184,685 research outputs found
Heat kernel transform for nilmanifolds associated to the Heisenberg group
We study the heat kernel transform on a nilmanifold of the Heisenberg
group. We show that the image of under this transform is a direct
sum of weighted Bergman spaces which are related to twisted Bergman and
Hermite-Bergman spaces.Comment: Revised version; to appear in Revista Mathematica Iberoamericana, 28
Arithmetic purity of strong approximation for homogeneous spaces
We prove that any open subset of a semi-simple simply connected quasi-split linear algebraic group with over a number field satisfies strong approximation by establishing a fibration of over a toric variety. We also prove a similar result of strong approximation with Brauer-Manin obstruction for a partial equivariant smooth compactification of a homogeneous space where all invertible functions are constant and the semi-simple part of the linear algebraic group is quasi-split. Some semi-abelian varieties of any given dimension where the complements of a rational point do not satisfy strong approximation with Brauer-Manin obstruction are given
Design of Photonic Crystal Klystrons
2D Photonic crystals (PC) with defects can act as standing-wave resonators, which offer benefit of high mode selectivity for building novel RF sources. We introduce our work on designing two-cavity single-beam and multi-beam klystrons using triangular lattice metallic PCs. We present the cold test results of the stub-coupled single-beam structure, which show that at resonance a very low reflection can be obtained, and the waves are well confined. We also present bead-pull measurement results of field strengths in the defect, using modified perturbation equation for small unit dielectric cylinder, which are in very good agreement to numerical results. A 6-beam klystron cavity is designed as a 6-coupled-defect structure with a central stub, which only couples to the in-phase mode at the lowest frequency. Finally, we present a feasibility discussion of using this multi-defect PC structure to construct an integrated klystron-accelerator cavity, along with numerical results showing a peak acceleration field of 22MV/m can be achieved
FFT-LB modeling of thermal liquid-vapor systems
We further develop a thermal LB model for multiphase flows. In the improved
model, we propose to use the FFT scheme to calculate both the convection term
and external force term. The usage of FFT scheme is detailed and analyzed. By
using the FFT algorithm spatiotemporal discretization errors are decreased
dramatically and the conservation of total energy is much better preserved. A
direct consequence of the improvement is that the unphysical spurious
velocities at the interfacial regions can be damped to neglectable scale.
Together with the better conservation of total energy, the more accurate flow
velocities lead to the more accurate temperature field which determines the
dynamical and final states of the system. With the new model, the phase diagram
of the liquid-vapor system obtained from simulation is more consistent with
that from theoretical calculation. Very sharp interfaces can be achieved. The
accuracy of simulation results are also verified by the Laplace law. The FFT
scheme can be easily applied to other models for multiphase flows.Comment: 34 pages, 21 figure
The asymmetric structure of the Galactic halo
Using the stellar photometry catalogue based on the latest data release (DR4)
of the Sloan Digital Sky Survey (SDSS), a study of the Galactic structure using
star counts is carried out for selected areas of the sky. The sample areas are
selected along a circle at a Galactic latitude of +60, and 10 strips of
high Galactic latitude along different longitudes. Direct statistics of the
data show that the surface densities of from to
are systematically higher than those of from
to , defining a region of overdensity (in the direction of Virgo)
and another one of underdensity (in the direction of Ursa Major) with respect
to an axisymmetric model. It is shown by comparing the results from star counts
in the colour that the density deviations are due to an asymmetry of
the stellar density in the halo. Theoretical models for the surface density
profile are built and star counts are performed using a triaxial halo of which
the parameters are constrained by observational data. Two possible reasons for
the asymmetric structure are discussed.Comment: 17 pages, 7 figures, 5 tables, MNRAS accepte
Comparison of different measures for quantum discord under non-Markovian noise
Two geometric measures for quantum discord were recently proposed by Modi et
al. [Phys. Rev. Lett. 104, 080501 (2010)] and Dakic et al. [Phys. Rev. Lett.
105, 190502 (2010)]. We study the similarities and differences for total
quantum correlations of Bell-diagonal states using these two geometry-based
quantum discord and the original quantum discord. We show that, under
non-Markovian dephasing channels, quantum discord and one of the geometric
measures stay constant for a finite amount of time, but not the other geometric
measure. However, all the three measures share a common sudden change point.
Our study on critical point of sudden transition might be useful for keeping
long time total quantum correlations under decoherence.Comment: 10 pages, 3 figures submitted for publicatio
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