11,445 research outputs found
Bosonization and density-matrix renormalization group studies of Fulde-Ferrell-Larkin-Ovchinnikov phase and irrational magnetization plateaus in coupled chains
We review the properties of two coupled fermionic chains, or ladders, under a
magnetic field parallel to the lattice plane. Results are computed by
complementary analytical (bosonization) and numerical (density-matrix
renormalization group) methods which allows a systematic comparison. Limiting
cases such as coupled bands and coupled chains regimes are discussed. We
particularly focus on the evolution of the superconducting correlations under
increasing field and on the presence of irrational magnetization plateaus. We
found the existence of large doping-dependent magnetization plateaus in the
weakly-interacting and strong-coupling limits and in the non-trivial case of
isotropic couplings. We report on the existence of extended
Fulde-Ferrell-Larkin-Ovchinnikov phases within the isotropic t-J and Hubbard
models, deduced from the evolution of different observables under magnetic
field. Emphasis is put on the variety of superconducting order parameters
present at high magnetic field. We have also computed the evolution of the
Luttinger exponent corresponding to the ungaped spin mode appearing at finite
magnetization. In the coupled chain regime, the possibility of having polarized
triplet pairing under high field is predicted by bosonization.Comment: 18 pages, 19 figure
Chemistry of the CO dimer at low temperatures
Researchers conducted a series of experiments on the chlorine-catalyzed photodecomposition of O sub 3 both in the gas and in inert solvents such as CF sub 4 and CO sub 2 in the temperature range about 190 to 225 K. The liquid medium was chosen in order to minimize possible surface loss of long-lived ClO dimer, and to aid in the stabilization of transient excited intermediates. The mechanism of dimer formation was as follows: (1) Cl sub 2 + hv yields Cl + Cl; (2) Cl + O sub 3 yields ClO + O sub 2; (3) ClO + ClO yields Cl sub 2 O sub 2. The experiments were done in cooled low temperature cells, with irradiation from an Osram high pressure mercury arc, filtered to remove radiation below 325 nm. Spectral analysis was by means of a Cary Model 2200 UV spectrometer. The principal objectives were: (1) to determine the lifetime of the dimer as a function of temperature; (2) to observe spectral changes in the mixtures which could be attributed to dimer or related products; and (3) to observe chemical or photochemical reactions of the dimer
Modelling the kinetics of transesterification reaction of sunflower oil with ethanol in microreactors
Transesterification reaction of vegetable oil with ethanol leads to ethyl esters, used to date for applications principally in food and cosmetic industry. To open the application field to biofuels (to substitute current fuels resulting from fossil resources), the process efficiency has to be developed to be economically profitable. In this work, the sunflower oil ethanolysis was performed in a micro-scaled continuous device, inducing better control for heat and mass transfer in comparison with batch processes. Moreover, this device ensures kinetic data acquisition at the first seconds of the reaction, which was not feasible in a conventional batch process. These data were used to model occurring phenomena and to determine kinetic constants and mass transfer coefficients. A single set of these parameters is able to represent the evolution of the reaction media composition function of time for five ethanol to oil molar ratios (6.0, 9.0, 16.2, 22.7 and 45.4). The model was validated in reaction and diffusion mode. Finally, it was subsequently used to simulate reactions with other operational conditions and to propose other process implementation
The distribution of the ratio of consecutive level spacings in random matrix ensembles
We derive expressions for the probability distribution of the ratio of two
consecutive level spacings for the classical ensembles of random matrices. This
ratio distribution was recently introduced to study spectral properties of
many-body problems, as, contrary to the standard level spacing distributions,
it does not depend on the local density of states. Our Wigner-like surmises are
shown to be very accurate when compared to numerics and exact calculations in
the large matrix size limit. Quantitative improvements are found through a
polynomial expansion. Examples from a quantum many-body lattice model and from
zeros of the Riemann zeta function are presented.Comment: 5 pages, 4 figure
Haldane charge conjecture in one-dimensional multicomponent fermionic cold atoms
A Haldane conjecture is revealed for spin-singlet charge modes in
2N-component fermionic cold atoms loaded into a one-dimensional optical
lattice. By means of a low-energy approach and DMRG calculations, we show the
emergence of gapless and gapped phases depending on the parity of for
attractive interactions at half-filling. The analogue of the Haldane phase of
the spin-1 Heisenberg chain is stabilized for N=2 with non-local string charge
correlation, and pseudo-spin 1/2 edge states. At the heart of this even-odd
behavior is the existence of a spin-singlet pseudo-spin operator which
governs the low-energy properties of the model for attractive interactions and
gives rise to the Haldane physics.Comment: 4 pages, 4 figure
Competing orders in one-dimensional half-filled multicomponent fermionic cold atoms: The Haldane-charge conjecture
We investigate the nature of the Mott-insulating phases of half-filled
2N-component fermionic cold atoms loaded into a one-dimensional optical
lattice. By means of conformal field theory techniques and large-scale DMRG
calculations, we show that the phase diagram strongly depends on the parity of
. First, we single out charged, spin-singlet, degrees of freedom, that carry
a pseudo-spin allowing to formulate a Haldane conjecture: for
attractive interactions, we establish the emergence of Haldane insulating
phases when is even, whereas a metallic behavior is found when is odd.
