3,628 research outputs found
Commensurate-incommensurate transitions of quantum Hall stripe states in double-quantum-well systems
In higher Landau levels (N>0) and around filling factors nu =4N+1, a
two-dimensional electron gas in a double-quantum-well system supports a stripe
groundstate in which the electron density in each well is spatially modulated.
When a parallel magnetic field is added in the plane of the wells, tunneling
between the wells acts as a spatially rotating effective Zeeman field coupled
to the ``pseudospins'' describing the well index of the electron states. For
small parallel fields, these pseudospins follow this rotation, but at larger
fields they do not, and a commensurate-incommensurate transition results.
Working in the Hartree-Fock approximation, we show that the combination of
stripes and commensuration in this system leads to a very rich phase diagram.
The parallel magnetic field is responsible for oscillations in the tunneling
matrix element that induce a complex sequence of transitions between
commensurate and incommensurate liquid or stripe states. The homogeneous and
stripe states we find can be distinguished by their collective excitations and
tunneling I-V, which we compute within the time-dependent Hartree-Fock
approximation.Comment: 23 pages including 8 eps figure
Lafortune L. et St-Pierre, L. (1994). Les processus mentaux et les émotions dans l’apprentissage. Montréal : Les Éditions Logiques. Lafortune L. et St-Pierre, L. (1994). La pensée et les émotions en mathématiques. Montréal : Les Éditions Logiques.
Finite volume analysis of reinforced concrete structure cracking using a thermo-plastic-damage model
This paper proposes modifications to the phenomenological model formulation called CDPM2, developed by Grassl et al. [1]. The proposed modifications are designed to enhance model performance with coupling to temperature effects. A very strong coupling between nonlinear elasticity, plasticity, nonlocal damage evolution and temperature gradient is used to simulate arbitrary crack propagation. The use of FVM to model solid damage is a numerical challenge. This approach presents some advantages such as: ensuring that discretization is conservative even when the geometry is changing; providing a simple formulation that can be obtained directly from a difference method; and employing unstructured meshes. Most authors have neglected the nonlinearity of concrete in the elastic domain from the start of loading to the plastic domain. In this paper we confirm that concrete rheology is not linear even under low loading. Also, since the so-called fracture energy is a key parameter needed to determine the size of cracks and how they propagate in space, we consider that the fracture energy is both material and geometrical parameter dependent. For this reason, we developed a new approach which includes adaptive mesh, nonlinear rheology and thermal effects to re-calculate fracture energy at each time step. Many authors use a constant value obtained from experiments to calculate fracture energy; others use a numerical correlation. In this study, the fracture energy parameter is not constant and can vary with temperature or/and with a change in geometry due to concrete failure. As is well known, the mesh quality of complex geometries is very important for making accurate predictions. A new meshing tool was developed using the C++ programming language. This tool is faster, more accurate and produces a high-quality structured mesh. The predictions obtained were compared to a wide variety of experimental data and showed good agreement
Skyrme Crystal In A Two-Dimensional Electron Gas
The ground state of a two-dimensional electron gas at Landau level filling
factors near is a Skyrme crystal with long range order in the
positions and orientations of the topologically and electrically charged
elementary excitations of the ferromagnetic ground state. The lowest
energy Skyrme crystal is a square lattice with opposing postures for
topological excitations on opposite sublattices. The filling factor dependence
of the electron spin-polarization, calculated for the square lattice Skyrme
crystal, is in excellent agreement with recent experiments.Comment: 3 pages, latex, 3 figures available upon request from
[email protected]
The formation of α-(1[right-facing arrow]3) D-glucosidic linkages by exocellular α-D-glucansucrases from Leuconostoc mesenteroides and Streptococcus mutans
Alternansucrase, an exocellular glucansucrase from Leuconostoc mesenteroides NRRL B-1355, which synthesizes an alternating (alpha)-(1(---\u3e)3), (alpha)-(1(---\u3e)6)-D-glucan from sucrose, was isolated from the culture supernatant fluid of cultures of this organism. The most effective method for accomplishing this was hydrophobic chromatography on phenoxyacetyl cellulose. Alternansucrase was shown to synthesize alternan and to form both (alpha)-(1(---\u3e)6) and (alpha)-(1(---\u3e)3) glucosidic bonds by acceptor reactions with low-molecular-weight saccharides in the presence of sucrose, but an (alpha)-(1(---\u3e)3) bond was synthesized only when an (alpha)-(1(---\u3e)6) glucosidic bond was present at the nonreducing end of the acceptor. Acceptor reactions occurred by transfer of glucosyl units from sucrose to the nonreducing ends of oligosaccharide acceptors;A mixture of two exocellular glucansucrases from L. mesenteroides NRRL B-742 was found to be