1,169 research outputs found
Collective modes of Fermi superfluid containing vortices along the BEC-BCS crossover
Using the coarse-grain averaged hydrodynamic approach, we calculate all low
energy transverse excitation spectrum of a rotating Fermi superfluid containing
vortex lattices for all regimes along the BEC-BCS crossover. In the fast
rotating regime, the molecular BEC enters into the lowest Landau level, but the
superfluid in the unitarity and the BCS regimes occupies many low-lying Landau
levels. The difference between the breathing mode frequencies at the BEC and
unitarity limit shrinks to zero as the rotation speed approaches the radial
trap frequency, in contrast to the finite difference in the non-rotating
systems.Comment: To appear in Physical Review
Auger decay, Spin-exchange, and their connection to Bose-Einstein condensation of excitons in Cu_2O
In view of the recent experiments of O'Hara, et al. on excitons in Cu_2O, we
examine the interconversion between the angular-momentum triplet-state excitons
and the angular-momentum singlet-state excitons by a spin-exchange process
which has been overlooked in the past. We estimate the rate of this
particle-conserving mechanism and find a substantially higher value than the
Auger process considered so far. Based on this idea, we give a possible
explanation of the recent experimental observations, and make certain
predictions, with the most important being that the singlet-state excitons in
Cu_2O is a very serious candidate for exhibiting the phenomenon of
Bose-Einstein condensation.Comment: 4 pages, RevTex, 1 ps figur
Cooling dynamics of ultracold two-species Fermi-Bose mixtures
We compare strategies for evaporative and sympathetic cooling of two-species
Fermi-Bose mixtures in single-color and two-color optical dipole traps. We show
that in the latter case a large heat capacity of the bosonic species can be
maintained during the entire cooling process. This could allow to efficiently
achieve a deep Fermi degeneracy regime having at the same time a significant
thermal fraction for the Bose gas, crucial for a precise thermometry of the
mixture. Two possible signatures of a superfluid phase transition for the Fermi
species are discussed.Comment: 4 pages, 3 figure
Three-body recombination in a three-state Fermi gas with widely tunable interactions
We investigate the stability of a three spin state mixture of ultracold
fermionic Li atoms over a range of magnetic fields encompassing three
Feshbach resonances. For most field values, we attribute decay of the atomic
population to three-body processes involving one atom from each spin state and
find that the three-body loss coefficient varies by over four orders of
magnitude. We observe high stability when at least two of the three scattering
lengths are small, rapid loss near the Feshbach resonances, and two unexpected
resonant loss features. At our highest fields, where all pairwise scattering
lengths are approaching , we measure a three-body loss
coefficient and a trend
toward lower decay rates for higher fields indicating that future studies of
color superfluidity and trion formation in a SU(3) symmetric Fermi gas may be
feasible
Numerical simulation of exciton dynamics in Cu2O at ultra low temperatures within a potential trap
We have studied theoretically the relaxation behaviour of excitons in cuprous
oxide (Cu2O) at ultra low temperatures when excitons are confined within a
potential trap by solving numerically the Boltzmann equation. As relaxation
processes, we have included in this paper deformation potential phonon
scattering, radiative and non-radiative decay and Auger decay. The relaxation
kinetics has been analysed for temperatures in the range between 0.3K and 5K.
Under the action of deformation potential phonon scattering only, we find for
temperatures above 0.5K that the excitons reach local equilibrium with the
lattice i.e. that the effective local temperature is coming down to bath
temperature, while below 0.5K a non-thermal energy distribution remains.
