728 research outputs found
Large magnetoresistance at room-temperature in small molecular weight organic semiconductor sandwich devices
We present an extensive study of a large, room temperature negative
magnetoresistance (MR) effect in tris-(8-hydroxyquinoline) aluminum sandwich
devices in weak magnetic fields. The effect is similar to that previously
discovered in polymer devices. We characterize this effect and discuss its
dependence on field direction, voltage, temperature, film thickness, and
electrode materials. The MR effect reaches almost 10% at fields of
approximately 10 mT at room temperature. The effect shows only a weak
temperature dependence and is independent of the sign and direction of the
magnetic field. Measuring the devices' current-voltage characteristics, we find
that the current depends on the voltage through a power-law. We find that the
magnetic field changes the prefactor of the power-law, whereas the exponent
remains unaffected. We also studied the effect of the magnetic field on the
electroluminescence (MEL) of the devices and analyze the relationship between
MR and MEL. We find that the largest part of MEL is simply a consequence of a
change in device current caused by the MR effect.Comment: 8 figure
On the stability of the primordial closed string gas
We recast the study of a closed string gas in a toroidal container in the
physical situation in which the single string density of states is independent
of the volume because energy density is very high. This includes the gas for
the well known Brandenberger-Vafa cosmological scenario. We describe the gas in
the grandcanonical and microcanonical ensembles. In the microcanonical
description, we find a result that clearly confronts the Brandenberger-Vafa
calculation to get the specific heat of the system. The important point is that
we use the same approach to the problem but a different regularization. By the
way, we show that, in the complex temperature formalism, at the Hagedorn
singularity, the analytic structure obtained from the so-called
F-representation of the free energy coincides with the one computed using the
S-representation.Comment: 20 pages and 1 figure. The final version that appeared in JHE
The future of pediatric pulmonology: A survey of division directors, assessment of current research funding, and discussion of workforce trends
Adequacy of the US workforce in pediatric pulmonology has been a source of serious concern within the field for some time, as it has been for several pediatric subspecialties. Contributing factors have been thought to include low fill rates of fellowship training programs, aging, and retirement rates of the subspecialist population, and the perception of insufficient numbers of specialists in some regions to meet clinical care needs. Several approaches to assessing workforce needs have already been described, and stakeholder groups are currently working on additional analyses. Although the recent report by Harris et al. captured a broad snapshot of workforce perceptions of 485 pediatric pulmonologists, this study (reporting data collected in 2014) did not address the future scope of practice
Instabilities in a Two-Component, Species Conserving Condensate
We consider a system of two species of bosons of equal mass, with
interactions and for bosons of the same and different
species respectively. We present a rigorous proof -- valid when the Hamiltonian
does not include a species switching term -- showing that, when
, the ground state is fully "polarized" (consists of
atoms of one kind only). In the unpolarized phase the low energy excitation
spectrum corresponds to two linearly dispersing modes that are even a nd odd
under species exchange. The polarization instability is signaled by the vani
shing of the velocity of the odd modes.Comment: To appear in Phys. Rev.
