721 research outputs found
Large wind energy converter: Growian 3 MW
The main features of the Growian wind energy converter are presented. Energy yield, environmental impact, and construction of the energy converter are discussed. Reliability of the windpowered system is assessed
The non-abelian D-brane effective action through order
Requiring the existence of certain BPS solutions to the equations of motion,
we determine the bosonic part of the non-abelian D-brane effective action
through order . We also propose an economic organizational
principle for the effective action.Comment: 12 pages, 2 figures, JHEP styl
Supersymmetric sources, integrability and generalized-structure compactifications
In the context of supersymmetric compactifications of type II supergravity to
four dimensions, we show that orientifold sources can be compatible with a
generalized SU(3) x SU(3)-structure that is neither strictly SU(3) nor static
SU(2). We illustrate this with explicit examples, obtained by suitably
T-dualizing known solutions on the six-torus. In addition we prove the
following integrability statements, valid under certain mild assumptions: (a)
for general type II supergravity backgrounds with orientifold and/or D-brane
generalized-calibrated sources, the source-corrected Einstein and dilaton
equations of motion follow automatically from the supersymmetry equations once
the likewise source-corrected form equations of motion and Bianchi identities
are imposed; (b) in the special case of supersymmetric compactifications to
four-dimensional Minkowski space, the equations of motion of all fields,
including the NSNS three-form, follow automatically once the supersymmetry and
the Bianchi identities of the forms are imposed. Both (a) and (b) are equally
valid whether the sources are smeared or localized. As a byproduct we obtain
the calibration form for a space-filling NS5-brane.Comment: 32 pages, 1 table, v2: added references, v3: corrected mistake in
(4.1) leading to factor 2 mistake in (B.6), corrected (B.5), smaller typo
Precision measurement of light shifts in a single trapped Ba ion
Using a single trapped barium ion we have developed an rf spectroscopy
technique to measure the ratio of the off-resonant vector ac Stark effect (or
light shift) in the 6S_{1/2} and 5D_{3/2} states to 0.1% precision. We find R =
Delta_S / Delta_D = -11.494(13) at 514.531 nm where Delta_{S,D} are the light
shifts of the m = +/- 1/2 splittings due to circularly polarized light.
Comparison of this result with an ab initio calculation of R would yield a new
test of atomic theory. By appropriately choosing an off-resonant light shift
wavelength one can emphasize the contribution of one or a few dipole matrix
elements and precisely determine their values.Comment: 4 pages, 5 figures, in submission to PR
Extracting the Single-Particle Gap in Carbon Nanotubes with Lattice Quantum Monte Carlo
We show how lattice Quantum Monte Carlo simulations can be used to calculate
electronic properties of carbon nanotubes in the presence of strong
electron-electron correlations. We employ the path integral formalism and use
methods developed within the lattice QCD community for our numerical work and
compare our results to empirical data of the Anti-Ferromagnetic Mott Insulating
gap in large diameter tubes.Comment: 8 pages, 5 figures, Lat2017 proceedin
From ten to four and back again: how to generalize the geometry
We discuss the four-dimensional N=1 effective approach in the study of warped
type II flux compactifications with SU(3)x SU(3)-structure to AdS_4 or flat
Minkowski space-time. The non-trivial warping makes it natural to use a
supergravity formulation invariant under local complexified Weyl
transformations. We obtain the classical superpotential from a standard
argument involving domain walls and generalized calibrations and show how the
resulting F-flatness and D-flatness equations exactly reproduce the full
ten-dimensional supersymmetry equations. Furthermore, we consider the effect of
non-perturbative corrections to this superpotential arising from gaugino
condensation or Euclidean D-brane instantons. For the latter we derive the
supersymmetry conditions in N=1 flux vacua in full generality. We find that the
non-perturbative corrections induce a quantum deformation of the internal
generalized geometry. Smeared instantons allow to understand KKLT-like AdS
vacua from a ten-dimensional point of view. On the other hand, non-smeared
instantons in IIB warped Calabi-Yau compactifications 'destabilize' the
Calabi-Yau complex structure into a genuine generalized complex one. This
deformation gives a geometrical explanation of the non-trivial superpotential
for mobile D3-branes induced by the non-perturbative corrections.Comment: LaTeX, 47 pages, v2, references, hyperref added, v3, correcting small
inaccuracies in eqs. (2.6a) and (5.16
D-branes on AdS flux compactifications
We study D-branes in N=1 flux compactifications to AdS_4. We derive their
supersymmetry conditions and express them in terms of background generalized
calibrations. Basically because AdS has a boundary, the analysis of stability
is more subtle and qualitatively different from the usual case of Minkowski
compactifications. For instance, stable D-branes filling AdS_4 may wrap trivial
internal cycles. Our analysis gives a geometric realization of the
four-dimensional field theory approach of Freedman and collaborators.
Furthermore, the one-to-one correspondence between the supersymmetry conditions
of the background and the existence of generalized calibrations for D-branes is
clarified and extended to any supersymmetric flux background that admits a
time-like Killing vector and for which all fields are time-independent with
respect to the associated time. As explicit examples, we discuss supersymmetric
D-branes on IIA nearly Kaehler AdS_4 flux compactifications.Comment: 43 pages, 2 pictures, 1 table; v2: added references, color to figure
and corrected typo in (6.21b
Derivative corrections to the Born-Infeld action through beta-function calculations in N=2 boundary superspace
We calculate the beta-functions for an open string sigma-model in the
presence of a U(1) background. Passing to N=2 boundary superspace, in which the
background is fully characterized by a scalar potential, significantly
facilitates the calculation. Performing the calculation through three loops
yields the equations of motion up to five derivatives on the fieldstrengths,
which upon integration gives the bosonic sector of the effective action for a
single D-brane in trivial bulk background fields through four derivatives and
to all orders in alpha'. Finally, the present calculation shows that demanding
ultra-violet finiteness of the non-linear sigma-model can be reformulated as
the requirement that the background is a deformed stable holomorphic U(1)
bundle.Comment: 25 pages, numerous figure
On the geometry of string duals with backreacting flavors
Making use of generalized calibrated geometry and G-structures we put the
problem of finding string-duals with smeared backreacting flavor branes in a
more mathematical setting. This more formal treatment of the problem allows us
to easily smear branes without good coordinate representations, establish
constraints on the smearing form and identify a topological central charge in
the SUSY algebra. After exhibiting our methods for a series of well known
examples, we apply them to the problem of flavoring a supergravity-dual to a
d=2+1 dimensional N=2 super Yang-Mills-like theory. We find new solutions to
both the flavored and unflavored systems. Interpretating these turns out to be
difficult.Comment: 38 pages - Typos corrected and references added - As published in
JHE
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