8,188 research outputs found
Intersubband magnetophonon resonances in quantum cascade structures
We report on our magnetotransport measurements of GaAs/GaAlAs quantum cascade
structures in a magnetic field of up to 62 T. We observe novel quantum
oscillations in tunneling current that are periodic in reciprocal magnetic
field. We explain these oscillations as intersubband magnetophonon resonance
due to electron relaxation by emission of either single optical or acoustic
phonons. Our work also provides a non-optical in situ measurement of
intersubband separations in quantum cascade structures.Comment: 5 pages, 4 figure
Cooling of the Cassiopeia A neutron star and the effect of diffusive nuclear burning
The study of how neutron stars cool over time can provide invaluable insights
into fundamental physics such as the nuclear equation of state and
superconductivity and superfluidity. A critical relation in neutron star
cooling is the one between observed surface temperature and interior
temperature. This relation is determined by the composition of the neutron star
envelope and can be influenced by the process of diffusive nuclear burning
(DNB). We calculate models of envelopes that include DNB and find that DNB can
lead to a rapidly changing envelope composition which can be relevant for
understanding the long-term cooling behavior of neutron stars. We also report
on analysis of the latest temperature measurements of the young neutron star in
the Cassiopeia A supernova remnant. The 13 Chandra observations over 18 years
show that the neutron star's temperature is decreasing at a rate of 2-3 percent
per decade, and this rapid cooling can be explained by the presence of a proton
superconductor and neutron superfluid in the core of the star.Comment: 7 pages, 7 figures; to appear in the AIP Conference Proceedings of
the Xiamen-CUSTIPEN Workshop on the EOS of Dense Neutron-Rich Matter in the
Era of Gravitational Wave Astronomy (January 3-7, 2019, Xiamen, China
Operator ordering and consistency of the wavefunction of the Universe
We demonstrate in the context of the minisuperspace model consisting of a
closed Friedmann-Robertson-Walker universe coupled to a scalar field that
Vilenkin's tunneling wavefunction can only be consistently defined for
particular choices of operator ordering in the Wheeler-DeWitt equation. The
requirement of regularity of the wavefunction has the particular consequence
that the probability amplitude, which has been used previously in the
literature in discussions of issues such as the prediction of inflation, is
likewise ill-defined for certain choices of operator ordering with Vilenkin's
boundary condition. By contrast, the Hartle-Hawking no-boundary wavefunction
can be consistently defined within these models, independently of operator
ordering. The significance of this result is discussed within the context of
the debate about the predictions of semiclassical quantum cosmology. In
particular, it is argued that inflation cannot be confidently regarded as a
"prediction" of the tunneling wavefunction, for reasons similar to those
previously invoked in the case of the no-boundary wavefunction. A synthesis of
the no-boundary and tunneling approaches is argued for.Comment: 9 pages, epsf, revTeX-3.1, 1 figure. In revised version (v2) a new
section etc with additional arguments increases the length of paper by 3
pages of Physical Review; several references added. v3: small typos fixe
The Luminosity Function Evolution of Soft X--ray selected AGN in the RIXOS survey
A sample of 198 soft X--ray selected active galactic nuclei (AGN) from the
ROSAT International X--ray Optical Survey (RIXOS), is used to investigate the
X--ray luminosity function and its evolution. RIXOS, with a flux limit of 3E-14
erg s-1 cm-2 (0.5 to 2.0 keV), samples a broad range in redshift over 20 deg^2
of sky, and is almost completely identified; it is used in combination with the
Einstein Extended Medium Sensitivity Survey (EMSS), to give a total sample of
over 600 AGN. We find the evolution of AGN with redshift to be consistent with
pure luminosity evolution (PLE) models in which the rate of evolution slows
markedly or stops at high redshifts z>1.8. We find that this result is not
affected by the inclusion, or exclusion, of narrow emission line galaxies at
low redshift in the RIXOS and EMSS samples, and is insensitive to uncertainties
in the conversion between flux values measured with ROSAT and Einstein. We
confirm, using a model independent Ve/Va test, that our survey is consistent
with no evolution at high redshifts.Comment: 10 pages, LaTeX file, PS figures and mn.sty. Accepted in MNRA
Black-hole radiation, the fundamental area unit, and the spectrum of particle species
Bekenstein and Mukhanov have put forward the idea that, in a quantum theory
of gravity a black hole should have a discrete mass spectrum with a concomitant
{\it discrete} line emission. We note that a direct consequence of this
intriguing prediction is that, compared with blackbody radiation, black-hole
radiance is {\it less} entropic. We calculate the ratio of entropy emission
rate from a quantum black hole to the rate of black-hole entropy decrease, a
quantity which, according to the generalized second law (GSL) of
thermodynamics, should be larger than unity. Implications of our results for
