1,125 research outputs found
Calabi-Yau 3-folds from 2-folds
We consider type IIA string theory on a Calabi-Yau 2-fold with D6-branes
wrapping 2-cycles in the 2-fold. We find a complete set of conditions on the
supergravity solution for any given wrapped brane configuration in terms of
SU(2) structures. We reduce the problem of finding a supergravity solution for
the wrapped branes to finding a harmonic function on RCY. We then
lift this solution to 11-dimensions as a product of R and a
Calabi-Yau 3-fold. We show how the metric on the 3-fold is determined in terms
of the wrapped brane solution. We write down the distinguished (3,0) form and
the K{\"a}hler form of the 3-fold in terms of structures defined on the base
2-d complex manifold. We discuss the topology of the 3-fold in terms of the
D6-branes and the underlying 2-fold. We show that in addition to the
non-trivial cycles inherited from the underlying 2-fold there are new
2-cycles. We construct closed (1,1) forms corresponding to these new cycles. We
also display some explicit examples. One of our examples is that of D6-branes
wrapping the 2-cycle in an A ALE space, the resulting 3-fold has
, where is the number of D6-branes.Comment: 30 page
Eddy Current Imaging for Material Surface Mapping
For most nondestructive inspections, quantitative evaluations have to be performed to correlate the measured signals and the desired material properties. In some applications, the relationship between the signals and the material parameters is simple and straightforward. An analytic mathematical function can be easily constructed and solved to describe the interaction. The inverse function also can be readily determined to transform the measured data into the desired information. In some cases, such as defect characterizations, the interaction between the sensing field and the test object is often too complicated for such an approach. Advanced signal analysis techniques with complex assumptions, approximations, and computations are required to interpret the signals. Such an intricate approach is frequently time consuming and beyond general comprehension. Alternative methods that enable the direct correlation of the signals with the test piece are being sought. Imaging techniques which provide a unique capability of correlating NDE signals with component geometry, are gaining in popularity
Raman Scattering Signatures of Kitaev Spin Liquids in A IrO Iridates
We study theoretically the Raman scattering response in the
gapless quantum spin liquid phase of the Kitaev-Heisenberg model. The dominant
polarization-independent contribution reflects the density of
states of the emergent Majorana fermions in the ground-state flux-sector. The
integrability-breaking Heisenberg exchange generates a second contribution,
whose dominant part has the form of a quantum quench
corresponding to an abrupt insertion of four gauge fluxes. This results
in a weakly polarization dependent response with a sharp peak at the energy of
the flux excitation accompanied by broad features, which can be related to
Majorana fermions in the presence of the perturbed gauge field. We discuss the
experimental situation and explore more generally the influence of
integrability breaking for Kitaev spin liquid response functions.Comment: 9 pages including supp. ma
On some geometric features of the Kramer interior solution for a rotating perfect fluid
Geometric features (including convexity properties) of an exact interior
gravitational field due to a self-gravitating axisymmetric body of perfect
fluid in stationary, rigid rotation are studied. In spite of the seemingly
non-Newtonian features of the bounding surface for some rotation rates, we
show, by means of a detailed analysis of the three-dimensional spatial
geodesics, that the standard Newtonian convexity properties do hold. A central
role is played by a family of geodesics that are introduced here, and provide a
generalization of the Newtonian straight lines parallel to the axis of
rotation.Comment: LaTeX, 15 pages with 4 Poscript figures. To be published in Classical
and Quantum Gravit
The causal structure of spacetime is a parameterized Randers geometry
There is a by now well-established isomorphism between stationary
4-dimensional spacetimes and 3-dimensional purely spatial Randers geometries -
these Randers geometries being a particular case of the more general class of
3-dimensional Finsler geometries. We point out that in stably causal
spacetimes, by using the (time-dependent) ADM decomposition, this result can be
extended to general non-stationary spacetimes - the causal structure (conformal
structure) of the full spacetime is completely encoded in a parameterized
(time-dependent) class of Randers spaces, which can then be used to define a
Fermat principle, and also to reconstruct the null cones and causal structure.Comment: 8 page
Two universal results for Wilson loops at strong coupling
We present results for Wilson loops in strongly coupled gauge theories. The
loops may be taken around an arbitrarily shaped contour and in any field theory
with a dual IIB geometry of the form M x S^5. No assumptions about
supersymmetry are made. The first result uses D5 branes to show how the loop in
any antisymmetric representation is computed in terms of the loop in the
fundamental representation. The second result uses D3 branes to observe that
each loop defines a rich sequence of operators associated with minimal surfaces
in S^5. The action of these configurations are all computable. Both results
have features suggesting a connection with integrability.Comment: 1+12 pages. LaTeX. No figure
Performance of a Large Diameter Tunnel in Weak Rocks
The correlation of tunnel movement versus rock mass quality was investigated using actual monitored data as well as theoretical studies. Results revealed that meaningful empirical correlation between the commonly used rock mass rating system and tunnel deformation can be obtained only if geological structure and in-situ stresses are taken into account. In this respect, the commonly used rock mass rating system is not very suitable for such purpose. A new parameter using rock mass strength normalized by in-situ stress level appears to be more suitable for establishing the relationship between tunnel deformation and rock mass quality
Carrier dynamics and coherent acoustic phonons in nitride heterostructures
We model generation and propagation of coherent acoustic phonons in
piezoelectric InGaN/GaN multi-quantum wells embedded in a \textit{pin} diode
structure and compute the time resolved reflectivity signal in simulated
pump-probe experiments. Carriers are created in the InGaN wells by ultrafast
pumping below the GaN band gap and the dynamics of the photoexcited carriers is
treated in a Boltzmann equation framework. Coherent acoustic phonons are
generated in the quantum well via both deformation potential electron-phonon
and piezoelectric electron-phonon interaction with photogenerated carriers,
with the latter mechanism being the dominant one. Coherent longitudinal
acoustic phonons propagate into the structure at the sound speed modifying the
optical properties and giving rise to a giant oscillatory differential
reflectivity signal. We demonstrate that coherent optical control of the
differential reflectivity can be achieved using a delayed control pulse.Comment: 14 pages, 11 figure
Oscillatons formed by non linear gravity
Oscillatons are solutions of the coupled Einstein-Klein-Gordon (EKG)
equations that are globally regular and asymptotically flat. By means of a
Legendre transformation we are able to visualize the behaviour of the
corresponding objects in non-linear gravity where the scalar field has been
absorbed by means of the conformal mapping.Comment: Revtex file, 6 pages, 3 eps figure; matches version published in PR
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