We point out that the cases do \emph{not} have the generic properties
of each family. The metallic phase for odd and larger than 1 has a
quasi-long range singlet pairing ordering with an interesting edge-state
structure. Moreover, the properties of the Haldane insulating phases with even
further depend on the parity of N/2. In this respect, within the low-energy
approach, we argue that the Haldane phases with N/2 even are not topologically
protected but equivalent to a topologically trivial insulating phase and thus
confirm the recent conjecture put forward by Pollmann {\it et al.} [Pollmann
{\it et al.}, arXiv:0909.4059 (2009)].Comment: 25 pages, 20 figure
Fast inactivation in Shaker K+ channels. Properties of ionic and gating currents.
Fast inactivating Shaker H4 potassium channels and nonconducting pore mutant Shaker H4 W434F channels have been used to correlate the installation and recovery of the fast inactivation of ionic current with changes in the kinetics of gating current known as "charge immobilization" (Armstrong, C.M., and F. Bezanilla. 1977. J. Gen. Physiol. 70:567-590.). Shaker H4 W434F gating currents are very similar to those of the conducting clone recorded in potassium-free solutions. This mutant channel allows the recording of the total gating charge return, even when returning from potentials that would largely inactivate conducting channels. As the depolarizing potential increased, the OFF gating currents decay phase at -90 mV return potential changed from a single fast component to at least two components, the slower requiring approximately 200 ms for a full charge return. The charge immobilization onset and the ionic current decay have an identical time course. The recoveries of gating current (Shaker H4 W434F) and ionic current (Shaker H4) in 2 mM external potassium have at least two components. Both recoveries are similar at -120 and -90 mV. In contrast, at higher potentials (-70 and -50 mV), the gating charge recovers significantly more slowly than the ionic current. A model with a single inactivated state cannot account for all our data, which strongly support the existence of "parallel" inactivated states. In this model, a fraction of the charge can be recovered upon repolarization while the channel pore is occupied by the NH2-terminus region
On-line monitoring of the transesterification reaction carried out in microreactors using near infrared spectroscopy
Biodiesel can be produced from vegetable oils, animal fats, and waste cooking oils by transesterification with ethanol (also called ethanolysis) in order to substitute fossil fuels. In this work, the batch ethanolysis of high oleic sunflower oil was transferred into a continuous microstructured device, which induces a better control of heat and mass transfers. Various parameters were studied, notably the initial ethanol to oil molar ratio. An innovative method using NIR spectroscopy was also developed to on-line monitor the transesterification reaction of high oleic sunflower oil with ethanol in microreactors (circular PFA tube 1/1600 OD, 0.0200 ID). The reactions were monitored directly in the microreactors through sequential scans of the reaction medium by the means of an adequate probe. The asset of the method is that no sample collection or preparation is necessary. Partial Least Squares regression was used to develop calibration and prediction models between NIR spectral data and analytical data obtained by a reference method (gas chromatography with flame ionization detection, GCâFID). This method is fast, safe, reliable, nondestructive and inexpensive contrary to conventional procedures, such as gas chromatography and high
performance liquid chromatography generally used to determine the composition of crude transesterification medium
Strain versus stress in a model granular material: a Devil's staircase
The series of equilibrium states reached by disordered packings of rigid,
frictionless discs in two dimensions, under gradually varying stress, are
studied by numerical simulations. Statistical properties of trajectories in
configuration space are found to be independent of specific assumptions ruling
granular dynamics, and determined by geometry only. A monotonic increase in
some macroscopic loading parameter causes a discrete sequence of
rearrangements. For a biaxial compression, we show that, due to the statistical
importance of such events of large magnitudes, the dependence of the resulting
strain on stress direction is a Levy flight in the thermodynamic limit.Comment: REVTeX, 4 pages, 5 included PostScript figures. New version altered
throughout text, very close to published pape
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