capable of transferring glucosyl units from sucrose to L. mesenteroides B-512F dextran, to form (alpha)-(1(---\u3e)3) branch linkages via acceptor reactions. It was demonstrated that only one of the two glucansucrases present in the mixture was responsible for these branching reactions; this enzyme is a dextransucrase which forms a dextran having an (alpha)-(1(---\u3e)6) backbone chain with a high percentage of single glucosyl branches linked (alpha)-(1(---\u3e)3) to the main chain. This percentage of branch points can vary, depending on the conditions under which the dextran is synthesized;A glucansucrase from Streptococcus mutans 6715, which produces a highly branched, water-soluble dextran, was found to be capable of forming (alpha)-(1(---\u3e)3) branch linkages in the same manner as the B-742 S dextransucrase, i.e., by acceptor reactions with relatively unbranched dextran, such as that from L. mesenteroides B-512F. This streptococcal dextransucrase was stimulated by the addition of exogenous dextrans. The stimulation was greatest with relatively unbranched dextrans, while more highly branched dextrans were less effective in their ability to stimulate S. mutans 6715 dextransucrase. Other (alpha)-D-glucans, such as glycogen, pullulan, and alternan, did not stimulate;The dextransucrase from S. mutans 6715 was able to utilize alternate glucosyl donors, such as dextran, maltotriose, panose, and isomaltodextrins containing three or more glucose units, in what could be considered the reverse of acceptor reactions
SYGMA: Stellar Yields for Galactic Modeling Applications
The stellar yields for galactic modeling applications (SYGMA) code is an
open-source module that models the chemical ejecta and feedback of simple
stellar populations (SSPs). It is intended for use in hydrodynamical
simulations and semi-analytic models of galactic chemical evolution. The module
includes the enrichment from asymptotic giant branch (AGB) stars, massive
stars, SNIa and neutron-star mergers. An extensive and extendable stellar
yields library includes the NuGrid yields with all elements and many isotopes
up to Bi. Stellar feedback from mechanic and frequency-dependent radiative
luminosities are computed based on NuGrid stellar models and their synthetic
spectra. The module further allows for customizable initial-mass functions and
supernova Ia (SNIa) delay-time distributions to calculate time-dependent ejecta
based on stellar yield input. A variety of r-process sites can be included. A
comparison of SSP ejecta based on NuGrid yields with those from Portinari et
al. (1998) and Marigo (2001) reveals up to a factor of 3.5 and 4.8 less C and N
enrichment from AGB stars at low metallicity, a result we attribute to NuGrid's
modeling of hot-bottom burning. Different core-collapse supernova explosion and
fallback prescriptions may lead to substantial variations for the accumulated
ejecta of C, O and Si in the first at . An online
interface of the open-source SYGMA module enables interactive simulations,
analysis and data extraction of the evolution of all species formed by the
evolution of simple stellar populations.Comment: 18 pages, 10 figures, 3 tables, published in ApJ
Dynamics of quantum Hall stripes in double-quantum-well systems
The collective modes of stripes in double layer quantum Hall systems are
computed using the time-dependent Hartree-Fock approximation. It is found that,
when the system possesses spontaneous interlayer coherence, there are two
gapless modes, one a phonon associated with broken translational invariance,
the other a pseudospin-wave associated with a broken U(1) symmetry. For large
layer separations the modes disperse weakly for wavevectors perpendicular to
the stripe orientation, indicating the system becomes akin to an array of
weakly coupled one-dimensional XY systems. At higher wavevectors the collective
modes develop a roton minimum associated with a transition out of the coherent
state with further increasing layer separation. A spin wave model of the system
is developed, and it is shown that the collective modes may be described as
those of a system with helimagnetic ordering.Comment: 16 pages including 7 postscript figure
Complex microwave conductivity of PrCeCuO thin films using a cavity perturbation method
We report a study of the microwave conductivity of electron-doped
PrCeCuO superconducting thin films using a
cavity perturbation technique. The relative frequency shifts obtained for the
samples placed at a maximum electric field location in the cavity are treated
using the high conductivity limit presented recently by Peligrad
Using two resonance modes, TE (16.5 GHz) and TE
(13 GHz) of the same cavity, only one adjustable parameter is needed
to link the frequency shifts of an empty cavity to the ones of a cavity loaded
with a perfect conductor. Moreover, by studying different sample
configurations, we can relate the substrate effects on the frequency shifts to
a scaling factor. These procedures allow us to extract the temperature
dependence of the complex penetration depth and the complex microwave
conductivity of two films with different quality. Our data confirm that all the
physical properties of the superconducting state are consistent with an order
parameter with lines of nodes. Moreover, we demonstrate the high sensitivity of
these properties on the quality of the films
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