Astrophysical Tests of Modified Gravity: A Screening Map of the Nearby Universe
Astrophysical tests of modified modified gravity theories in the nearby
universe have been emphasized recently by Hui, Nicolis and Stubbs (2009) and
Jain and VanderPlas (2011). A key element of such tests is the screening
mechanism whereby general relativity is restored in massive halos or high
density environments like the Milky Way. In chameleon theories of gravity,
including all f(R) models, field dwarf galaxies may be unscreened and therefore
feel an extra force, as opposed to screened galaxies. The first step to study
differences between screened and unscreened galaxies is to create a 3D
screening map. We use N-body simulations to test and calibrate simple
approximations to determine the level of screening in galaxy catalogs. Sources
of systematic errors in the screening map due to observational inaccuracies are
modeled and their contamination is estimated. We then apply our methods to
create a map out to 200 Mpc in the Sloan Digital Sky Survey footprint using
data from the Sloan survey and other sources. In two companion papers this map
will be used to carry out new tests of gravity using distance indicators and
the disks of dwarf galaxies. We also make our screening map publicly available.Comment: 21 pages, 10 figure
Duality for symmetric second rank tensors. II. The linearized gravitational field
The construction of dual theories for linearized gravity in four dimensions
is considered. Our approach is based on the parent Lagrangian method previously
developed for the massive spin-two case, but now considered for the zero mass
case. This leads to a dual theory described in terms of a rank two symmetric
tensor, analogous to the usual gravitational field, and an auxiliary
antisymmetric field. This theory has an enlarged gauge symmetry, but with an
adequate partial gauge fixing it can be reduced to a gauge symmetry similar to
the standard one of linearized gravitation. We present examples illustrating
the general procedure and the physical interpretation of the dual fields. The
zero mass case of the massive theory dual to the massive spin-two theory is
also examined, but we show that it only contains a spin-zero excitation.Comment: 20 pages, no figure
Ferromagnetic phase transition and Bose-Einstein condensation in spinor Bose gases
Phase transitions in spinor Bose gases with ferromagnetic (FM) couplings are
studied via mean-field theory. We show that an infinitesimal value of the
coupling can induce a FM phase transition at a finite temperature always above
the critical temperature of Bose-Einstein condensation. This contrasts sharply
with the case of Fermi gases, in which the Stoner coupling can not lead
to a FM phase transition unless it is larger than a threshold value . The
FM coupling also increases the critical temperatures of both the ferromagnetic
transition and the Bose-Einstein condensation.Comment: 4 pages, 4 figure
Characterization of density fluctuations during the search for an I-mode regime on the DIII-D tokamak
The I-mode regime, routinely observed on the Alcator C-Mod tokamak, is characterized by an edge energy transport barrier without an accompanying particle barrier and with broadband instabilities, known as weakly coherent modes (WCM), believed to regulate particle transport at the edge. Recent experiments on the DIII-D tokamak exhibit I-mode characteristics in various physical quantities. These DIII-D plasmas evolve over long periods, lasting several energy confinement times, during which the edge electron temperature slowly evolves towards an H-mode-like profile, while maintaining a typical L-mode edge density profile. During these periods, referred to as I-mode phases, the radial electric field at the edge also gradually reaches values typically observed in H-mode. Density fluctuations measured with the phase contrast imaging diagnostic during I-mode phases exhibit three features typically observed in H-mode on DIII-D, although they develop progressively with time and without a sharp transition: the intensity of the fluctuations is reduced; the frequency spectrum is broadened and becomes non-monotonic; two dimensional space-time spectra appear to approach those in H-mode, showing phase velocities of density fluctuations at the edge increasing to about 10 km sâ1. However, in DIII-D there is no clear evidence of the WCM. Preliminary linear gyro-kinetic simulations are performed in the pedestal region with the GS2 code and its recently upgraded model collision operator that conserves particles, energy and momentum. The increased bootstrap current and flow shear generated by the temperature pedestal are shown to decrease growth rates, thus possibly generating a feedback mechanism that progressively stabilizes fluctuations.United States. Department of Energy. Office of Fusion Energy Sciences (Award DE-FG02- 94ER54235)United States. Department of Energy. Office of Fusion Energy Sciences (Award DE-FG02-94ER54084)United States. Department of Energy. Office of Fusion Energy Sciences (Award DE-FG02-08ER54984)United States. Department of Energy. Office of Fusion Energy Sciences (Award DE-FC02-04ER54698
Momentum transfer using chirped standing wave fields: Bragg scattering
We consider momentum transfer using frequency-chirped standing wave fields.
Novel atom-beam splitter and mirror schemes based on Bragg scattering are
presented. It is shown that a predetermined number of photon momenta can be
transferred to the atoms in a single interaction zone.Comment: 4 pages, 3 figure
Coherent matter wave inertial sensors for precision measurements in space
We analyze the advantages of using ultra-cold coherent sources of atoms for
matter-wave interferometry in space. We present a proof-of-principle experiment
that is based on an analysis of the results previously published in [Richard et
al., Phys. Rev. Lett., 91, 010405 (2003)] from which we extract the ratio h/m
for 87Rb. This measurement shows that a limitation in accuracy arises due to
atomic interactions within the Bose-Einstein condensate
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