22,813 research outputs found
On tachyons, gauged linear sigma models, and flip transitions
We study systems of multiple localized closed string tachyons and the
phenomena associated with their condensation, in C3/ZN nonsupersymmetric
noncompact orbifold singularities using gauged linear sigma model
constructions, following hep-th/0406039. Our study reveals close connections
between the combinatorics of nonsupersymmetric flip transitions (between
topologically distinct resolutions of the original singularity), the physics of
tachyons of different degrees of relevance and the singularity structure of the
corresponding residual endpoint geometries. This in turn can be used to study
the stability of the phases of gauged linear sigma models and gain qualitative
insight into the closed string tachyon potential.Comment: Latex, 38 pages, 7 eps figs. v2. minor modification
Beaming effect from increased-index photonic crystal waveguides
We study the beaming effect of light for the case of increased-index photonic
crystal (PhC) waveguides, formed through the omission of low-dielectric media
in the waveguide region. We employ the finite-difference time-domain numerical
method for characterizing the beaming effect and determining the mechanisms of
loss and the overall efficiency of the directional emission. We find that,
while this type of PhC waveguides is capable of producing a highly collimated
emission as was demonstrated experimentally, the inherent characteristics of
the structure result in a restrictively low efficiency in the coupling of light
into the collimated beam of light.Comment: 4 pages, 5 figures, submitted to Applied Physics
Modular Frobenius manifolds and their invariant flows
The space of Frobenius manifolds has a natural involutive symmetry on it:
there exists a map which send a Frobenius manifold to another Frobenius
manifold. Also, from a Frobenius manifold one may construct a so-called almost
dual Frobenius manifold which satisfies almost all of the axioms of a Frobenius
manifold. The action of on the almost dual manifolds is studied, and the
action of on objects such as periods, twisted periods and flows is studied.
A distinguished class of Frobenius manifolds sit at the fixed point of this
involutive symmetry, and this is made manifest in certain modular properties of
the various structures. In particular, up to a simple reciprocal
transformation, for this class of modular Frobenius manifolds, the flows are
invariant under the action of $I\,.
The role of coherent structures in the generation of noise for subsonic jets
Acoustic measurements were made in the 'near' (r/D 60, x/D 60) field for high Reynolds number (184,000 to 262,000) axisymmetric cold air jets exhausting at atmospheric pressure. These measurements were in conjunction with an investigation which characterized the large scale coherent structure in the flow field of Mach number 0.6 to 0.8 jets. Natural jets as well as artificially excited jets were studied. Directivity plots were made for both natural jets and jets excited at various frequencies. Overall noise radiated by the jets reached a maximum value around 30 deg from the jet axis. However, individual frequencies emitted maximum sound pressure level at different angles from the jet axis. As the angle from the jet axis increased, the spectra of the noise shifted to higher frequencies
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