47,231 research outputs found
Circular Dichroism of RbHe and RbN Molecules
We present measurements of the circular dichroism of optically pumped Rb
vapor near the D1 resonance line. Collisions with the buffer gases He and
N reduce the transparency of the vapor, even when fully polarized. We use
two methods to measure this effect, show that the He results can be understood
from RbHe potential curves, and show how this effect conspires with the
spectral profile of the optical pumping light to increase the laser power
demands for optical pumping of very optically thick samples
Effects of Nitrogen Quenching Gas on Spin-Exchange Optical Pumping of He-3
We consider the degree of conservation of nuclear spin polarization in the
process of optical pumping under typical spin-exchange optical pumping
conditions. Previous analyses have assumed that negligible nuclear spin
precession occurs in the brief periods of time the alkali-metal atoms are in
the excited state after absorbing photons and before undergoing quenching
collisions with nitrogen molecules. We include excited-state hyperfine
interactions, electronic spin relaxation in collisions with He and N_2,
spontaneous emission, quenching collisions, and a simplified treatment of
radiation trapping
Alfalfa in the Delta - Delta Branch Station
Mode of access: Internet
Report of work at the Delta Branch Experiment Station for 1911
Mode of access: Internet
The Origin of Anomalous Low-Temperature Downturns in the Thermal Conductivity of Cuprates
We show that the anomalous decrease in the thermal conductivity of cuprates
below 300 mK, as has been observed recently in several cuprate materials
including PrCeCuO in the field-induced normal state,
is due to the thermal decoupling of phonons and electrons in the sample. Upon
lowering the temperature, the phonon-electron heat transfer rate decreases and,
as a result, a heat current bottleneck develops between the phonons, which can
in some cases be primarily responsible for heating the sample, and the
electrons. The contribution that the electrons make to the total low- heat
current is thus limited by the phonon-electron heat transfer rate, and falls
rapidly with decreasing temperature, resulting in the apparent low- downturn
of the thermal conductivity. We obtain the temperature and magnetic field
dependence of the low- thermal conductivity in the presence of
phonon-electron thermal decoupling and find good agreement with the data in
both the normal and superconducting states.Comment: 8 pages, 5 figure
A Statistical Description of AGN Jet Evolution from the VLBA Imaging and Polarimetry Survey (VIPS)
A detailed analysis of the evolution of the properties of core-jet systems
within the VLBA Imaging and Polarimetry Survey (VIPS) is presented. We find a
power-law relationship between jet intensity and width that suggests for the
typical jet, little if any energy is lost as it moves away from its core. Using
VLA images at 1.5 GHz, we have found evidence that parsec-scale jets tend to be
aligned with the the direction of emission on kiloparsec scales. We also found
that this alignment improves as the jets move farther from their cores on
projected scales as small as ~50-100 pc. This suggests that realignment of jets
on these projected scales is relatively common. We typically find a modest
amount of bending (a change in jet position angle of ~5 deg.) on these scales,
suggesting that this realignment may typically occur relatively gradually.Comment: Accepted to ApJ, 20 pages, 8 figure
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