50 research outputs found
Quasi Two-dimensional Transfer of Elastic Waves
A theory for multiple scattering of elastic waves is presented in a random
medium bounded by two ideal free surfaces, whose horizontal size is infinite
and whose transverse size is smaller than the mean free path of the waves. This
geometry is relevant for seismic wave propagation in the Earth crust. We derive
a time-dependent, quasi-2D radiative transfer equation, that describes the
coupling of the eigenmodes of the layer (surface Rayleigh waves, SH waves, and
Lamb waves). Expressions are found that relate the small-scale fluctuations to
the life time of the modes and to their coupling rates. We discuss a diffusion
approximation that simplifies the mathematics of this model significantly, and
which should apply at large lapse times. Finally, coherent backscattering is
studied within the quasi-2D radiative transfer equation for different source
and detection configurations.Comment: REVTeX, 36 pages with 10 figures. Submitted to Phys. Rev.
Diffusion and Localization of Cold Atoms in 3D Optical Speckle
In this work we re-formulate and solve the self-consistent theory for
localization to a Bose-Einstein condensate expanding in a 3D optical speckle.
The long-range nature of the fluctuations in the potential energy, treated in
the self-consistent Born approximation, make the scattering strongly velocity
dependent, and its consequences for mobility edge and fraction of localized
atoms have been investigated numerically.Comment: 8 pages, 11 figure
Optical response of superfluid state in dilute atomic Fermi-Dirac gases
We theoretically study the propagation of light in a Fermi-Dirac gas in the
presence of a superfluid state. BCS pairing between atoms in different
hyperfine levels may significantly increase the optical linewidth and line
shift of a quantum degenerate Fermi-Dirac gas and introduce a local-field
correction that, under certain conditions, dramatically dominates over the
Lorentz-Lorenz shift. These optical properties could possibly unambiguously
sign the presence of the superfluid state and determine the value of the BCS
order parameter.Comment: 5 pages, 2 figure
Magneto-electric response functions for simple atomic systems
We consider a simple atomic two-body bound state system that is overall charge neutral
and placed in a static electric and magnetic field, and calculate the magneto-electric
response function as a function of frequency. This is done from first principles using a
two-particle Hamiltonian for both an harmonic oscillator and Coulomb binding potential. In
the high frequency limit, the response function falls off as
1/ω2 whilst at low frequencies it tends to a constant
value