31,809 research outputs found
Curvature Spectra and Nongaussianities in the Roulette Inflation Model
Using the gradient expansion method of Rigopoulos, Shellard and van Tent
which treats cosmological perturbations as gradients on top of a homogeneous
and isotropic FRW background, we study the production of nongaussianities in
the roulette model of inflation. Investigating a number of trajectories within
this two-field model of inflation, we find that while the superhorizon
influence of the isocurvature modes on the curvature bispectrum produces
nonzero contribution to f_NL, the effect is negligible next to the standard
inflationary prediction |f_NL| ~ n_s - 1. This is the case in both the squeezed
and equilateral configurations of the bispectrum, although the former is
slightly larger in the trajectories under consideration.Comment: 23 pages, 6 figures, 3 tables, 1 appendix; Added references, slightly
extended section
The Atmospheric Monitoring Strategy for the Cherenkov Telescope Array
The Imaging Atmospheric Cherenkov Technique (IACT) is unusual in astronomy as
the atmosphere actually forms an intrinsic part of the detector system, with
telescopes indirectly detecting very high energy particles by the generation
and transport of Cherenkov photons deep within the atmosphere. This means that
accurate measurement, characterisation and monitoring of the atmosphere is at
the very heart of successfully operating an IACT system. The Cherenkov
Telescope Array (CTA) will be the next generation IACT observatory with an
ambitious aim to improve the sensitivity of an order of magnitude over current
facilities, along with corresponding improvements in angular and energy
resolution and extended energy coverage, through an array of Large (23m),
Medium (12m) and Small (4m) sized telescopes spread over an area of order
~km. Whole sky coverage will be achieved by operating at two sites: one in
the northern hemisphere and one in the southern hemisphere. This proceedings
will cover the characterisation of the candidate sites and the atmospheric
calibration strategy. CTA will utilise a suite of instrumentation and analysis
techniques for atmospheric modelling and monitoring regarding pointing
forecasts, intelligent pointing selection for the observatory operations and
for offline data correction.Comment: 6 pages. To appear in the proceedings of the Adapting to the
Atmosphere conference 201
Ageing and relaxation times in disordered insulators
We focus on the slow relaxations observed in the conductance of disordered
insulators at low temperature (especially granular aluminum films). They
manifest themselves as a temporal logarithmic decrease of the conductance after
a quench from high temperatures and the concomitant appearance of a field
effect anomaly centered on the gate voltage maintained. We are first interested
in ageing effects, i.e. the age dependence of the dynamical properties of the
system. We stress that the formation of a second field effect anomaly at a
different gate voltage is not a "history free" logarithmic (lnt) process, but
departs from lnt in a way which encodes the system's age. The apparent
relaxation time distribution extracted from the observed relaxations is thus
not "constant" but evolves with time. We discuss what defines the age of the
system and what external perturbation out of equilibrium does or does not
rejuvenate it. We further discuss the problem of relaxation times and comment
on the commonly used "two dip" experimental protocol aimed at extracting
"characteristic times" for the glassy systems (granular aluminum, doped indium
oxide...). We show that it is inoperable for systems like granular Al and
probably highly doped InOx where it provides a trivial value only determined by
the experimental protocol. But in cases where different values are obtained
like in lightly doped InOx or some ultra thin metal films, potentially
interesting information can be obtained, possibly about the "short time"
dynamics of the different systems. Present ideas about the effect of doping on
the glassiness of disordered insulators may also have to be reconsidered.Comment: to appear in the proceedings of the 14th International Conference on
Transport and Interactions in Disordered Systems (TIDS14
Squeezed Light and Entangled Images from Four-Wave-Mixing in Hot Rubidium Vapor
Entangled multi-spatial-mode fields have interesting applications in quantum
information, such as parallel quantum information protocols, quantum computing,
and quantum imaging. We study the use of a nondegenerate four-wave mixing
process in rubidium vapor at 795 nm to demonstrate generation of
quantum-entangled images. Owing to the lack of an optical resonator cavity, the
four-wave mixing scheme generates inherently multi-spatial-mode output fields.
We have verified the presence of entanglement between the multi-mode beams by
analyzing the amplitude difference and the phase sum noise using a dual
homodyne detection scheme, measuring more than 4 dB of squeezing in both cases.
