20,721 research outputs found
Monolayers of 3He on the Surface of Bulk Superfluid 4He
We have used quantum evaporation to investigate the two-dimensional fermion
system that forms at the free surface of (initially isotopically pure) 4He when
small quantities of 3He are added to it. By measuring the first-arrival times
of the evaporated atoms, we have determined that the 3He-3He potential in this
system is V_3S/k_B=(0.23+/-0.02) K nm^2 (repulsive) and estimated a value of
m_3S=(1.53+/-0.02)m_3 for the zero-coverage effective mass. We have also
observed the predicted second layer-state which becomes occupied once the first
layer-state density exceeds about 0.6 monolayers.Comment: 2 pages, 3 figures. Submitted to Proc. LT-22 (1999) to appear in
Physica
Relative Evaporation Probabilities of 3He and 4He from the Surface of Superfluid 4He
We report a preliminary experiment which demonstrates that 3He atoms in
Andreev states are evaporated by high-energy (E/k_B ~ 10.2 K) phonons in a
quantum evaporation process similar to that which occurs in pure 4He. Under
conditions of low 3He coverage, high-energy phonons appear to evaporate 3He and
4He atoms with equal probability. However, we have not managed to detect any
3He atoms that have been evaporated by rotons, and conclude that the
probability of a roton evaporating a 3He atom is less than 2% of the
probability that it evaporates a 4He atom.Comment: 2 pages, 3 figures. Submitted to Proc. LT-22 (1999) Physica
Preshaping command inputs to reduce telerobotic system oscillations
The results of using a new technique for shaping inputs to a model of the space shuttle Remote Manipulator System (RMS) are presented. The shapes inputs move the system to the same location that was originally commanded, however, the oscillations of the machine are considerably reduced. An overview of the new shaping method is presented. A description of RMS model is provided. The problem of slow joint servo rates on the RMS is accommodated with an extension of the shaping method. The results and sample data are also presented for both joint and three-dimensional cartesian motions. The results demonstrate that the new shaping method performs well on large, telerobotic systems which exhibit significant structural vibration. The new method is shown to also result in considerable energy savings during operations of the RMS manipulator
Spectroscopy of the optical Einstein ring 0047-2808
We present optical and near-infrared spectroscopic observations of the
optical Einstein ring 0047-2808. We detect both [OIII] lines 4959, 5007 near
2.3 micron, confirming the redshift of the lensed source as z=3.595. The Ly-a
line is redshifted relative to the [OIII] line by 140+-20 km/s. Similar
velocity shifts have been seen in nearby starburst galaxies. The [OIII] line is
very narrow, 130 km/s FWHM. If the ring is the image of the centre of a galaxy
the one-dimensional stellar velocity dispersion sigma=55 km/s is considerably
smaller than the value predicted by Baugh et al. (1998) for the somewhat
brighter Lyman-break galaxies. The Ly-a line is significantly broader than the
[OIII] line, probably due to resonant scattering. The stellar central velocity
dispersion of the early-type deflector galaxy at z=0.485 is 250+-30 km/s. This
value is in good agreement both with the value predicted from the radius of the
Einstein ring (and a singular isothermal sphere model for the deflector), and
the value estimated from the D_n-sigma relation.Comment: 7 pages, 3 figures, accepted for publication in MNRA
On-demand microwave generator of shaped single photons
We demonstrate the full functionality of a circuit that generates single
microwave photons on demand, with a wave packet that can be modulated with a
near-arbitrary shape. We achieve such a high tunability by coupling a
superconducting qubit near the end of a semi-infinite transmission line. A dc
superconducting quantum interference device shunts the line to ground and is
employed to modify the spatial dependence of the electromagnetic mode structure
in the transmission line. This control allows us to couple and decouple the
qubit from the line, shaping its emission rate on fast time scales. Our
decoupling scheme is applicable to all types of superconducting qubits and
other solid-state systems and can be generalized to multiple qubits as well as
to resonators.Comment: 10 pages, 7 figures. Published versio
Seed Yield Prediction Models of Four Common Moist-Soil Plant Species in Texas
Seed production by moist-soil plant species often varies within and among managed wetlands and on larger landscapes. Quantifying seed production of moist-soil plants can be used to evaluate wetland management strategies and estimate wetland energetic carrying capacity, specifically for waterfowl. In the past, direct estimation techniques were used, but due to excessive personnel and time costs, other indirect methods have been developed. Because indirect seed yield models do not exist for moist-soil plant species in east-central or coastal Texas, we developed direct and indirect methods to model seed production on regional managed wetlands. In September 2004 and 2005, we collected Echinochloa crusgalli (barnyard grass), E. walterii (wild millet), E. colona (jungle rice), and Oryza sativa (cultivated rice) for phytomorphological measurements and seed yield modeling. Initial simple linear and point of origin regression analyses demonstrate strong relationships (P \u3c 0.001) among phytomorphological and dot grid methods in predicting seed production for all four species. These models should help regional wetland managers evaluate moist-soil management success and create models for seed production for other moist-soil plants in this region
Distribution of the second virial coefficients of globular proteins
George and Wilson [Acta. Cryst. D 50, 361 (1994)] looked at the distribution
of values of the second virial coefficient of globular proteins, under the
conditions at which they crystallise. They found the values to lie within a
fairly narrow range. We have defined a simple model of a generic globular
protein. We then generate a set of proteins by picking values for the
parameters of the model from a probability distribution. At fixed solubility,
this set of proteins is found to have values of the second virial coefficient
that fall within a fairly narrow range. The shape of the probability
distribution of the second virial coefficient is Gaussian because the second
virial coefficient is a sum of contributions from different patches on the
protein surface.Comment: 5 pages, including 3 figure
- …