20,851 research outputs found
Doping dependent optical properties of Bi2201
An experimental study of the in-plane optical conductivity of
(Pb,Bi)(LaSr)CuO (Bi2201) is presented
for a broad doping and temperature range. The in-plane conductivity is analyzed
within a strong coupling formalism. We address the interrelationship between
the optical conductivity (), the single particle self energy,
and the electron-boson spectral function. We find that the frequency and
temperature dependence can be well described within this formalism. We present
a universal description of optical, ARPES and tunneling spectra. The full
frequency and temperature dependence of the optical spectra and single particle
self-energy is shown to result from an electron-boson spectral function, which
shows a strong doping dependence and weak temperature dependence.Comment: 20 pages, 9 figures. To appear in special focus issue
"Superconductors with Exotic Symmetries", New Journal of Physic
Control and coherence of the optical transition of single defect centers in diamond
We demonstrate coherent control of the optical transition of single
Nitrogen-Vacancy defect centers in diamond. On applying short resonant laser
pulses, we observe optical Rabi oscillations with a half-period as short as 1
nanosecond, an order of magnitude shorter than the spontaneous emission time.
By studying the decay of Rabi oscillations, we find that the decoherence is
dominated by laser-induced spectral jumps. By using a low-power probe pulse as
a detuning sensor and applying post-selection, we demonstrate that spectral
diffusion can be overcome in this system to generate coherent photons.Comment: 5 pages,4 figure
Influence of an Internal Magnetar on Supernova Remnant Expansion
Most of the proposed associations between magnetars and supernova remnant
suffer from age problems. Usually, supernova remnants ages are determined from
an approximation of the Sedov-Taylor phase relation between radius and age, for
a fixed energy of the explosion ~ 10^{51} erg. Those ages do not generally
agree with the characteristic ages of the (proposed) associated magnetars. We
show quantitatively that, by taking into account the energy injected on the
supernova remnant by magnetar spin-down, a faster expansion results, improving
matches between characteristic ages and supernova remnants ages. However, the
magnetar velocities inferred from observations would inviabilize some
associations. Since characteristic ages may not be good age estimators, their
influence on the likelihood of the association may not be as important.
In this work we present simple numerical simulations of supernova remnants
expansion with internal magnetars, and apply it to the observed objects. A
short initial spin period, thought to be important for the very generation of
the magnetic field, is also relevant for the modified expansion of the remnant.
We next analyze all proposed associations case-by-case, addressing the
likelyhood of each one, according to this perspective. We consider a larger
explosion energy and reasses the characteristic age issue, and conclude that
about 50% of the associations can be true ones, provided SGRs and AXPs are
magnetars.Comment: 30 pages, AAStex, 5 figures, format fixe
Modelling an isolated dust grain in a plasma using matched asymptotic expansions
The study of dusty plasmas is of significant practical use and scientific interest. A characteristic feature of dust grains in a plasma is that they are typically smaller than the electron Debye distance, a property which we exploit using the technique of matched asymptotic expansions. We first consider the case of a spherical dust particle in a stationary plasma, employing the Allen–Boyd–Reynolds theory, which assumes cold, collisionless ions. We derive analytical expressions for the electric potential, the ion number density and ion velocity. This requires only one computation that is not specific to a single set of dust–plasma parameters, and sheds new light on the shielding distance of a dust grain. The extension of this calculation to the case of uniform ion streaming past the dust grain, a scenario of interest in many dusty plasmas, is less straightforward. For streaming below a certain threshold we again establish asymptotic solutions but above the streaming threshold there appears to be a fundamental change in the behaviour of the system
AN EVALUATION OF THE PRIA GRAZING FEE FORMULA
The federal grazing fee is currently set using the Public Rangeland Improvement Act (PRIA) fee formula established in 1978 and modified in 1986. The formula is adjusted annually using indices of private land grazing lease rates (Forage Value Index, FVI), prices received for beef cattle (Beef Cattle Price Index, BCPI), and costs of beef production (Prices Paid Index, PPI). The FVI tracks price movement in the private forage market and was the only index originally proposed to be included in the fee formula. Public land ranchers and an Interdepartmental Grazing Fee Technical Committee assigned to study grazing fee alternatives in the 1960s questioned the ability of the FVI to account for short-term demand, supply, and price equilibrium, and, for this reason, the BCPI and PPI were added to the fee formula. Over 30 years of data are now available to evaluate whether adding the BCPI and PPI did, in fact, help explain short-term market fluctuations. This analysis shows, as earlier studies did, that, if tracking the private forage market is the primary objective, then the fee formula should have included only the FVI. Including the BCPI and, especially, the PPI has caused calculated grazing fees to fall further and further behind private land lease rates. Had the 3.84/AUM instead of $1.35/AUM in 2000. It is time to consider the feasibility of a competitive bid system for public lands, or, at the very least, adopt a new fee formula that generates more equitable grazing fees.Land Economics/Use,
Modelling the Pioneer anomaly as modified inertia
This paper proposes an explanation for the Pioneer anomaly: an unexplained
Sunward acceleration of 8.74 +/- 1.33 x 10^-10 m s^-2 seen in the behaviour of
the Pioneer probes. Two hypotheses are made: (1) Inertia is a reaction to Unruh
radiation and (2) this reaction is weaker for low accelerations because some
wavelengths in the Unruh spectrum do not fit within a limiting scale (twice the
Hubble distance) and are disallowed: a process similar to the Casimir effect.
