830 research outputs found
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
Bamboo-like Chained Cavities and Other Halogen-Bonded Complexes from Tetrahaloethynyl Cavitands with Simple Ditopic Halogen Bond Acceptors
Halogen bonding provides a useful complement to hydrogen bonding and metal-coordination as a tool for organizing supramolecular systems. Resorcinarenes, tetrameric bowl-shaped cavitands, have been previously shown to function as efficient scaffolds for generating dimeric capsules in both solution and solid-phase, and complicated one-, two-, and three-dimensional frameworks in the solid phase. Tetrahaloethynyl resorcinarenes (bromide and iodide) position the halogen atoms in a very promising crown-like orientation for acting as organizing halogen-bond donors to help build capsules and higher-order networks. Symmetric divalent halogen bond acceptors including bipyridines, 1,4-dioxane, and 1,4-diazabicyclo[2.2.2]octane are very promising halogen bond accepting partners for creating these systems. This report describes the complex structures arising from combining these various systems including self-included dimers, herringbone-packed architectures enclosing medium (186 Å3) cavities, and a very intriguing bamboo-like one-dimensional rod with large (683 Å3) cavities between adjacent dimeric units. These various structures, all organized through host-host, host-acceptor, and host-solvent interactions highlight the emergent complexity of these types of complexes. As halogen bonds are weaker than hydrogen-bonds, the resulting architectures are harder to predict, and these results provide additional insight into the parameters requiring consideration when designing crystalline supramolecular systems using halogen-bonds as the core organizing principle
Halogen-bonded solvates of tetrahaloethynyl cavitands
The formation and structures of halogen-bonded solvates of three different tetrahaloethynyl cavitands with acetone, chloroform, acetonitrile, DMF and DMSO were prepared and investigated. The inclusion and host-guest behaviour of the resorcinarene cavitands was found to be highly dependent on the flexibility of the ethylene-bridging unit
Hartree-Fock Theory of Hole Stripe States
We report on Hartree-Fock theory results for stripe states of two-dimensional
hole systems in quantum wells grown on GaAs (311)A substrates. We find that the
stripe orientation energy has a rich dependence on hole density, and on
in-plane field magnitude and orientation. Unlike the electron case, the
orientation energy is non-zero for zero in-plane field, and the ground state
orientation can be either parallel or perpendicular to a finite in-plane field.
We predict an orientation reversal transition in in-plane fields applied along
the direction.Comment: 5 pages including 4 figure
Momentum--dependent nuclear mean fields and collective flow in heavy ion collisions
We use the Boltzmann-Uehling-Uhlenbeck model to simulate the dynamical
evolution of heavy ion collisions and to compare the effects of two
parametrizations of the momentum--dependent nuclear mean field that have
identical properties in cold nuclear matter. We compare with recent data on
nuclear flow, as characterized by transverse momentum distributions and flow
() variables for symmetric and asymmetric systems. We find that the precise
functional dependence of the nuclear mean field on the particle momentum is
important. With our approach, we also confirm that the difference between
symmetric and asymmetric systems can be used to pin down the density and
momentum dependence of the nuclear self consistent one--body potential,
independently. All the data can be reproduced very well with a
momentum--dependent interaction with compressibility K = 210 MeV.Comment: 15 pages in ReVTeX 3.0; 12 postscript figures uuencoded; McGill/94-1
A Calibration Method for the Integrated Design of Finishing Robotic Workcells in the Aerospace Industry
Industrial robotics provides high flexibility and reconfigurability, cost effectiveness and user friendly programming for many applications but still lacks in accuracy. An effective workcell calibration reduces the errors in robotic manufacturing and contributes to extend the use of industrial robots to perform high quality finishing of complex parts in the aerospace industry. A novel workcell calibration method is embedded in an integrated design framework for an in-depth exploitation of CAD-based simulation and offline programming.