the GSL, for the value of the fundamental area unit in quantum gravity, and for
the spectrum of massless particles in nature are discussed.Comment: 4 page
Space for Both No-Boundary and Tunneling Quantum States of the Universe
At the minisuperspace level of homogeneous models, the bare probability for a
classical universe has a huge peak at small universes for the Hartle-Hawking
`no-boundary' wavefunction, in contrast to the suppression at small universes
for the `tunneling' wavefunction. If the probability distribution is cut off at
the Planck density (say), this suggests that the former quantum state is
inconsistent with our observations. For inhomogeneous models in which
stochastic inflation can occur, it is known that the idea of including a volume
factor in the observational probability distribution can lead to arbitrarily
large universes' being likely. Here this idea is shown to be sufficient to save
the Hartle-Hawking proposal even at the minisuperspace level (for suitable
inflaton potentials), by giving it enough space to be consistent with
observations.Comment: LaTeX, 20 pages, no figures, blank lines removed, page break inserte
Developing bus transfer facilities for maximum transit agency and community benefit
Bus transfer centers are often regarded as âundesirable neighborsâ that are difficult to site and difficult to gain support for due to noise, exhaust, traffic congestion, and the presence of unwanted passengers. In fact, there are some locations where major bus transfer activities are not only not welcome, but are probably not in the best interests of the surrounding development. However, many transit agencies are elevating the acceptance and relevance of transit in their service areas by making their transfer centers true community assets rather than nuisances. This often means finding the right location for a transit center for both the surrounding community and the passengers, replacing run down development with new facilities, incorporating exciting and inspiring architecture and design, and improving the pedestrian amenities and safety and security of the immediate area around the transfer center. Some communities have greatly expanded the concept of bus transfer centers by using them as locations for vital health and human services, as well as other conveniences that improve the quality of life for the residents of the surrounding community. In addition, others have utilized the advantage of their Federal grants to build transit centers that provide opportunities for joint development that help to generate revenues that can be used for other public improvements in the immediate area, which help to attract additional private investment and positive development where there once was blight. This report highlights how four transit agencies used their bus transfer centers to not only improve their image and community relations, but to serve as catalysts for positive development in the surrounding areas.U.S. Department of Transportation, Washington, D.C.Florida Department of Transportation, Tallahassee, FLhttp://deepblue.lib.umich.edu/bitstream/2027.42/64893/1/102506.pd
Number counts and clustering properties of bright Distant Red Galaxies in the UKIDSS Ultra Deep Survey Early Data Release
We describe the number counts and spatial distribution of 239 Distant Red
Galaxies (DRGs), selected from the Early Data Release of the UKIDSS Ultra Deep
Survey. The DRGs are identified by their very red infrared colours with
(J-K)AB>1.3, selected over 0.62 sq degree to a 90% completeness limit of
KAB~20.7. This is the first time a large sample of bright DRGs has been studied
within a contiguous area, and we provide the first measurements of their number
counts and clustering. The population shows strong angular clustering,
intermediate between those of K-selected field galaxies and
optical/infrared-selected Extremely Red Galaxies. Adopting the redshift
distributions determined from other recent studies, we infer a high correlation
length of r0~11 h-1 Mpc. Such strong clustering could imply that our galaxies
are hosted by very massive dark matter halos, consistent with the progenitors
of present-day L>L* elliptical galaxies.Comment: 5 pages, 4 figures, revised version accepted to MNRAS.
Higher-resolution figures available from the authors on reques
A schematic model for QCD at finite temperature
The simplest version of a class of toy models for QCD is presented. It is a
Lipkin-type model, for the quark-antiquark sector, and, for the gluon sector,
gluon pairs with spin zero are treated as elementary bosons. The model
restricts to mesons with spin zero and to few baryonic states. The
corresponding energy spectrum is discussed. We show that ground state
correlations are essential to describe physical properties of the spectrum at
low energies. Phase transitions are described in an effective manner, by using
coherent states. The appearance of a Goldstone boson for large values of the
interaction strength is discussed, as related to a collective state. The
formalism is extended to consider finite temperatures. The partition function
is calculated, in an approximate way, showing the convenience of the use of
coherent states. The energy density, heat capacity and transitions from the
hadronic phase to the quark-gluon plasma are calculated.Comment: 33 pages, 11 figure
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