Interestingly, for all temperatures the global spatial distribution of excitons
does not reach the equilibrium distribution, but stays at a much higher
effective temperature. If we include further a finite lifetime of the excitons
and the two-particle Auger decay, we find that both the local and the global
effective temperature are not coming down to bath temperature. In the first
case we find a Bose-Einstein condensation (BEC) to occur for all temperatures
in the investigated range. Comparing our results with the thermal equilibrium
case, we find that BEC occurs for a significantly higher number of excitons in
the trap. This effect could be related to the higher global temperature, which
requires an increased number of excitons within the trap to observe the BEC. In
case of Auger decay, we do not find at any temperature a BEC due to the heating
of the exciton gas
Glycerol carbonate as green solvent for pretreatment of sugarcane bagasse
Background\ud
Pretreatment of lignocellulosic biomass is a prerequisite for effective saccharification to produce fermentable sugars. We have previously reported an effective low temperature (90 °C) process at atmospheric pressure for pretreatment of sugarcane bagasse with acidified mixtures of ethylene carbonate (EC) and ethylene glycol (EG). In this study, “greener” solvent systems based on acidified mixtures of glycerol carbonate (GC) and glycerol were used to treat sugarcane bagasse and the roles of each solvent in deconstructing biomass were determined. \ud
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Results\ud
Pretreatment of sugarcane bagasse at 90 °C for only 30 min with acidified GC produced a solid residue having a glucan digestibility of 90% and a glucose yield of 80%, which were significantly higher than a glucan digestibility of 16% and a glucose yield of 15% obtained for bagasse pretreated with acidified EC. Biomass compositional analyses showed that GC pretreatment removed more lignin than EC pretreatment (84% vs 54%). Scanning electron microscopy (SEM) showed that fluffy and size-reduced fibres were produced from GC pretreatment whereas EC pretreatment produced compact particles of reduced size. The maximal glucan digestibility and glucose yield of GC/glycerol systems were about 7% lower than those of EC/ethylene glycol (EG) systems. Replacing up to 50 wt% of GC with glycerol did not negatively affect glucan digestibility and glucose yield. The results from pretreatment of microcrystalline cellulose (MCC) showed that (1) pretreatment with acidified alkylene glycol (AG) alone increased enzymatic digestibility compared to pretreatments with acidified alkylene carbonate (AC) alone and acidified mixtures of AC and AG, (2) pretreatment with acidified GC alone slightly increased, but with acidified EC alone significantly decreased, enzymatic digestibility compared to untreated MCC, and (3) there was a good positive linear correlation of enzymatic digestibility of treated and untreated MCC samples with congo red (CR) adsorption capacity.\ud
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Conclusions\ud
Acidified GC alone was a more effective solvent for pretreatment of sugarcane bagasse than acidified EC alone. The higher glucose yield obtained with GC-pretreated bagasse is possibly due to the presence of one hydroxyl group in the GC molecular structure, resulting in more significant biomass delignification and defibrillation, though both solvent pretreatments reduced bagasse particles to a similar extent. The maximum glucan digestibility of GC/glycerol systems was less than that of EC/EG systems, which is likely attributed to glycerol being less effective than EG in biomass delignification and defibrillation. Acidified AC/AG solvent systems were more effective for pretreatment of lignin-containing biomass than MCC
Stiff knots
We report on the geometry and mechanics of knotted stiff strings. We discuss
both closed and open knots. Our two main results are: (i) Their equilibrium
energy as well as the equilibrium tension for open knots depend on the type of
knot as the square of the bridge number; (ii) Braid localization is found to be
a general feature of stiff strings entanglements, while angles and knot
localization are forbidden. Moreover, we identify a family of knots for which
the equilibrium shape is a circular braid. Two other equilibrium shapes are
found from Monte Carlo simulations. These three shapes are confirmed by
rudimentary experiments. Our approach is also extended to the problem of the
minimization of the length of a knotted string with a maximum allowed
curvature.Comment: Submitted to Phys. Rev.
Stress Dependence of Exciton Relaxation Processes in Cu2O
A comprehensive study of the exciton relaxation processes in Cu2O has led to
some surprises. We find that the ortho-para conversion rate becomes slower at
high stress, and that the Auger nonradiative recombination rate increases with
stress, with apparently no Auger recombination at zero stress. These results
have important consequences for the pursuit of Bose-Einstein condensation of
excitons in a harmonic potential.Comment: 10 figures, 1 tabl
Development of an apparatus for cooling 6Li-87Rb Fermi-Bose mixtures in a light-assisted magnetic trap
We describe an experimental setup designed to produce ultracold trapped gas
clouds of fermionic 6Li and bosonic 87Rb. This combination of alkali metals has
the potential to reach deeper Fermi degeneracy with respect to other mixtures
since it allows for improved heat capacity matching which optimizes sympathetic
cooling efficiency. Atomic beams of the two species are independently produced
and then decelerated by Zeeman slowers. The slowed atoms are collected into a
magneto-optical trap and then transferred into a quadrupole magnetic trap. An
ultracold Fermi gas with temperature in the 10^-3 T_F range should be
attainable through selective confinement of the two species via a properly
detuned laser beam focused in the center of the magnetic trap.Comment: Presented at LPHYS'06, 8 figure
Very long storage times and evaporative cooling of cesium atoms in a quasi-electrostatic dipole trap
We have trapped cesium atoms over many minutes in the focus of a CO-laser
beam employing an extremely simple laser system. Collisional properties of the
unpolarized atoms in their electronic ground state are investigated. Inelastic
binary collisions changing the hyperfine state lead to trap loss which is
quantitatively analyzed. Elastic collisions result in evaporative cooling of
the trapped gas from 25 K to 10 K over a time scale of about 150 s.Comment: 5 pages, 3 figure
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