2,099 research outputs found
Intermodal Four-Wave-Mixing and Parametric Amplification in km-long Fibers
We theoretically and numerically investigate intermodal four-wave-mixing in
km-long fibers, where random birefringence fluctuations are present along the
fiber length. We identify several distinct regimes that depend on the relative
magnitude between the length scale of the random fluctuations and the
beat-lengths of the interacting quasi-degenerate modes. In addition, we analyze
the impact of polarization mode-dispersion and we demonstrate that random
variations of the core radius, which are typically encountered during the
drawing stage of the fiber, can represent the major source of bandwidth
impairment. These results set a boundary on the limits of validity of the
classical Manakov model and may be useful for the design of multimode
parametric amplifiers and wavelength converters, as well as for the analysis of
nonlinear impairments in long-haul spatial division multiplexed transmission
CHIRON - A Fiber Fed Spectrometer for Precise Radial Velocities
The CHIRON optical high-resolution echelle spectrometer was commissioned at
the 1.5m telescope at CTIO in 2011. The instrument was designed for high
throughput and stability, with the goal of monitoring radial velocities of
bright stars with high precision and high cadence for the discovery of low-mass
exoplanets. Spectral resolution of R=79,000 is attained when using a slicer
with a total (including telescope and detector) efficiency of 6% or higher,
while a resolution of R=136,000 is available for bright stars. A fixed spectral
range of 415 to 880 nm is covered. The echelle grating is housed in a vacuum
enclosure and the instrument temperature is stabilized to +-0.2deg. Stable
illumination is provided by an octagonal multimode fiber with excellent
light-scrambling properties. An iodine cell is used for wavelength calibration.
We describe the main optics, fiber feed, detector, exposure-meter, and other
aspects of the instrument, as well as the observing procedure and data
reduction.Comment: 15 pages, 10 figures. Accepted by PAS
Quantum Monte Carlo calculations of electronic excitation energies: the case of the singlet (CO) transition in acrolein
We report state-of-the-art quantum Monte Carlo calculations of the singlet (CO) vertical excitation energy in the acrolein molecule, extending
the recent study of Bouab\c{c}a {\it et al.} [J. Chem. Phys. {\bf 130}, 114107
(2009)]. We investigate the effect of using a Slater basis set instead of a
Gaussian basis set, and of using state-average versus state-specific
complete-active-space (CAS) wave functions, with or without reoptimization of
the coefficients of the configuration state functions (CSFs) and of the
orbitals in variational Monte Carlo (VMC). It is found that, with the Slater
basis set used here, both state-average and state-specific CAS(6,5) wave
functions give an accurate excitation energy in diffusion Monte Carlo (DMC),
with or without reoptimization of the CSF and orbital coefficients in the
presence of the Jastrow factor. In contrast, the CAS(2,2) wave functions
require reoptimization of the CSF and orbital coefficients to give a good DMC
excitation energy. Our best estimates of the vertical excitation energy are
between 3.86 and 3.89 eV.Comment: 6 pages, 1 figure, 2 tables, to appear in Progress in Theoretical
Chemistry and Physic
Quantum Monte Carlo facing the Hartree-Fock symmetry dilemma: The case of hydrogen rings
When using Hartree-Fock (HF) trial wave functions in quantum Monte Carlo
calculations, one faces, in case of HF instabilities, the HF symmetry dilemma
in choosing between the symmetry-adapted solution of higher HF energy and
symmetry-broken solutions of lower HF energies. In this work, we have examined
the HF symmetry dilemma in hydrogen rings which present singlet instabilities
for sufficiently large rings. We have found that the symmetry-adapted HF wave
function gives a lower energy both in variational Monte Carlo and in fixed-node
diffusion Monte Carlo. This indicates that the symmetry-adapted wave function
has more accurate nodes than the symmetry-broken wave functions, and thus
suggests that spatial symmetry is an important criterion for selecting good
trial wave functions.Comment: 6 pages, 3 figures, 2 tables, to appear in "Advances in Quantum Monte
Carlo", AC
Power-law cosmic expansion in f(R) gravity models
We show that within the class of f(R) gravity theories, FLRW power-law
perfect fluid solutions only exist for R^n gravity. This significantly
restricts the set of exact cosmological solutions which have similar properties
to what is found in standard General Relativity.Comment: 4 pages, 2 figure
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Interstitial lung abnormalities in patients with early rheumatoid arthritis: A pilot study evaluating prevalence and progression.
