82 research outputs found
Transport Measurements on Nano-engineered Two Dimensional Superconducting Wire Networks
Superconducting triangular Nb wire networks with high normal-state resistance
are fabricated by using a negative tone hydrogen silsesquioxane (HSQ) resist.
Robust magnetoresistance oscillations are observed up to high magnetic fields
and maintained at low temperatures, due to the eective reduction of wire
dimensions. Well-defined dips appear at integral and rational values (1/2, 1/3,
1/4) of the reduced flux f = Phi/Phi_0, which is the first observation in the
triangular wire networks. These results are well consistent with theoretical
calculations for the reduced critical temperature as a function of f.Comment: 4 pages, 3 figure
Cosmic Microwave Background anisotropies from second order gravitational perturbations
This paper presents a complete analysis of the effects of second order
gravitational perturbations on Cosmic Microwave Background anisotropies, taking
explicitly into account scalar, vector and tensor modes. We also consider the
second order perturbations of the metric itself obtaining them, for a universe
dominated by a collision-less fluid, in the Poisson gauge, by transforming the
known results in the synchronous gauge. We discuss the resulting second order
anisotropies in the Poisson gauge, and analyse the possible relevance of the
different terms. We expect that, in the simplest scenarios for structure
formation, the main effect comes from the gravitational lensing by scalar
perturbations, that is known to give a few percent contribution to the
anisotropies at small angular scales.Comment: 15 pages, revtex, no figures. Version to be published in Phys. Rev.
Witnessing the formation of a brightest cluster galaxy at z>2
We present deep observations taken with the HST Advanced Camera for Surveys
of the central massive galaxy in a forming cluster at z=2.2. The galaxy hosting
the powerful radio source MRC 1138-262 is associated with one of the most
extensive merger systems known in the early universe. Our HST/ACS image shows
many star-forming galaxies merging within a ~200 kpc region that emits both
diffuse line emission and continuum in the rest-frame UV. Because this galaxy
lives in an overdense environment, it represents a rare view of a brightest
cluster galaxy in formation at z>2 which may serve as a testbed for predictions
of massive cluster galaxy formation.Comment: Contribution to the proceedings of "The Fate of Gas in Galaxies",
Dwingeloo, July 2006, with 2 colour figures. To appear in New Astronomy
Reviews, Vol. 51 (2007), eds. Morganti, Oosterloo, Villar-Martin & van Gorko
Giant Superfluorescent Bursts from a Semiconductor Magnetoplasma
Currently, considerable resurgent interest exists in the concept of
superradiance (SR), i.e., accelerated relaxation of excited dipoles due to
cooperative spontaneous emission, first proposed by Dicke in 1954. Recent
authors have discussed SR in diverse contexts, including cavity quantum
electrodynamics, quantum phase transitions, and plasmonics. At the heart of
these various experiments lies the coherent coupling of constituent particles
to each other via their radiation field that cooperatively governs the dynamics
of the whole system. In the most exciting form of SR, called superfluorescence
(SF), macroscopic coherence spontaneously builds up out of an initially
incoherent ensemble of excited dipoles and then decays abruptly. Here, we
demonstrate the emergence of this photon-mediated, cooperative, many-body state
in a very unlikely system: an ultradense electron-hole plasma in a
semiconductor. We observe intense, delayed pulses, or bursts, of coherent
radiation from highly photo-excited semiconductor quantum wells with a
concomitant sudden decrease in population from total inversion to zero. Unlike
previously reported SF in atomic and molecular systems that occur on nanosecond
time scales, these intense SF bursts have picosecond pulse-widths and are
delayed in time by tens of picoseconds with respect to the excitation pulse.
They appear only at sufficiently high excitation powers and magnetic fields and
sufficiently low temperatures - where various interactions causing decoherence
are suppressed. We present theoretical simulations based on the relaxation and
recombination dynamics of ultrahigh-density electron-hole pairs in a quantizing
magnetic field, which successfully capture the salient features of the
experimental observations.Comment: 21 pages, 4 figure
On the sensitivity of the HAWC observatory to gamma-ray bursts
We present the sensitivity of HAWC to Gamma Ray Bursts (GRBs). HAWC is a very
high-energy gamma-ray observatory currently under construction in Mexico at an
altitude of 4100 m. It will observe atmospheric air showers via the water
Cherenkov method. HAWC will consist of 300 large water tanks instrumented with
4 photomultipliers each. HAWC has two data acquisition (DAQ) systems. The main
DAQ system reads out coincident signals in the tanks and reconstructs the
direction and energy of individual atmospheric showers. The scaler DAQ counts
the hits in each photomultiplier tube (PMT) in the detector and searches for a
statistical excess over the noise of all PMTs. We show that HAWC has a
realistic opportunity to observe the high-energy power law components of GRBs
that extend at least up to 30 GeV, as it has been observed by Fermi LAT. The
two DAQ systems have an energy threshold that is low enough to observe events
similar to GRB 090510 and GRB 090902b with the characteristics observed by
Fermi LAT. HAWC will provide information about the high-energy spectra of GRBs
which in turn could help to understanding about e-pair attenuation in GRB jets,
extragalactic background light absorption, as well as establishing the highest
energy to which GRBs accelerate particles
Local Luminous Infrared Galaxies: Spatially resolved mid-infrared observations with Spitzer/IRS
Luminous Infrared (IR) Galaxies (LIRGs) are an important cosmological class
of galaxies as they are the main contributors to the co-moving star formation
rate density of the universe at z=1. In this paper we present a GTO Spitzer IRS
program aimed to obtain spectral mapping of a sample of 14 local (d<76Mpc)
LIRGs. The data cubes map, at least, the central 20arcsec x 20arcsec to
30arcsec x 30arcsec regions of the galaxies, and use all four IRS modules
covering the full 5-38micron spectral range. The final goal of this project is
to characterize fully the mid-IR properties of local LIRGs as a first step to
understanding their more distant counterparts. In this paper we present the
first results of this GTO program. The IRS spectral mapping data allow us to
build spectral maps of the bright mid-IR emission lines (e.g., [NeII], [NeIII],
[SIII], H_2), continuum, the 6.2 and 11.3micron PAH features, and the 9.7micron
silicate feature, as well as to extract 1D spectra for regions of interest in
each galaxy. The IRS data are used to obtain spatially resolved measurements of
the extinction using the 9.7micron silicate feature, and to trace star forming
regions using the neon lines and the PAH features. We also investigate a number
of AGN indicators, including the presence of high excitation emission lines and
a strong dust continuum emission at around 6micron. We finally use the
integrated Spitzer/IRS spectra as templates of local LIRGs. We discuss several
possible uses for these templates, including the calibration of the star
formation rate of IR-bright galaxies at high redshift. We also predict the
intensities of the brightest mid-IR emission lines for LIRGs as a function of
redshift, and compare them with the expected sensitivities of future space IR
missions.Comment: Accepted for publication in Advances in Space Researc
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