When these ideas are used to model the Pioneer crafts' trajectories there is a
slight reduction in their inertial mass, causing an anomalous Sunward
acceleration of 6.9 +/- 3.5 x 10^-10 m s^-2 which agrees within error bars with
the observed Pioneer anomaly beyond 10 AU from the Sun. This new scheme is
appealingly simple and does not require adjustable parameters. However, it also
predicts an anomaly within 10 AU of the Sun, which has not been observed.
Various observational tests for the idea are proposed.Comment: 15 pages, 2 bw figures, accepted by MNRAS 19th December 200
Dipole Interactions and Electrical Polarity in Nanosystems -- the Clausius-Mossotti and Related Models
Point polarizable molecules at fixed spatial positions have solvable
electrostatic properties in classical approximation, the most familiar being
the Clausius-Mossotti (CM) formula. This paper generalizes the model and
imagines various applications to nanosystems. The behavior is worked out for a
sequence of octahedral fragments of simple cubic crystals, and the crossover to
the bulk CM law is found. Some relations to fixed moment systems are discussed
and exploited. The one-dimensional dipole stack is introduced as an important
model system. The energy of interaction of parallel stacks is worked out, and
clarifies the diverse behavior found in different crystal structures. It also
suggests patterns of self-organization which polar molecules in solution might
adopt. A sum rule on the stack interaction is found and tested. Stability of
polarized states under thermal fluctuations is discussed, using the
one-dimensional domain wall as an example. Possible structures for polar hard
ellipsoids are considered. An idea is formulated for enhancing polarity of
nanosystems by intentionally adding metallic coatings.Comment: 18 pages (includes 6 embedded figures and 3 tables). New references,
and other small improvements. Scheduled for publication by J. Chem. Phys.,
Jan. 200
Photons in polychromatic rotating modes
We propose a quantum theory of rotating light beams and study some of its
properties. Such beams are polychromatic and have either a slowly rotating
polarization or a slowly rotating transverse mode pattern. We show there are,
for both cases, three different natural types of modes that qualify as
rotating, one of which is a new type not previously considered. We discuss
differences between these three types of rotating modes on the one hand and
non-rotating modes as viewed from a rotating frame of reference on the other.
We present various examples illustrating the possible use of rotating photons,
mostly for quantum information processing purposes. We introduce in this
context a rotating version of the two-photon singlet state.Comment: enormously expanded: 12 pages, 3 figures; a new, more informative,
but less elegant title, especially designed for Phys. Rev.
Where Do Cooling Flows Cool?
Although only about 5 percent of the total baryonic mass in luminous
elliptical galaxies is in the form of cooled interstellar gas, it is
concentrated within the optical effective radius r_e where it influences the
local dynamical mass. The mass of cooled gas must be spatially distributed
since it greatly exceeds the masses of central black holes. We explore here the
proposition that a population of low mass, optically dark stars is created from
the cooled gas. We consider a wide variety of radial distributions for the
interstellar cooling, but only a few are consistent with observed X-ray surface
brightness profiles. In a region of concentrated interstellar cooling, the
X-ray emission can exceed that observed, suggesting the presence of additional
support by magnetic stresses or non-thermal pressure. In general we find that
the mass of cooled gas contributes significantly to stellar dynamical mass to
light ratios which vary with galactic radius. If the stars formed from cooled
interstellar gas are optically luminous, their influence on the the mass to
light ratio would be reduced. The mass of cooled gas inside r_e is sensitive to
the rate that old stars lose mass, which is nearly independent of the initial
mass function of the old stellar population.Comment: 18 pages with 6 figures; accepted by Astrophysical Journa
Zoonotic realism, computational cognitive science and pandemic prevention
Using animals in food and food production systems is one of many drivers of novel zoonoses. Moving toward less dependence on animal proteins is a possible avenue for reducing pandemic risk, but we think that Wiebers & Feigin’s proposed change to food policy (phasing out animal meat production) is unrealistic in its political achievability and its current capacity to feed the world in a cost-effective and sustainable manner. We suggest that improvements in communication strategies, precipitated by developments in computational cognitive neuroscience, can lead the way to a safer future and are feasible now
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