3,802 research outputs found
Atom-molecule coherence in a one-dimensional system
We study a model of one-dimensional fermionic atoms that can bind in pairs to
form bosonic molecules. We show that at low energy, a coherence develops
between the molecule and fermion Luttinger liquids. At the same time, a gap
opens in the spin excitation spectrum. The coherence implies that the order
parameters for the molecular Bose-Einstein Condensation and the atomic BCS
pairing become identical. Moreover, both bosonic and fermionic charge density
wave correlations decay exponentially, in contrast with a usual Luttinger
liquid. We exhibit a Luther-Emery point where the systems can be described in
terms of noninteracting pseudofermions. At this point, we provide closed form
expressions for the density-density response functions.Comment: 5 pages, no figures, Revtex 4; (v2) added a reference to
cond-mat/0505681 where related results are reported; (v3) Expression of
correlation functions given in terms of generalized hypergeometric function
A performance evaluation of commercial fibrinogen reference preparations and assays for Clauss and PT-derived fibrinogen
The wide availability of fibrinogen estimations based on the prothrombin time (PT-Fg) has caused concern about the variability and clinical utility of fibrinogen assays. In a multi-centre study, we investigated fibrinogen assays using various reagents and analysers, Clauss assays generally gave good agreement, although one reagent gave 15-30% higher values in DIC and thrombolysis. Two commercial reference preparations had much lower potencies than the manufacturers declared, and plasma turbidity influenced parallelism in some Clauss assays, PT-Fg assays gave higher values than Clauss and showed calibrant dependent effects, the degree of disparity correlating with calibrant and test sample turbidity. Analyser and thromboplastin dependent differences were noted. The relationship between Clauss and PT-Fg assays was sigmoid, and the plateau of maximal PT-Fg differed by about 2 g/l between reagents. ELISA and immunonephelometric assays correlated well, but with a high degree of scatter. Antigen levels were higher than Clauss, but slightly lower than PT-Fg assays, which appeared to be influenced by degraded fibrinogen. Clauss assays are generally reproducible between centres, analysers and reagents, but PT-Fg assays are not reliable in clinical settings
Clustering of Fermi particles with arbitrary spin
A single l-shell model is investigated for a system of fermions of spin s and
an attractive s-wave, spin channel independent, interaction. The spectra and
eigenvectors are determined exactly for different l, s values and particle
numbers N. As a generalization of Cooper pairing it is shown that when
N=mu(2s+1), mu=1,2,...,2l+1, the ground state consists of clusters of (2s+1)
particles. The relevance of the results for more general situations including
the homogeneous system is briefly discussed.Comment: Submitted for publication, 4 pages, 1 figur
HfC(310) High Brightness Sources for Advanced Imaging Applications
The authors report on electron emission from HfC(310) operating in extended Schottky emission mode. Data are gathered from test stands as well as through operation in a commercial scanning electron microscope. Emitter end-form geometry consisted of rounded, via electrochemical etching, and truncated, via ion milling. The authors demonstrate high angular intensity operation of \u3e60 mA/sr especially for the rounded end-form emitters. Advantages include robustness of the material, which is not reliant on material supply as is the case with standard ZrO/W(100) sources. Hence, operation is available over a much larger range of temperatures, fields, and potentially pressures. Operation in a commercial scanning electron microscope gave ten times higher beam currents for identical operational parameters over a standard Schottky source
The Evolution and Development of Coloniality in Hydrozoans
Hydrozoan colonies display a variety of shapes and sizes including encrusting, upright and pelagic forms. Phylogenetic patterns reveal a complex evolutionary history of these distinct colony forms, as well as colony loss. Within a species, phenotypic variation in colonies as a response to changing environmental cues and resources has been documented. The patterns of branching of colony specific tissue, called stolons in encrusting colonies and stalks in upright colonies, are likely under the control of signaling mechanisms whose changing expression in evolution and development are responsible for the diversity of hydrozoan colony forms. Although mechanisms of polyp development have been well studied, little research has focused on colony development and patterning. In the few studies that investigated mechanisms governing colony patterning, the Wnt signaling pathway has been implicated. The diversity of colony form, evolutionary patterns and mechanisms of colony variation in Hydrozoa are reviewed here
Dissipative Particle Dynamics with Energy Conservation
The stochastic differential equations for a model of dissipative particle
dynamics with both total energy and total momentum conservation in the
particle-particle interactions are presented. The corresponding Fokker-Planck
equation for the evolution of the probability distribution for the system is
deduced together with the corresponding fluctuation-dissipation theorems
ensuring that the ab initio chosen equilibrium probability distribution for the
relevant variables is a stationary solution. When energy conservation is
included, the system can sustain temperature gradients and heat flow can be
modeled.Comment: 7 pages, submitted to Europhys. Let
Electron Emission from Hafnium Carbide
Abstract: HfC was evaluated as a cold field emission source. Single crystal HfC was produced and fabricated into cold field emitters, then angular intensity and reduced brightness were determined from experimental I(V) data. Energy distribution data were in agreement with a theoretical model. The reduced brightness, energy distribution, and emission stability are compared to commercially available sources which show that HfC produced a higher brightness and a lower energy spread than a W cold field source or a ZrO/W Schottky emitter. HfC maintains its emission level for one hour in moderate UHV condition; a dramatic improvement over the stability of W. This combined with a durability that allows for frequent flash cleaning without degradation of the emitter end form make HfC a highly promising source. We compared stability and noise to emission from a tungsten tip at the same angular intensity. By increasing the emitter temperature slightly, stability is improved while maintaining a low energy spread
Diffraction of a superfluid Fermi gas by an atomic grating
An atomic grating generated by a pulsed standing wave laser field is proposed
to manipulate the superfluid state in a quantum degenerate gas of fermionic
atoms. We show that in the presence of atomic Cooper pairs, the density
oscillations of the gas caused by the atomic grating exhibit a much longer
coherence time than that in the normal Fermi gas. Our result indicates that the
technique of a pulsed atomic grating can be a potential candidate to detect the
atomic superfluid state in a quantum degenerate Fermi gas.Comment: 4 pages, 2 figure
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