27,137 research outputs found
D-branes and Discrete Torsion II
We derive D-brane gauge theories for C^3/Z_n x Z_n orbifolds with discrete
torsion and study the moduli space of a D-brane at a point. We show that, as
suggested in previous work, closed string moduli do not fully resolve the
singularity, but the resulting space -- containing n-1 conifold singularities
-- is somewhat surprising. Fractional branes also have unusual properties.
We also define an index which is the CFT analog of the intersection form in
geometric compactification, and use this to show that the elementary D6-brane
wrapped about T^6/Z_n x Z_n must have U(n) world-volume gauge symmetry.Comment: harvmac, 25 p
Stability and BPS branes
We define the concept of Pi-stability, a generalization of mu-stability of
vector bundles, and argue that it characterizes N=1 supersymmetric brane
configurations and BPS states in very general string theory compactifications
with N=2 supersymmetry in four dimensions.Comment: harvmac, 18 p
On D0-branes in Gepner models
We show why and when D0-branes at the Gepner point of Calabi-Yau manifolds
given as Fermat hypersurfaces exist.Comment: 22 pages, substantial improvements in sections 2 and 3, references
added, version to be publishe
Longitudinal phase space manipulation in energy recovering linac-driven free-electron lasers
Energy recovering an electron beam after it has participated in a
free-electron laser (FEL) interaction can be quite challenging because of the
substantial FEL-induced energy spread and the energy anti-damping that occurs
during deceleration. In the Jefferson Lab infrared FEL driver-accelerator, such
an energy recovery scheme was implemented by properly matching the longitudinal
phase space throughout the recirculation transport by employing the so-called
energy compression scheme. In the present paper,after presenting a
single-particle dynamics approach of the method used to energy-recover the
electron beam, we report on experimental validation of the method obtained by
measurements of the so-called "compression efficiency" and "momentum
compaction" lattice transfer maps at different locations in the recirculation
transport line. We also compare these measurements with numerical tracking
simulations.Comment: 31 pages, 13 figures, submitted to Phys. Rev. Special Topics A&
Invariant expectations and vanishing of bounded cohomology for exact groups
We study exactness of groups and establish a characterization of exact groups
in terms of the existence of a continuous linear operator, called an invariant
expectation, whose properties make it a weak counterpart of an invariant mean
on a group. We apply this operator to show that exactness of a finitely
generated group implies the vanishing of the bounded cohomology of with
coefficients in a new class of modules, which are defined using the Hopf
algebra structure of .Comment: Final version, to appear in the Journal of Topology and Analysi
Theory of the spatial structure of non-linear lasing modes
A self-consistent integral equation is formulated and solved iteratively
which determines the steady-state lasing modes of open multi-mode lasers. These
modes are naturally decomposed in terms of frequency dependent biorthogonal
modes of a linear wave equation and not in terms of resonances of the cold
cavity. A one-dimensional cavity laser is analyzed and the lasing mode is found
to have non-trivial spatial structure even in the single-mode limit. In the
multi-mode regime spatial hole-burning and mode competition is treated exactly.
The formalism generalizes to complex, chaotic and random laser media.Comment: 4 pages, 3 figure
The supporting-cell antigen: a receptor-like protein tyrosine phosphatase expressed in the sensory epithelia of the inner ear
After noise- or drug-induced hair-cell loss, the sensory epithelia of the avian inner ear can regenerate new hair cells. Few molecular markers are available for the supporting-cell precursors of the hair cells that regenerate, and little is known about the signaling mechanisms underlying this regenerative response. Hybridoma methodology was used to obtain a monoclonal antibody (mAb) that stains the apical surface of supporting cells in the sensory epithelia of the inner ear. The mAb recognizes the supporting-cell antigen (SCA), a protein that is also found on the apical surfaces of retinal MĂĽller cells, renal tubule cells, and intestinal brush border cells. Expression screening and molecular cloning reveal that the SCA is a novel receptor-like protein tyrosine phosphatase (RPTP), sharing similarity with human density-enhanced phosphatase, an RPTP thought to have a role in the density-dependent arrest of cell growth. In response to hair-cell damage induced by noise in vivo or hair-cell loss caused by ototoxic drug treatment in vitro, some supporting cells show a dramatic decrease in SCA expression levels on their apical surface. This decrease occurs before supporting cells are known to first enter S-phase after trauma, indicating that it may be a primary rather than a secondary response to injury. These results indicate that the SCA is a signaling molecule that may influence the potential of nonsensory supporting cells to either proliferate or differentiate into hair cell
A Point's Point of View of Stringy Geometry
The notion of a "point" is essential to describe the topology of spacetime.
Despite this, a point probably does not play a particularly distinguished role
in any intrinsic formulation of string theory. We discuss one way to try to
determine the notion of a point from a worldsheet point of view. The derived
category description of D-branes is the key tool. The case of a flop is
analyzed and Pi-stability in this context is tied in to some ideas of
Bridgeland. Monodromy associated to the flop is also computed via Pi-stability
and shown to be consistent with previous conjectures.Comment: 15 pages, 3 figures, ref adde
Assessing the transferability of ReaxFF for alumina polymorphs
The objective of this study is to assess the transferability of the ReaxFF interatomic potential for alumina polymorphs using energy calculations and virtual diffraction characterization. This study includes simulations of bulk, surface, and interface alumina structures containing the ?-, ?-, ?-, and a-Al2O3 phases. Transferability of the ReaxFF interatomic potential is determined by comparing computed bulk, surface, and interface energies to prior experimental and simulation results. In addition, minimum energy structures are characterized using a virtual diffraction method that generates experimentally comparable X-ray diffraction and electron diffraction patterns. Analysis of these results identifies a key limitation of the ReaxFF interatomic potential for alumina that biases formation of an amorphous phase. Although data show ReaxFF correctly predicts the energetic stability of a-Al2O3 among the bulk crystalline alumina phases, it incorrectly predicts an even lower energy amorphous phase. Experimental comparison of the virtual X-ray diffraction patterns constructed for each bulk phase validates the minimum energy bulk crystalline structures. However, virtual selected area electron diffraction patterns of alumina surface models show significant surface reconstructions at temperature due to the energetic bias for amorphization. Discrete peaks within these virtual diffraction patterns show that complete amorphization of the surfaces does not occur due to the constraints imposed by the interior atoms
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