10,169 research outputs found
Unitary Fermi gas at finite temperature in the epsilon expansion
Thermodynamics of the unitary Fermi gas at finite temperature is investigated
from the perspective of the expansion over epsilon=4-d with d being the
dimensionality of space. We show that the thermodynamics is dominated by
bosonic excitations in the low temperature region T<<Tc. Analytic formulas for
the thermodynamic functions as functions of the temperature are derived to the
lowest order in epsilon in this region. In the high temperature region where T
Tc, bosonic and fermionic quasiparticles are excited. We determine the critical
temperature Tc of the superfluid phase transition and the thermodynamic
functions around Tc to the leading and next-to-leading orders in epsilon.Comment: 13 pages, 7 figures, revtex4; version to appear in Phys. Rev.
BCS-BEC crossover in a relativistic superfluid and its significance to quark matter
The character change of a superfluid state due to the variation of the
attractive force is investigated in the relativistic framework with a massive
fermion. Two crossovers are found. One is a crossover from the usual BCS state
to the Bose-Einstein condensation (BEC) of bound fermion pairs. The other is
from the BEC to the relativistic Bose-Einstein condensation (RBEC) of nearly
massless bound pairs where antiparticles as well as particles dominate the
thermodynamics. Possible realization of the BEC and RBEC states in the quark
matter is also pointed out.Comment: 5 pages, 1 figure, revtex4; (v2) text has been clarified, references
updated; (v3) final version to appear in Phys. Rev.
Liberating Efimov physics from three dimensions
When two particles attract via a resonant short-range interaction, three
particles always form an infinite tower of bound states characterized by a
discrete scaling symmetry. It has been considered that this Efimov effect
exists only in three dimensions. Here we review how the Efimov physics can be
liberated from three dimensions by considering two-body and three-body
interactions in mixed dimensions and four-body interaction in one dimension. In
such new systems, intriguing phenomena appear, such as confinement-induced
Efimov effect, Bose-Fermi crossover in Efimov spectrum, and formation of
interlayer Efimov trimers. Some of them are observable in ultracold atom
experiments and we believe that this study significantly broadens our horizons
of universal Efimov physics.Comment: 17 pages, 5 figures, contribution to a special issue of Few-Body
Systems devoted to Efimov Physic
Small scale noise and wind tunnel tests of upper surface blowing nozzle flap concepts. Volume 1. Aerodynamic test results
The results and analyses of aerodynamic and acoustic studies conducted on the small scale noise and wind tunnel tests of upper surface blowing nozzle flap concepts are presented. Various types of nozzle flap concepts were tested. These are an upper surface blowing concept with a multiple slot arrangement with seven slots (seven slotted nozzle), an upper surface blowing type with a large nozzle exit at approximately mid-chord location in conjunction with a powered trailing edge flap with multiple slots (split flow or partially slotted nozzle). In addition, aerodynamic tests were continued on a similar multi-slotted nozzle flap, but with 14 slots. All three types of nozzle flap concepts tested appear to be about equal in overall aerodynamic performance but with the split flow nozzle somewhat better than the other two nozzle flaps in the landing approach mode. All nozzle flaps can be deflected to a large angle to increase drag without significant loss in lift. The nozzle flap concepts appear to be viable aerodynamic drag modulation devices for landing
Zooplankton diversity analysis through single-gene sequencing of a community sample
<p>Abstract</p> <p>Background</p> <p>Oceans cover more than 70% of the earth's surface and are critical for the homeostasis of the environment. Among the components of the ocean ecosystem, zooplankton play vital roles in energy and matter transfer through the system. Despite their importance, understanding of zooplankton biodiversity is limited because of their fragile nature, small body size, and the large number of species from various taxonomic phyla. Here we present the results of single-gene zooplankton community analysis using a method that determines a large number of mitochondrial <it>COI </it>gene sequences from a bulk zooplankton sample. This approach will enable us to estimate the species richness of almost the entire zooplankton community.</p> <p>Results</p> <p>A sample was collected from a depth of 721 m to the surface in the western equatorial Pacific off Pohnpei Island, Micronesia, with a plankton net equipped with a 2-m<sup>2 </sup>mouth opening. A total of 1,336 mitochondrial <it>COI </it>gene sequences were determined from the cDNA library made from the sample. From the determined sequences, the occurrence of 189 species of zooplankton was estimated. BLASTN search results showed high degrees of similarity (>98%) between the query and database for 10 species, including holozooplankton and merozooplankton.</p> <p>Conclusion</p> <p>In conjunction with the Census of Marine Zooplankton and Barcode of Life projects, single-gene zooplankton community analysis will be a powerful tool for estimating the species richness of zooplankton communities.</p
Settlement rehabilitation of a 35 year old building : case study integrated with analysis and implementation
This paper presents a rehabilitation project concerning the settlement of a 35 year old building. The foundation system of the northwest wing of the building consists of strip footings and slabon-grade. Differential settlement results in significant cracking of the masonry partition walls located on the footing and hence rehabilitation of the footing is required to stabilize the foundation system. Geotechnical and structural investigations are conducted, including site borings and analytical modeling based on one-dimensional consolidation theory that is incorporated into a finite element analysis. The predictive model exhibits that the differential settlement does not cause noticeable distress for the primary structural members, whereas the continued settlement affects use of the building. Site implementation is performed with the pushpile method to terminate the continuous settlement of the foundation
Pairing instabilities in quasi-two-dimensional Fermi gases
We study non-equilibrium dynamics of ultracold two-component Fermi gases in
low-dimensional geometries after the interactions are quenched from weakly
interacting to strongly interacting regime. We develop a T-matrix formalism
that takes into account the interplay between Pauli blocking and tight
confinement in low-dimensional geometries. We employ our formalism to study the
formation of molecules in quasi-two-dimensional Fermi gases near Feshbach
resonance and show that the rate at which molecules form depends strongly on
the transverse confinement. Furthermore, Pauli blocking gives rise to a sizable
correction to the binding energy of molecules.Comment: 6 pages, 3 figure
Marginally unstable Holmboe modes
Marginally unstable Holmboe modes for smooth density and velocity profiles
are studied. For a large family of flows and stratification that exhibit
Holmboe instability, we show that the modes with phase velocity equal to the
maximum or the minimum velocity of the shear are marginally unstable. This
allows us to determine the critical value of the control parameter R
(expressing the ratio of the velocity variation length scale to the density
variation length scale) that Holmboe instability appears R=2. We then examine
systems for which the parameter R is very close to this critical value. For
this case we derive an analytical expression for the dispersion relation of the
complex phase speed c(k) in the unstable region. The growth rate and the width
of the region of unstable wave numbers has a very strong (exponential)
dependence on the deviation of R from the critical value. Two specific examples
are examined and the implications of the results are discussed.Comment: Submitted to Physics of Fluid
Design, construction and evaluation of a 12.045 GHz, 2.0 kW-cw permanent-magnet focused klystron amplifier
An analytical and experimental program to demonstrate the technical feasibility of a lightweight, high-efficiency, 1-2 kW cw, permanent magnet focused klystron operating at 12.0 GHz was described. The design is based on use of a samarium-cobalt permanent magnet for focusing of the electron beam and choice of the most optimum parameters for maximum efficiency. A filter-loaded output circuit is used for the required bandwidth. The design incorporates a collector which is demountable from the tube to facilitate multistage depressed collector experiments, permitting replacement with a NASA-designed axisymmetric, electrostatic collector for linear beam microwave tubes. A further requirement is that the focusing field between the last interaction gap and the collector decay in a prescribed manner referred to as adiabatic expansion
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