25,003 research outputs found
Indirect Dissociative Recombination of LiH Molecules Fueled by Complex Resonance Manifolds
The LiH molecule is prototypical of the indirect dissociative
recombination (DR) process, in which a colliding electron destroys the molecule
through Rydberg capture pathways. This Letter develops the first quantitative
test of the Siegert state multichannel quantum defect theory description of
indirect DR for a diatomic molecular ion. The R-matrix approach is adopted to
calculate ab-initio quantum defects, functions of the internuclear distance
that characterize both Rydberg states and the zero-energy collisions of
electrons with LiH ions. The calculated DR rate coefficient agrees
accurately with recent experimental data (S. Krohn et al, Phys. Rev. Lett. 86,
4005). We identify the doorways to fast indirect DR as complex resonance
manifolds, which couple closed channels having both high and low principal
quantum numbers. This sheds new light on the competition between direct and
indirect DR pathways, and suggests the reason why previous theory
underestimated the DR rate by an order of magnitude.Comment: Submitted to PR
Hall effect encoding of brushless dc motors
Encoding mechanism integral to the motor and using the permanent magnets embedded in the rotor eliminates the need for external devices to encode information relating the position and velocity of the rotating member
Physical Properties of the Narrow-Line Region of Low-Mass Active Galaxies
We present spectroscopic observations of 27 active galactic nuclei (AGN) with
some of the lowest black hole (BH) masses known. We use the high spectral
resolution and small aperture of our Keck data, taken with the Echellette
Spectrograph and Imager, to isolate the narrow-line regions (NLRs) of these
low-mass BHs. We investigate their emission-line properties and compare them
with those of AGN with higher-mass black holes. While we are unable to
determine absolute metallicities, some of our objects plausibly represent
examples of the low-metallicity AGN described by Groves et al. (2006), based on
their [N II]/H_alpha ratios and their consistency with the Kewley & Ellison
(2008) mass-metallicity relation. We find tentative evidence for steeper far-UV
spectral slopes in lower-mass systems. Overall, NLR emission lines in these
low-mass AGN exhibit trends similar to those seen in AGN with higher-mass BHs,
such as increasing blueshifts and broadening with increasing ionization
potential. Additionally, we see evidence of an intermediate line region whose
intensity correlates with L/L_Edd, as seen in higher-mass AGN. We highlight the
interesting trend that, at least in these low-mass AGN, the [O III] equivalent
width (EW) is highest in symmetric NLR lines with no blue wing. This trend of
increasing [O III] EW with line symmetry could be explained by a high covering
factor of lower ionization gas in the NLR. In general, low-mass AGN preserve
many well-known trends in the structure of the NLR, while exhibiting steeper
ionizing continuum slopes and somewhat lower gas-phase metallicities.Comment: 46 pages, 14 figures, 7 table
Vortex dynamics
Vortex flows of interest to aerodynamicists cover a wide range of scales from a fraction of an inch in boundary layer flows to many feet in wake flows. In many applications these flows are poorly understood and, due to their complexity, present a challenge both analytically and experimentally. Four topics representing the spectrum of experimental and analytical vortex research are presented
Infrared Signature of the Superconducting State in Pr(2-x)Ce(x)CuO(4)
We measured the far infrared reflectivity of two superconducting
Pr(2-x)Ce(x)CuO(4) films above and below Tc. The reflectivity in the
superconducting state increases and the optical conductivity drops at low
energies, in agreement with the opening of a (possibly) anisotropic
superconducting gap. The maximum energy of the gap scales roughly with Tc as 2
Delta_{max} / kB Tc ~ 4.7. We determined absolute values of the penetration
depth at 5 K as lambda_{ab} = (3300 +/- 700) A for x = 0.15 and lambda_{ab} =
(2000 +/- 300) A for x = 0.17. A spectral weight analysis shows that the
Ferrell-Glover-Tinkham sum rule is satisfied at conventional low energy scales
\~ 4 Delta_{max}.Comment: 4 pages, 4 figure
Correlation between incoherent phase fluctuations and disorder in YPrBaCuO epitaxial films from Nernst effect measurements
Measurements of Nernst effect, resistivity and Hall angle on epitaxial films
of YPrBaCuO(Pr-YBCO, 00.4) are
reported over a broad range of temperature and magnetic field. While the Hall
and resistivity data suggest a broad pseudogap regime in accordance with
earlier results, these first measurements of the Nernst effect on Pr-YBCO show
a large signal above the superconducting transition temperature(T). This
effect is attributed to vortex-like excitations in the phase incoherent
condensate existing above T. A correlation between disorder and the width
of the phase fluctuation regime has been established for the YBCO family of
cuprates, which suggests a T110K for disorder-free
YBaCuO.Comment: 5 pages, 6 figure
- …