3,289 research outputs found
"Supersolid" self-bound Bose condensates via laser-induced interatomic forces
We show that the dipole-dipole interatomic forces induced by a single
off-resonant running laser beam can lead to a self-bound pencil-shaped Bose
condensate, even if the laser beam is a plane-wave. For an appropriate laser
intensity the ground state has a quasi-one dimensional density modulation --- a
Bose "supersolid".Comment: 4 pages, 3 eps figure
Classical versus quantum dynamics of the atomic Josephson junction
We compare the classical (mean-field) dynamics with the quantum dynamics of
atomic Bose-Einstein condensates in double-well potentials. The quantum
dynamics are computed using a simple scheme based upon the Raman-Nath
equations. Two different methods for exciting a non-equilbrium state are
considered: an asymmetry between the wells which is suddenly removed, and a
periodic time oscillating asymmetry. The first method generates wave packets
that lead to collapses and revivals of the expectation values of the
macroscopic variables, and we calculate the time scale for these revivals. The
second method permits the excitation of a single energy eigenstate of the
many-particle system, including Schroedinger cat states. We also discuss a band
theory interpretation of the energy level structure of an asymmetric
double-well, thereby identifying analogies to Bloch oscillations and Bragg
resonances. Both the Bloch and Bragg dynamics are purely quantum and are not
contained in the mean-field treatment.Comment: 31 pages, 14 figure
Measurements with the Chandra X-Ray Observatory's flight contamination monitor
NASA's Chandra X-ray Observatory includes a Flight Contamination Monitor
(FCM), a system of 16 radioactive calibration sources mounted to the inside of
the Observatory's forward contamination cover. The purpose of the FCM is to
verify the ground-to-orbit transfer of the Chandra flux scale, through
comparison of data acquired during the ground calibration with those obtained
in orbit, immediately prior to opening the Observatory's sun-shade door. Here
we report results of these measurements, which place limits on the change in
mirror--detector system response and, hence, on any accumulation of molecular
contamination on the mirrors' iridium-coated surfaces.Comment: 7pages,8figures,for SPIE 4012, paper 7
Collective excitation frequencies and stationary states of trapped dipolar Bose-Einstein condensates in the Thomas-Fermi regime
We present a general method for obtaining the exact static solutions and
collective excitation frequencies of a trapped Bose-Einstein condensate (BEC)
with dipolar atomic interactions in the Thomas-Fermi regime. The method
incorporates analytic expressions for the dipolar potential of an arbitrary
polynomial density profile, thereby reducing the problem of handling non-local
dipolar interactions to the solution of algebraic equations.
We comprehensively map out the static solutions and excitation modes,
including non-cylindrically symmetric traps, and also the case of negative
scattering length where dipolar interactions stabilize an otherwise unstable
condensate. The dynamical stability of the excitation modes gives insight into
the onset of collapse of a dipolar BEC. We find that global collapse is
consistently mediated by an anisotropic quadrupolar collective mode, although
there are two trapping regimes in which the BEC is stable against quadrupole
fluctuations even as the ratio of the dipolar to s-wave interactions becomes
infinite. Motivated by the possibility of fragmented BEC in a dipolar Bose gas
due to the partially attractive interactions, we pay special attention to the
scissors modes, which can provide a signature of superfluidity, and identify a
long-range restoring force which is peculiar to dipolar systems. As part of the
supporting material for this paper we provide the computer program used to make
the calculations, including a graphical user interface.Comment: 23 pages, 11 figure
Dynamical diffraction in sinusoidal potentials: uniform approximations for Mathieu functions
Eigenvalues and eigenfunctions of Mathieu's equation are found in the short
wavelength limit using a uniform approximation (method of comparison with a
`known' equation having the same classical turning point structure) applied in
Fourier space. The uniform approximation used here relies upon the fact that by
passing into Fourier space the Mathieu equation can be mapped onto the simpler
problem of a double well potential. The resulting eigenfunctions (Bloch waves),
which are uniformly valid for all angles, are then used to describe the
semiclassical scattering of waves by potentials varying sinusoidally in one
direction. In such situations, for instance in the diffraction of atoms by
gratings made of light, it is common to make the Raman-Nath approximation which
ignores the motion of the atoms inside the grating. When using the
eigenfunctions no such approximation is made so that the dynamical diffraction
regime (long interaction time) can be explored.Comment: 36 pages, 16 figures. This updated version includes important
references to existing work on uniform approximations, such as Olver's method
applied to the modified Mathieu equation. It is emphasised that the paper
presented here pertains to Fourier space uniform approximation
Scaling at the OTOC Wavefront: Integrable versus chaotic models
Out of time ordered correlators (OTOCs) are useful tools for investigating
foundational questions such as thermalization in closed quantum systems because
they can potentially distinguish between integrable and nonintegrable dynamics.
Here we discuss the properties of wavefronts of OTOCs by focusing on the region
around the main wavefront at , where is the butterfly
velocity. Using a Heisenberg spin model as an example, we find that a
propagating Gaussian with the argument
gives an excellent fit for both the integrable case and the chaotic case.
However, the scaling in these two regimes is very different: in the integrable
case the coefficients and have an inverse power law dependence on
whereas in the chaotic case they decay exponentially. In fact, the
wavefront in the integrable case is a rainbow caustic and catastrophe theory
can be invoked to assert that power law scaling holds rigorously in that case.
Thus, we conjecture that exponential scaling of the OTOC wavefront is a robust
signature of a nonintegrable dynamics.Comment: 8 pages, 2 figure
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