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

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 $\nu =1$ is a Skyrme crystal with long range order in the positions and orientations of the topologically and electrically charged elementary excitations of the $\nu=1$ 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 $10^7\, \mathrm{yr}$ at $Z=0.001$. 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 Pr1.85_{1.85}Ce0.15_{0.15}CuO4−δ_{4-\delta} thin films using a cavity perturbation method

We report a study of the microwave conductivity of electron-doped Pr$_{1.85}$Ce$_{0.15}$CuO$_{4-\delta}$ 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 $\textit{et}$ $\textit{al.}$ Using two resonance modes, TE$_{102}$ (16.5 GHz) and TE$_{101}$ (13 GHz) of the same cavity, only one adjustable parameter $\Gamma$ 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