The method is composed of two steps: a first offline calibration of the workpiece-independent elements in the workcell layout and a final automated online calibration of workpiece-dependent elements. The method is finally applied to a robotic workcell for finishing aluminum housings of helicopter gear transmissions, characterized by complex and non-repetitive shapes, and by severe dimensional and geometrical accuracy demands. Experimental results
demonstrate enhanced performances of the robotic workcell and improved final quality of the housings
Kondo effect of non-magnetic impurities and the co-existing charge order in the cuprate superconductors
We present a theory of Kondo effect caused by an induced magnetic moment near
non-magnetic impurities such as Zn and Li in the cuprate superconductors. Based
on the co-existence of charge order and superconductivity, a natural
description of the induced moment and the resulting Kondo effect is obtained in
the framework of bond-operator theory of microscopic t-J-V Hamiltonian. The
local density of state near impurities is computed in a self-consistent
Bogoliubov-de Gennes theory which shows a low-energy peak in the middle of
superconducting gap. Our theory also suggests that the charge order can be
enhanced near impuries.Comment: 5 pages, 4 figure
Correlations Between Charge Ordering and Local Magnetic Fields in Overdoped YBaCuO
Zero-field muon spin relaxation (ZF-SR) measurements were undertaken on
under- and overdoped samples of superconducting YBaCuO to
determine the origin of the weak static magnetism recently reported in this
system. The temperature dependence of the muon spin relaxation rate in
overdoped crystals displays an unusual behavior in the superconducting state. A
comparison to the results of NQR and lattice structure experiments on highly
doped samples provides compelling evidence for strong coupling of charge, spin
and structural inhomogeneities.Comment: 4 pages, 4 figures, new data, new figures and modified tex
Muon spin relaxation studies of incommensurate magnetism and superconductivity in stage-4 LaCuO and LaSrCuO
This paper reports muon spin relaxation (MuSR) measurements of two single
crystals of the title high-Tc cuprate systems where static incommensurate
magnetism and superconductivity coexist. By zero-field MuSR measurements and
subsequent analyses with simulations, we show that (1) the maximum ordered Cu
moment size (0.36 Bohr magneton) and local spin structure are identical to
those in prototypical stripe spin systems with the 1/8 hole concentration; (2)
the static magnetism is confined to less than a half of the volume of the
sample, and (3) regions with static magnetism form nano-scale islands with the
size comparable to the in-plane superconducting coherence length. By
transverse-field MuSR measurements, we show that Tc of these systems is related
to the superfluid density, in the same way as observed in cuprate systems
without static magnetism. We discuss a heuristic model involving percolation of
these nanoscale islands with static magnetism as a possible picture to
reconcile heterogeneity found by the present MuSR study and long-range spin
correlations found by neutron scattering.Comment: 19 pages, 15 figures, submitted to Phys. Rev. B. E-mail:
[email protected]
Directed flow in Au+Au, Xe+CsI and Ni+Ni collisions and the nuclear equation of state
We present new experimental data on directed flow in collisions of Au+Au,
Xe+CsI and Ni+Ni at incident energies from 90 to 400A MeV. We study the
centrality and system dependence of integral and differential directed flow for
particles selected according to charge. All the features of the experimental
data are compared with Isospin Quantum Molecular Dynamics (IQMD) model
calculations in an attempt to extract information about the nuclear matter
equation of state (EoS). We show that the combination of rapidity and
transverse momentum analysis of directed flow allow to disentangle various
parametrizations in the model. At 400A MeV, a soft EoS with momentum dependent
interactions is best suited to explain the experimental data in Au+Au and
Xe+CsI, but in case of Ni+Ni the model underpredicts flow for any EoS. At 90A
MeV incident beam energy, none of the IQMD parametrizations studied here is
able to consistently explain the experimental data.Comment: RevTeX, 20 pages, 30 eps figures, accepted for publication in Phys.
Rev. C. Data files available at http://www.gsi.de/~fopiwww/pub
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