7,687 research outputs found
Fields in nonaffine bundles. III. Effective symmetries and conserved currents in strings and higher branes
The principles of a previously developed formalism for the covariant
treatment of multi-scalar fields for which (as in a nonlinear sigma model) the
relevant target space is not of affine type -- but curved -- are recapitulated.
Their application is extended from ordinary harmonic models to a more general
category of "harmonious" field models, with emphasis on cases in which the
field is confined to a string or higher brane worldsheet, and for which the
relevant internal symmetry group is non Abelian, so that the conditions for
conservation of the corresponding charge currents become rather delicate,
particularly when the symmetry is gauged. Attention is also given to the
conditions for conservation of currents of a different kind -- representing
surface fluxes of generalised momentum or energy -- associated with symmetries
not of the internal target space but of the underlying spacetime background
structure, including the metric and any relevant gauge field. For the
corresponding current to be conserved the latter need not be manifestly
invariant: preservation modulo a gauge adjustment will suffice. The simplest
case is that of "strong" symmetry, meaning invariance under the action of an
effective Lie derivative (an appropriately gauge adjusted modification of an
ordinary Lie derivative). When the effective symmetry is of the more general
"weak" kind, the kinetic part of the current is not conserved by itself but
only after being supplemented by a suitable contribution from the background.Comment: 27 pages Latex (color
Molecular crystal global phase diagrams. II. Reference lattices
In the first part of this series [Keith et al. (2004). Cryst. Growth Des. 4, 1009-1012; Mettes et al. (2004). Acta Cryst. A60, 621-636], a method was developed for constructing global phase diagrams (GPDs) for molecular crystals in which crystal structure is presented as a function of intermolecular potential parameters. In that work, a face-centered-cubic center-of-mass lattice was arbitrarily adopted as a reference state. In part two of the series, experimental crystal structures composed of tetrahedral point group molecules are classified to determine what fraction of structures are amenable to inclusion in the GPDs and the number of reference lattices necessary to span the observed structures. It is found that 60% of crystal structures composed of molecules with T_d point-group symmetry are amenable and that eight reference lattices are sufficient to span the observed structures. Similar results are expected for other cubic point groups
On-board processing concepts for future satellite communications systems
The initial definition of on-board processing for an advanced satellite communications system to service domestic markets in the 1990's is discussed. An exemplar system with both RF on-board switching and demodulation/remodulation baseband processing is used to identify important issues related to system implementation, cost, and technology development. Analyses of spectrum-efficient modulation, coding, and system control techniques are summarized. Implementations for an RF switch and baseband processor are described. Among the major conclusions listed is the need for high gain satellites capable of handling tens of simultaneous beams for the efficient reuse of the 2.5 GHz 30/20 frequency band. Several scanning beams are recommended in addition to the fixed beams. Low power solid state 20 GHz GaAs FET power amplifiers in the 5W range and a general purpose digital baseband processor with gigahertz logic speeds and megabits of memory are also recommended
Magnetically generated spin-orbit coupling for ultracold atoms
We present a new technique for producing two- and three-dimensional
Rashba-type spin-orbit couplings for ultracold atoms without involving light.
The method relies on a sequence of pulsed inhomogeneous magnetic fields
imprinting suitable phase gradients on the atoms. For sufficiently short pulse
durations, the time-averaged Hamiltonian well approximates the Rashba
Hamiltonian. Higher order corrections to the energy spectrum are calculated
exactly for spin-1/2 and perturbatively for higher spins. The pulse sequence
does not modify the form of rotationally symmetric atom-atom interactions.
Finally, we present a straightforward implementation of this pulse sequence on
an atom chip
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