22,680 research outputs found
Reduced Dynamics from the Unitary Group to Some Flag Manifolds : Interacting Matrix Riccati Equations
In this paper we treat the time evolution of unitary elements in the N level
system and consider the reduced dynamics from the unitary group U(N) to flag
manifolds of the second type (in our terminology). Then we derive a set of
differential equations of matrix Riccati types interacting with one another and
present an important problem on a nonlinear superposition formula that the
Riccati equation satisfies.
Our result is a natural generalization of the paper {\bf Chaturvedi et al}
(arXiv : 0706.0964 [quant-ph]).Comment: Latex ; 13 pages ; no figur
Near- to mid-infrared picosecond optical parametric oscillator based on periodically poled RbTiOAsO4
We describe a Ti:sapphire-pumped picosecond optical parametric oscillator based on periodically poled RbTiOAsO4 that is broadly tunable in the near to mid infrared. A 4.5-mm single-grating crystal at room temperature in combination with pump wavelength tuning provided access to a continuous-tuning range from 3.35 to 5 mu m, and a pump power threshold of 90 mW was measured. Average mid-infrared output powers in excess of 100 mW and total output powers of 400 mW in similar to 1-ps pulses were obtained at 33% extraction efficiency. (C) 1998 Optical Society of America.</p
5-micron photometry of late-type dwarfs
We present narrowband-M photometry of nine low-mass dwarfs with spectral
types ranging from M2.5 to L0.5. Combining the (L'-M') colours derived from our
observations with data from the literature, we find colours consistent with a
Rayleigh-Jeans flux distribution for spectral types earlier than M5, but
enhanced F_3.8/F_4.7 flux ratios (negative (L'-M') colours) at later spectral
types. This probably reflects increased absorption at M' due to the CO
fundamental band. We compare our results against recent model predictions and
briefly discuss the implications.Comment: accepted for the Astronomical Journa
On the Computation of Power in Volume Integral Equation Formulations
We present simple and stable formulas for computing power (including
absorbed/radiated, scattered and extinction power) in current-based volume
integral equation formulations. The proposed formulas are given in terms of
vector-matrix-vector products of quantities found solely in the associated
linear system. In addition to their efficiency, the derived expressions can
guarantee the positivity of the computed power. We also discuss the application
of Poynting's theorem for the case of sources immersed in dissipative
materials. The formulas are validated against results obtained both with
analytical and numerical methods for scattering and radiation benchmark cases
Brown dwarfs in the Hyades and beyond?
We have used both the Low-Resolution Imaging Spectrograph and the HIRES
echelle spectrograph on the Keck telescopes to obtain spectra of twelve
candidate members of the Hyades cluster identified by Leggett and Hawkins
(1988, 1989). All of the objects are chromospherically-active, late-type
M-dwarfs, with H equivalent widths varying from 1 to 30\AA. Based on
our measured radial velocities, the level of stellar activity and other
spectroscopic features, only one of the twelve stars has properties consistent
with cluster membership. We consider how this result affects estimates of the
luminosity and mass function of the Hyades cluster. Five of the eleven field
stars have weak K I 7665/7699\AA and CaH absorption as compared with M-dwarf
standards of the same spectral type, suggesting a lower surface gravity. Two of
these sources, LH0416+14 and LH0419+15, exhibit significant lithium 6708 \AA
absorption. Based partly on parallax measurements by the US Naval Observatory
(Harris et al, 1998), we identify all five as likely to be young, pre-main
sequence objects in or near the Taurus-Auriga association at distances of
between 150 and 250 parsecs. A comparison with theoretical models of pre-main
sequence stars indicates masses of less than 0.05 M.Comment: to appear in AJ, January 1999; 34 pages, (Latex format), including 10
embedded postscript figures and two table
From d-wave to s-wave pairing in the iron-pnictide superconductor (Ba,K)Fe2As2
The nature of the pairing state in iron-based superconductors is the subject
of much debate. Here we argue that in one material, the stoichiometric iron
pnictide KFe2As2, there is overwhelming evidence for a d-wave pairing state,
characterized by symmetry-imposed vertical line nodes in the superconducting
gap. This evidence is reviewed, with a focus on thermal conductivity and the
strong impact of impurity scattering on the critical temperature Tc. We then
compare KFe2As2 to Ba0.6K0.4Fe2As2, obtained by Ba substitution, where the
pairing symmetry is s-wave and the Tc is ten times higher. The transition from
d-wave to s-wave within the same crystal structure provides a rare opportunity
to investigate the connection between band structure and pairing mechanism. We
also compare KFe2As2 to the nodal iron-based superconductor LaFePO, for which
the pairing symmetry is probably not d-wave, but more likely s-wave with
accidental line nodes
Symptom screen: diagnostic usefulness in detecting pulmonary tuberculosis in HIV-infected pregnant women in Kenya
OBJECTIVE: To determine the diagnostic usefulness of tuberculosis (TB) symptom screening to detect active pulmonary TB among human immunodeficiency virus (HIV) infected pregnant women in two PMTCT (prevention of mother-to-child transmission) clinics in western Kenya that are supported by the United States Agency for International Development–Academic Model Providing Access to Healthcare partnership. DESIGN: Cross-sectional study. Participants were interviewed for TB symptoms with a standardized questionnaire (cough >2 weeks, fever, night sweats, weight loss or failure to gain weight). Those with cough submitted sputum specimens for smear microscopy for acid-fast bacilli and mycobacterial culture. Women at >14 weeks gestation underwent shielded chest radiography (CXR). RESULTS: Of 187 HIV-infected women, 38 (20%) were symptom screen-positive. Of these, 21 had a cough for >2 weeks, but all had negative sputum smears and mycobacterial cultures. CXRs were performed in 26 symptomatic women: three were suggestive of TB (1 miliary, 1 infiltrates and 1 cavitary). Of 149 women with a negative symptom screen, 100 had a CXR and seven had a CXR suggestive of TB (1 cavitary, 2 miliary and 4 infiltrates). CONCLUSION: This study did not support the utility of isolated symptom screening in identification of TB disease in our PMTCT setting. CXR was useful in identification of TB suspects in both symptomatic and asymptomatic women
A consistent scalar-tensor cosmology for inflation, dark energy and the Hubble parameter
The authors are grateful for financial support to the Cruickshank Trust (CW), EPSRC/GG-Top (CW, JR), Omani Government (MA), Science Without Borders programme, CNPq, Brazil (DR), and STFC/CfFP (CW, AM, RB, JM). CW and AM acknowledge the hospitality of CERN, where this work was started. The University of Aberdeen and University of Edinburgh are charitable bodies registered in Scotland, with respective registration numbers SC013683 and SC005336.Peer reviewedPostprin
- …