217 research outputs found
Study of pinholes and nanotubes in AlInGaN films by cathodoluminescence and atomic force microscopy
Cathodoluminescence (CL) in the scanning electron microscope and atomic force microscopy (AFM) have been used to study the formation of pinholes in tensile and compressively strained AlInGaN films grown on Al2O3 substrates by plasma-induced molecular beam epitaxy. Nanotubes, pits, and V-shaped pinholes are observed in a tensile strained sample. CL images show an enhanced emission around the pits and a lower intensity at the V-shaped pinholes. Rounded pinholes appear in compressively strained samples in island-like regions with higher In concentration. The grain structure near the pinholes is resolved by AFM. (C) 2004 American Institute of Physics
Electrically tunable photonic true-time-delay line
We present a new application of the acousto-optic superlattice modulation of a fiber Bragg grating based on the dynamic phase and group delay properties of this fiber-optic component. We demonstrate a tunable photonic true-time-delay line based on the group delay change of the light reflected from the grating sidebands. The delay is electrically tuned by adjusting the voltage applied to a piezoelectric transducer that generates the acoustic wave propagating along the grating. In our experiments, a truetime delay of 400 ps is continuously adjusted (300 ps within the 3 dB amplitude range of the first sideband), using a 12 cm long uniform grating
Fermion Zero Modes in Odd Dimensions
We study the zero modes of the Abelian Dirac operator in any odd dimension.
We use the stereographic projection between a dimensional space and a
sphere embedded in a dimensional space. It is shown that the
Dirac operator with a gauge field of uniform field strengths in has
symmetries of SU()U(1) which is a subgroup of SO(). Using group
representation theory, we obtain the number of fermion zero modes, as well as
their explicit forms, in a simple way.Comment: 14 page
Building MSSM Flux Vacua
We construct N=1 and N=0 chiral four-dimensional vacua of flux
compactification in Type IIB string theory. These vacua have the common
features that they are free of tadpole instabilities (both NSNS and RR) even
for models with N=0 supersymmetry. In addition, the dilaton/complex structure
moduli are stabilised and the supergravity background metric is warped. We
present an example in which the low energy spectrum contains the MSSM spectrum
with three generations of chiral matter. In the N=0 models, the background
fluxes which stabilise the moduli also induce soft supersymmetry breaking terms
in the gauge and chiral sectors of the theory, while satisfying the equation of
motion. We also discuss some phenomenological features of these three
generation MSSM flux vacua. Our techniques apply to other closed string
backgrounds as well and, in fact, also allow to find new N=1 D-brane models
which were believed not to exist. Finally, we discuss in detail the consistency
conditions of these flux compactifications. Cancellation of K-theory charges
puts additional constraints on the consistency of the models, which render some
chiral D-brane models in the literature inconsistent.Comment: 33 pages, 1 figure. Minor correction
Femtosecond parabolic pulse shaping in normally dispersive optical fibers
Formation of parabolic pulses at femtosecond time scale by means of passive nonlinear reshaping in normally dispersive optical fibers is analyzed. Two approaches are examined and compared: the parabolic waveform formation in transient propagation regime and parabolic waveform formation in the steady-state propagation regime. It is found that both approaches could produce parabolic pulses as short as few hundred femtoseconds applying commercially available fibers, specially designed all-normal dispersion photonic crystal fiber and modern femtosecond lasers for pumping. The ranges of parameters providing parabolic pulse formation at the femtosecond time scale are found depending on the initial pulse duration, chirp and energy. Applicability of different fibers for femtosecond pulse shaping is analyzed. Recommendation for shortest parabolic pulse formation is made based on the analysis presented
Fluxes, moduli fixing and MSSM-like vacua in a simple IIA orientifold
We study the effects of adding RR, NS and metric fluxes on a T^6/(\Omega
(-1)^{F_L} I_3) Type IIA orientifold. By using the effective flux-induced
superpotential we obtain Minkowski or AdS vacua with broken or unbroken
supersymmetry. In the Minkowski case some combinations of real moduli remain
undetermined, whereas all can be stabilized in the AdS solutions. Many flux
parameters are available which are unconstrained by RR tadpole cancellation
conditions allowing to locate the minima at large volume and small dilaton. We
also find that in AdS supersymmetric vacua with metric fluxes, the overall flux
contribution to RR tadpoles can vanish or have opposite sign to that of
D6-branes, allowing for new model-building possibilities. In particular, we
construct the first N=1 supersymmetric intersecting D6-brane models with
MSSM-like spectrum and with all closed string moduli stabilized. Some
axion-like fields remain undetermined but they are precisely required to give
St\"uckelberg masses to (potentially anomalous) U(1) brane fields. We show that
the cancellation of the Freed-Witten anomaly guarantees that the axions with
flux-induced masses are orthogonal to those giving masses to the U(1)'s.
Cancellation of such anomalies also guarantees that the D6-branes in our N=1
supersymmetric AdS vacua are calibrated so that they are forced to preserve one
unbroken supersymmetry.Comment: 61 pages, Latex, v2: added references, v3: minor correction
A World-Volume Perspective on the Recombination of Intersecting Branes
We study brane recombination for supersymmetric configurations of
intersecting branes in terms of the world-volume field theory. This field
theory contains an impurity, corresponding to the degrees of freedom localized
at the intersection. The Higgs branch, on which the impurity fields condense,
consists of vacua for which the intersection is deformed into a smooth
calibrated manifold. We show this explicitly using a superspace formalism for
which the calibration equations arise naturally from F- and D-flatness.Comment: References adde
Instanton effects in N=1 brane models and the Kahler metric of twisted matter
We consider locally consistent systems of magnetized D9 branes on an
orbifolded six-torus which support N=1 gauge theories. In such realizations,
the matter multiplets arise from "twisted" strings connecting different stacks
of branes. The introduction of Euclidean 5 branes (E5) wrapped on the
six-dimensional compact space leads to instanton effects. For instance, if the
system is engineered so as to yield SQCD, a single E5 brane may account for the
ADS/TVY superpotential. We discuss the subtle interplay that exists between the
annuli diagrams with an E5 boundary and the holomorphicity properties of the
effective low-energy action of the N=1 theory. The consistency of this picture
allows to obtain information on the Kahler metric of the chiral matter
multiplets arising from twisted strings.Comment: 33 pages, 4 figures. V2: improved discussion, clarifyng comments and
references added. Version to be published in JHE
The LHC String Hunter's Companion (II): Five-Particle Amplitudes and Universal Properties
We extend the study of scattering amplitudes presented in ``The LHC String
Hunter's Companion'' to the case of five-point processes that may reveal the
signals of low mass strings at the LHC and are potentially useful for detailed
investigations of fundamental Regge excitations. In particular, we compute the
full-fledged string disk amplitudes describing all 2->3 parton scattering
subprocesses leading to the production of three hadronic jets. We cast our
results in a form suitable for the implementation of stringy partonic cross
sections in the LHC data analysis. We discuss the universal, model-independent
features of multi-parton processes and point out the existence of even stronger
universality relating N-gluon amplitudes to the amplitudes involving N-2 gluons
and one quark-antiquark pair. We construct a particularly simple basis of two
functions describing all universal five-point amplitudes. We also discuss
model-dependent amplitudes involving four fermions and one gauge boson that may
be relevant for studying jets associated to Drell-Yan pairs and other processes
depending on the spectrum of Kaluza-Klein particles, thus on the geometry of
compact dimensions.Comment: 71 pages, LaTeX v2: very few typos removed, final version to appear
in NP
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