spin Glasses with Discrete Couplings
We study link-diluted Ising spin glass models on the hierarchical
lattice and on a three-dimensional lattice close to the percolation threshold.
We show that previously computed zero temperature fixed points are unstable
with respect to temperature perturbations and do not belong to any critical
line in the dilution-temperature plane. We discuss implications of the presence
of such spurious unstable fixed points on the use of optimization algorithms,
and we show how entropic effects should be taken into account to obtain the
right physical behavior and critical points.Comment: 4 pages, 4 figures. A major typo error in formula (8) has been
correcte
Tailoring superradiance to design artificial quantum systems
Cooperative phenomena arising due to the coupling of individual atoms via the
radiation field are a cornerstone of modern quantum and optical physics. Recent
experiments on x-ray quantum optics added a new twist to this line of research
by exploiting superradiance in order to construct artificial quantum systems.
However, so far, systematic approaches to deliberately design superradiance
properties are lacking, impeding the desired implementation of more advanced
quantum optical schemes. Here, we develop an analytical framework for the
engineering of single-photon superradiance in extended media applicable across
the entire electromagnetic spectrum, and show how it can be used to tailor the
properties of an artificial quantum system. This "reverse engineering" of
superradiance not only provides an avenue towards non-linear and quantum
mechanical phenomena at x-ray energies, but also leads to a unified view on and
a better understanding of superradiance across different physical systems.Comment: 6 pages + supplemental materia
Flow dependence of high parton energy loss in heavy-ion collisions
The measured transverse momentum spectra and HBT correlations of bulk (i.e.
low ) matter can be well explained by assuming that the soft sector of
particles produced in ultrarelativistic heavy-ion collisions is (approximately)
thermalized and undergoes collective accelerated expansion in both longitudinal
and transverse direction. However, this implies that bulk matter will have a
non-vanishing flow component transverse to the trajectory of a high
partonic jets. In general, this will increase the energy loss experienced by
the jet parton and modify the shape of the jet cone. In this paper, we present
a systematic study of the magnitude of the additional energy loss induced by
flow under realistic assumptions for the medium evolution. We argue that a
perturbative QGP description may be sufficient for the measured if
flow during the medium evolution is taken into account properly
Yoctosecond photon pulses from quark-gluon plasmas
Present ultra-fast laser optics is at the frontier between atto- and
zeptosecond photon pulses, giving rise to unprecedented applications. We show
that high-energetic photon pulses down to the yoctosecond timescale can be
produced in heavy ion collisions. We focus on photons produced during the
initial phase of the expanding quark-gluon plasma. We study how the time
evolution and properties of the plasma may influence the duration and shape of
the photon pulse. Prospects for achieving double peak structures suitable for
pump-probe experiments at the yoctosecond timescale are discussed.Comment: 4 pages, 2 figures; final version as accepted by PR
Streaking At High Energies With Electrons And Positrons
State-of-the-art attosecond metrology deals with the detection and
characterization of photon pulses with typical energies up to the hundreds of
eV and time resolution of several tens of attoseconds. Such short pulses are
used for example to control the motion of electrons on the atomic scale or to
measure inner-shell atomic dynamics. The next challenge of time-resolving the
inner-nuclear dynamics, transient meson states and resonances requires photon
pulses below attosecond duration and with energies exceeding the MeV scale.
Here we discuss a detection scheme for time-resolving high-energy gamma ray
pulses down to the zeptosecond timescale. The scheme is based on the concept of
attosecond streak imaging, but instead of conversion of photons into electrons
in a nonlinear medium, the high-energy process of electron-positron pair
creation is utilized. These pairs are produced in vacuum through the collision
of a test pulse to be characterized with an intense laser pulse, and they
acquire additional energy and momentum depending on their phase in the
streaking pulse at the moment of production. A coincidence measurement of the
electron and positron momenta after the interaction provides information on the
pair production phase within the streaking pulse. We examine the limitations
imposed by quantum radiation reaction in multiphoton Compton scattering on this
detection scheme, and discuss other necessary conditions to render the scheme
feasible in the upcoming Extreme Light Infrastructure (ELI) laser facility.Comment: 6 pages, 2 figures, contribution to the AIP proceedings of "Light at
Extreme Intensities" (LEI 2011), Szeged, Hungary, Nov 14-18, 201
Revealed Price Preference: Theory and Empirical Analysis
With the aim of determining the welfare implications of price change in
consumption data, we introduce a revealed preference relation over prices. We
show that an absence of cycles in this preference relation characterizes a
model of demand where consumers trade-off the utility of consumption against
the disutility of expenditure. This model is appropriate whenever a consumer's
demand over a {\em strict} subset of all available goods is being analyzed. For
the random utility extension of the model, we devise nonparametric statistical
procedures for testing and welfare comparisons. The latter requires the
development of novel tests of linear hypotheses for partially identified
parameters. In doing so, we provide new algorithms for the calculation and
statistical inference in nonparametric counterfactual analysis for a general
partially identified model. Our applications on national household expenditure
data provide support for the model and yield informative bounds concerning
welfare rankings across different prices.Comment: 53 page
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