OBJECTIVE:Pulmonary disease is a leading cause of morbidity and mortality in rheumatoid arthritis (RA). In this study, we investigated the prevalence and progression of interstitial lung abnormalities (ILA) in a prospective cohort study of 18 subjects with early RA. METHODS:Eighteen adults diagnosed with anti-citrullinated protein-antibody-positive RA within the prior year underwent baseline high-resolution computed tomography (HRCT), symptom assessment, and pulmonary function and laboratory testing. The follow-up HRCT and clinical assessment were completed after 1 year. RESULTS:Seven of the 18 patients (39%) had baseline HRCT abnormalities including septal thickening, honeycombing, ground glass opacities, and/or traction bronchiectasis. At follow-up, 6 out of the 7 subjects (86%) with ILAs at baseline exhibited progression, while 10 out of 11 (91%) without ILAs at baseline remained stable. A higher Clinical Chronic Obstructive Pulmonary Disease Questionnaire score was associated with both the presence and progression of HRCT abnormalities (10 vs 2, p=0.045; 10 vs 2, p=0.009, respectively). C-reactive protein (CRP) trended higher in patients with radiologic abnormalities (3.5 mg/L vs 1.1 mg/L, p=0.08) and was significantly higher in those with progression (3.5 mg/L vs 1 mg/L, p=0.024). Smoking, pulmonary function, and autoantibodies were not associated with HRCT abnormalities. CONCLUSION:ILAs are prevalent in patients with early RA. If identified at baseline, radiographic progression of ILAs after 1 year is likely, while those without ILAs at baseline are unlikely to develop new ILAs. In addition, early respiratory symptoms and higher CRP levels may correlate with the presence and progression of underlying ILAs
Aquatic and terrestrial proxy evidence for Middle Pleistocene palaeolake and lakeâshore development at two Lower Palaeolithic sites of Schöningen, Germany
The archaeological sites in the openâcast mine of Schöningen, Germany, represent outstanding archives for understanding Middle Pleistocene interglacialâglacial transitions and human adaption. Aquatic microfossil and pollen assemblages from the âReinsdorf sequenceâ, likely correlated to Marine Isotope Stage 9, document environmental changes from a thermal maximum to succeeding glacial conditions recorded in two sequences of excavation sites 12 II and 13 II. Multiâproxy analyses enable detailed reconstruction of lakeâshore and landscape developments despite variable microfossil preservation in changing carbonateâ and organicârich deposits. Rich aquatic vegetation with abundant charophytes suggests repeated phases with water depths of 0.5â2 m at site 13 II, while even greater temporary depths are deduced for 12 II DB. Mesorheophilic and mesotitanophilic ostracod species indicate stream inflows with mediumâlow calcium contents of >18 mg Ca L â1 originating from nearby springs. Diatoms point to mesoâeutrophic conditions and an alkaline pH of the lake water. Interglacial conditions with thermophile forests but no aquatic microfossils preserved, suggesting a dry or only temporarily flooded site, mark the beginning of the sequence. Continuous presence of aquatic organisms and overall dominance of small tychoplanktonic diatoms during a subsequent cool steppe phase provide evidence for increased water depths and unstable habitats characterized by erosion and probably prolonged periods of lake ice cover. During the succeeding boreal forestâsteppe phase, surface runoff into the productive, shallow lake decreased due to a more extensive vegetation cover. Concurrently, intensified groundwater input in contact with the nearby salt wall caused elevated salinities. Following a lake level drop, stream inflows and lake levels increased again towards the end of the Reinsdorf sequence and promoted development of a diverse fauna and flora at the lake shore; thereby maintaining an attractive living and hunting environment for early humans during a phase of generally cooler temperatures and landscape instability at the transition into a glacial period
Pulmonary capillary hemangiomatosis: a lesson learned
Pulmonary capillary hemangiomatosis (PCH) is a rare and controversial entity that is known to be a cause of pulmonary hypertension and is microscopically characterized by proliferation of dilated capillary-sized channels along and in the alveolar walls. Clinically, it is mostly seen in adults. Clinical features are characterized by nonspecific findings such as shortness of breath, cough, chest pain, and fatigue. It can be clinically indistinguishable from pre-capillary pulmonary arterial hypertension disorders such as primary pulmonary arterial hypertension (PAH) or chronic thromboembolic pulmonary hypertension. However, the diagnostic distinction, which usually requires a multidisciplinary approach, is crucial in order to avoid inappropriate treatment with vasodilator medications usually used for PAH treatment. Prognosis of PCH remains poor with lung transplant being the only definitive treatment. We report an autopsy case of pulmonary capillary hemangiomatosis unmasked at autopsy that was treated with a prostacyclin analog, usually contraindicated in such patients. We emphasize that this entity should always be on the differential diagnosis in a patient with pulmonary hypertension and requires great vigilance on the part of the clinician, radiologist and pathologist to make the diagnosis and guide appropriate management
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