82,736 research outputs found
Magnetic fluxes and moduli stabilization
Stabilization of closed string moduli in toroidal orientifold
compactifications of type IIB string theory are studied using constant internal
magnetic fields on D-branes and 3-form fluxes that preserve N=1 supersymmetry
in four dimensions. Our analysis corrects and extends previous work by us, and
indicates that charged scalar VEV's need to be turned on, in addition to the
fluxes, in order to construct a consistent supersymmetric model. As an explicit
example, we first show the stabilization of all Kahler class and complex
structure moduli by turning on magnetic fluxes on different sets of D9-branes
that wrap the internal space T^6 in a compactified type I string theory, when a
charged scalar on one of these branes acquires a non-zero VEV. The latter can
also be determined by adding extra magnetized branes, as we demonstrate in a
subsequent example. In a different model with magnetized D7-branes, in a IIB
orientifold on T^6/Z_2, we show the stabilization of all the closed string
moduli, including the axion-dilaton at weak string coupling g_s, by turning on
appropriate closed string 3-form fluxes.Comment: v2: minor changes, added discussio
Moduli stabilization with open and closed string fluxes
We study the stabilization of all closed string moduli in the T^6/Z_2
orientifold, using constant internal magnetic fields and 3-form fluxes that
preserve N=1 supersymmetry in four dimensions. We first analyze the
stabilization of Kahler class and complex structure moduli by turning on
magnetic fluxes on different sets of D9 branes that wrap the internal space
T^6/Z_2. We present explicit consistent string constructions, satisfying in
particular tadpole cancellation, where the radii can take arbitrarily large
values by tuning the winding numbers appropriately. We then show that the
dilaton-axion modulus can also be fixed by turning on closed string constant
3-form fluxes, consistently with the supersymmetry preserved by the magnetic
fields, providing at the same time perturbative values for the string coupling.
Finally, several models are presented combining open string magnetic fields
that fix part of Kahler class and complex structure moduli, with closed string
3-form fluxes that stabilize the remaining ones together with the dilaton.Comment: 49 pages, a new model added, as well as improvements and reference
Strings, Junctions and Stability
Identification of string junction states of pure SU(2) Seiberg-Witten theory
as B-branes wrapped on a Calabi-Yau manifold in the geometric engineering limit
is discussed. The wrapped branes are known to correspond to objects in the
bounded derived category of coherent sheaves on the projective line \cp{1} in
this limit. We identify the pronged strings with triangles in the underlying
triangulated category using Pi-stability. The spiral strings in the weak
coupling region are interpreted as certain projective resolutions of the
invertible sheaves. We discuss transitions between the spiral strings and
junctions using the grade introduced for Pi-stability through the central
charges of the corresponding objects.Comment: 15 pages, LaTeX; references added. typos correcte
Dynamic Normalization for Compact Binary Coalescence Searches in Non-Stationary Noise
The output of gravitational-wave interferometers, such as LIGO and Virgo, can be highly non-stationary. Broadband detector noise can affect the detector sensitivity on the order of tens of seconds. Gravitational-wave transient searches, such as those for colliding black holes, estimate this noise in order to identify gravitational-wave events. During times of non-stationarity we see a higher rate of false events being reported. To accurately separate signal from noise, it is imperative to incorporate the changing detector state into gravitational-wave searches. We develop a new statistic which estimates the variation of the interferometric detector noise. We use this statistic to re-rank candidate events identified during LIGO-Virgo's second observing run by the PyCBC search pipeline. This results in a 7% improvement in the sensitivity volume for low mass binaries, particularly binary neutron stars mergers
Ultrafast photoinduced enhancement of nonlinear optical response in 15-atom gold clusters on indium tin oxide conducting film
We show that the third order optical nonlinearity of 15-atom gold clusters is
significantly enhanced when in contact with indium tin oxide (ITO) conducting
film. Open and close aperture z-scan experiments together with non-degenerate
pump-probe differential transmission experiments were done using 80 fs laser
pulses centered at 395 nm and 790 nm on gold clusters encased inside
cyclodextrin cavities. We show that two photon absorption coefficient is
enhanced by an order of magnitude as compared to that when the clusters are on
pristine glass plate. The enhancement for the nonlinear optical refraction
coefficient is ~3 times. The photo-induced excited state absorption using
pump-probe experiments at pump wavelength of 395 nm and probe at 790 nm also
show an enhancement by an order of magnitude. These results attributed to the
excited state energy transfer in the coupled gold cluster-ITO system are
different from the enhancement seen so far in charge donor-acceptor complexes
and nanoparticle-conjugate polymer composites.Comment: To appear in Optics Express (2013);
http://dx.doi.org/10.1364/OE.21.00848
Wire tomography in the H-1NF heliac for investigation of fine structure of magnetic islands
Electron beam wire tomography in the H-1NF heliac enables high resolution mapping of vacuum flux surfaces with minimal disruption of the plasma operations schedule. Recent experimental results have proven this technique to be a highly accurate and high resolution method for mapping vacuum magnetic islands. Islands of width as small as delta approximately 8 mm have been measured, providing estimates of the internal rotational transform of the island. Point-to-point comparison of the mapping results with computer tracing, in conjunction with an image warping technique, enables systematic exploration of magnetic islands and surfaces of interest. Recent development of a fast mapping technique significantly reduced the mapping time and made this technique suitable for mapping at higher magnetic fields. This article presents recent experimental results and associated techniques.with support from
the Australian Research Council Grant No. DP0344361
Duality, Marginal Perturbations and Gauging
We study duality transformations for two-dimensional sigma models with
abelian chiral isometries and prove that generic such transformations are
equivalent to integrated marginal perturbations by bilinears in the chiral
currents, thus confirming a recent conjecture by Hassan and Sen formulated in
the context of Wess-Zumino-Witten models. Specific duality transformations
instead give rise to coset models plus free bosons.Comment: 15 page
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