This paper will discuss the quantum properties of amplifiers based on
four-wave-mixing, along with the multi mode properties of such devices.Comment: 11 pages, 8 figures. SPIE Optics and Photonics 2008 proceeding (San
Diego, CA
Five Things You Didn’t Want to Know about Hydraulic Fractures
It is common to envision and design hydraulic fractures as if they were simple, planar features that are relatively consistent in width and durable in their flow capacity. Production forecasting is frequently based on a simplified description of the reservoir as a homogeneous single productive layer. In rare instances the pay intervals may be simulated with as many as a dozen layered strata, but even the most meticulous reservoir engineer may mistakenly assign each layer a highly conductive, durable connection with the wellbore. When analyzing the resulting production data, similar assumptions are made, which can erroneously reinforce these misconceptions
Energy Efficient User Association and Power Allocation in Millimeter Wave Based Ultra Dense Networks with Energy Harvesting Base Stations
Millimeter wave (mmWave) communication technologies have recently emerged as
an attractive solution to meet the exponentially increasing demand on mobile
data traffic. Moreover, ultra dense networks (UDNs) combined with mmWave
technology are expected to increase both energy efficiency and spectral
efficiency. In this paper, user association and power allocation in mmWave
based UDNs is considered with attention to load balance constraints, energy
harvesting by base stations, user quality of service requirements, energy
efficiency, and cross-tier interference limits. The joint user association and
power optimization problem is modeled as a mixed-integer programming problem,
which is then transformed into a convex optimization problem by relaxing the
user association indicator and solved by Lagrangian dual decomposition. An
iterative gradient user association and power allocation algorithm is proposed
and shown to converge rapidly to an optimal point. The complexity of the
proposed algorithm is analyzed and the effectiveness of the proposed scheme
compared with existing methods is verified by simulations.Comment: to appear, IEEE Journal on Selected Areas in Communications, 201
A comprehensive study of electric, thermoelectric and thermal conductivities of Graphene with short range unitary and charged impurities
Motivated by the experimental measurement of electrical and hall
conductivity, thermopower and Nernst effect, we calculate the longitudinal and
transverse electrical and heat transport in graphene in the presence of unitary
scatterers as well as charged impurities. The temperature and carrier density
dependence in this system display a number of anomalous features that arise due
to the relativistic nature of the low energy fermionic degrees of freedom. We
derive the properties in detail including the effect of unitary and charged
impurities self-consistently, and present tables giving the analytic
expressions for all the transport properties in the limit of small and large
temperature compared to the chemical potential and the scattering rates. We
compare our results with the available experimental data. While the qualitative
variations with temperature and density of carriers or chemical potential of
all transport properties can be reproduced, we find that a given set of
parameters of the impurities fits the Hall conductivity, Thermopower and the
Nernst effect quantitatively but cannot fit the conductivity quantitatively. On
the other hand a single set of parameters for scattering from Coulomb
impurities fits conductivity, hall resistance and thermopower but not Nernst
Leptons from Dark Matter Annihilation in Milky Way Subhalos
Numerical simulations of dark matter collapse and structure formation show
that in addition to a large halo surrounding the baryonic component of our
galaxy, there also exists a significant number of subhalos that extend hundreds
of kiloparsecs beyond the edge of the observable Milky Way. We find that for
dark matter (DM) annihilation models, galactic subhalos can significantly
modify the spectrum of electrons and positrons as measured at our galactic
position. Using data from the recent Via Lactea II simulation we include the
subhalo contribution of electrons and positrons as boundary source terms for
simulations of high energy cosmic ray propagation with a modified version of
the publicly available GALPROP code. Focusing on the DM DM -> 4e annihilation
channel, we show that including subhalos leads to a better fit to both the
Fermi and PAMELA data. The best fit gives a dark matter particle mass of 1.2
TeV, for boost factors of 90 in the main halo and 1950-3800 in the subhalos
(depending on assumptions about the background), in contrast to the 0.85 TeV
mass that gives the best fit in the main halo-only scenario. These fits suggest
that at least a third of the observed electron cosmic rays from DM annihilation
could come from subhalos, opening up the possibility of a relaxation of recent
stringent constraints from inverse Compton gamma rays originating from the
high-energy leptons.Comment: 8 pages, 13 figures; added referenc
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