22,464 research outputs found
Optical squeezing of a mechanical oscillator by dispersive interaction
We consider a small partially reflecting vibrating mirror coupled
dispersively to a single optical mode of a high finesse cavity. We show this
arrangement can be used to implement quantum squeezing of the mechanically
oscillating mirror.Comment: 8 pages, 3 figure
Contribution from unresolved discrete sources to the Extragalactic Gamma-Ray Background (EGRB)
The origin of the extragalactic gamma-ray background (EGRB) is still an open
question, even after nearly forty years of its discovery. The emission could
originate from either truly diffuse processes or from unresolved point sources.
Although the majority of the 271 point sources detected by EGRET (Energetic
Gamma Ray Experiment Telescope) are unidentified, of the identified sources,
blazars are the dominant candidates. Therefore, unresolved blazars may be
considered the main contributor to the EGRB, and many studies have been carried
out to understand their distribution, evolution and contribution to the EGRB.
Considering that gamma-ray emission comes mostly from jets of blazars and that
the jet emission decreases rapidly with increasing jet to line-of-sight angle,
it is not surprising that EGRET was not able to detect many large inclination
angle active galactic nuclei (AGNs). Though Fermi could only detect a few large
inclination angle AGNs in the first three months' survey, it is expected to
detect many such sources in the near future. Since non-blazar AGNs are expected
to have higher density as compared to blazars, these could also contribute
significantly to the EGRB. In this paper we discuss contributions from
unresolved discrete sources including normal galaxies, starburst galaxies,
blazars and off-axis AGNs to the EGRB.Comment: 11 pages, 4 figures, accepted for publication in RA
Strain induced band gap deformation of H/F passivated graphene and h-BN sheet
Strain induced band gap deformations of hydrogenated/fluorinated graphene and
hexagonal BN sheet have been investigated using first principles density
functional calculations. Within harmonic approximation, the deformation is
found to be higher for hydrogenated systems than for the fluorinated systems.
Interestingly, our calculated band gap deformation for hydrogenated/fluorinated
graphene and BN sheets are positive, while those for pristine graphene and BN
sheet are found to be negative. This is due to the strong overlap between
nearest neighbor {\pi} orbitals in the pristine sheets, that is absent in the
passivated systems. We also estimate the intrinsic strength of these materials
under harmonic uniaxial strain, and find that the in-plane stiffness of
fluorinated and hydrogenated graphene are close, but larger in magnitude as
compared to those of fluorinated and hydrogenated BN sheet.Comment: Submitted to PR
Multicanonical Methods vs. Molecular Dynamics vs. Monte Carlo: Comparison for Lennard-Jones Glasses
We applied a multicanonical algorithm (entropic sampling) to a
two-dimensional and a three-dimensional Lennard-Jones system with
quasicrystalline and glassy ground states. Focusing on the ability of the
algorithm to locate low lying energy states, we compared the results of the
multicanonical simulations with standard Monte Carlo simulated annealing and
molecular dynamics methods. We find slight benefits to using entropic sampling
in small systems (less than 80 particles), which disappear with larger systems.
This is disappointing as the multicanonical methods are designed to surmount
energy barriers to relaxation. We analyze this failure theoretically, and show
(1) the multicanonical method is reduced in the thermodynamic limit (large
systems) to an effective Monte Carlo simulated annealing with a random
temperature vs. time, and (2) the multicanonical method gets trapped by
unphysical entropy barriers in the same metastable states whose energy barriers
trap the traditional quenches. The performance of Monte Carlo and molecular
dynamics quenches were remarkably similar.Comment: 12 pages, 6 figures, REVTEX, epsf.st
Collective decision making in cohesive flocks
Most of us must have been fascinated by the eye catching displays of
collectively moving animals. Schools of fish can move in a rather orderly
fashion and then change direction amazingly abruptly. There are a huge number
of further examples both from the living and the non-living world for phenomena
during which the many interacting, permanently moving units seem to arrive at a
common behavioural pattern taking place in a short time. As a paradigm of this
type of phenomena we consider the problem of how birds arrive at a decision
resulting in their synchronized landing. We introduce a simple model to
interpret this process. Collective motion prior to landing is modelled using a
simple self-propelled particle (SPP) system with a new kind of boundary
condition, while the tendency and the sudden propagation of the intention of
landing is introduced through rules analogous to the random field Ising model
in an external field. We show that our approach is capable of capturing the
most relevant features of collective decision making in a system of units with
a variance of individual intentions and being under an increasing level of
pressure to switch states. We find that as a function of the few parameters of
our model the collective switching from the flying to the landing state is
indeed much sharper than the distribution of the individual landing intentions.
The transition is accompanied by a number of interesting features discussed in
this report
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