62 research outputs found
The Moment of Inertia and the Scissors Mode of a Bose-condensed Gas
We relate the frequency of the scissors mode to the moment of inertia of a
trapped Bose gas at finite temperature in a semi-classical approximation. We
apply these theoretical results to the data obtained in our previous study of
the properties of the scissors mode of a trapped Bose-Einstein condensate of
Rb atoms as a function of the temperature. The frequency shifts that we
measured show quenching of the moment of inertia of the Bose gas at
temperatures below the transition temperature - the system has a lower moment
of inertia that of a rigid body with the same mass distribution, because of
superfluidity.Comment: 14 pages, 5 fig
Surface-enhanced Raman spectroscopy in 3D electrospun nanofiber mats coated with gold nanorods
Nanofibers functionalized by metal nanostructures and particles are exploited
as effective flexible substrates for SERS analysis. Their complex
three-dimensional structure may provide Raman signals enhanced by orders of
magnitude compared to untextured surfaces. Understanding the origin of such
improved performances is therefore very important for pushing nanofiber-based
analytical technologies to their upper limit. Here we report on polymer
nanofiber mats which can be exploited as substrates for enhancing the Raman
spectra of adsorbed probe molecules. The increased surface area and the
scattering of light in the nanofibrous system are individually analyzed as
mechanisms to enhance Raman scattering. The deposition of gold nanorods on the
fibers further amplifies Raman signals due to SERS. This study suggests that
Raman signals can be finely tuned in intensity and effectively enhanced in
nanofiber mats and arrays by properly tailoring the architecture, composition,
and light-scattering properties of the complex networks of filaments.Comment: 29 pages, 9 figures, 1 Tabl
The experimental observation of Beliaev damping in a Bose condensed gas
We report the first experimental observation of Beliaev damping of a
collective excitation in a Bose-condensed gas. Beliaev damping is not predicted
by the Gross-Pitaevskii equation and so this is one of the few experiments that
tests BEC theory beyond the mean field approximation. Measurements of the
amplitude of a high frequency scissors mode, show that the Beliaev process
transfers energy to a lower lying mode and then back and forth between these
modes. These characteristics are quite distinct from those of Landau damping,
which leads to a monotonic decrease in amplitude. To enhance the Beliaev
process we adjusted the geometry of the magnetic trapping potential to give a
frequency ratio of 2 to 1 between two of the scissors modes of the condensate.
The ratios of the trap oscillation frequencies and
were changed independently, so that we could investigate
the resonant coupling over a range of conditions.Comment: 4 pages including 5 fig
Finite-temperature simulations of the scissors mode in Bose-Einstein condensed gases
The dynamics of a trapped Bose-condensed gas at finite temperatures is
described by a generalized Gross-Pitaevskii equation for the condensate order
parameter and a semi-classical kinetic equation for the thermal cloud, solved
using -body simulations. The two components are coupled by mean fields as
well as collisional processes that transfer atoms between the two. We use this
scheme to investigate scissors modes in anisotropic traps as a function of
temperature. Frequency shifts and damping rates of the condensate mode are
extracted, and are found to be in good agreement with recent experiments.Comment: 4 pages, 3 figure
Observation of harmonic generation and nonlinear coupling in the collective dynamics of a Bose condensate
We report the observation of harmonic generation and strong nonlinear
coupling of two collective modes of a condensed gas of rubidium atoms. Using a
modified TOP trap we changed the trap anisotropy to a value where the frequency
of the m=0 high-lying mode corresponds to twice the frequency of the m=0
low-lying mode, thus leading to strong nonlinear coupling between these modes.
By changing the anisotropy of the trap and exciting the low-lying mode we
observed significant frequency shifts of this fundamental mode and also the
generation of its second harmonic.Comment: 4 pages,3 figure
Vortex nucleation in Bose-Einstein condensates in an oblate, purely magnetic potential
We have investigated the formation of vortices by rotating the purely
magnetic potential confining a Bose-Einstein condensate. We modified the bias
field of an axially symmetric TOP trap to create an elliptical potential that
rotates in the radial plane. This enabled us to study the conditions for vortex
nucleation over a wide range of eccentricities and rotation rates.Comment: 4 pages 4 figure
Bose-Einstein condensation in a stiff TOP trap with adjustable geometry
We report on the realisation of a stiff magnetic trap with independently
adjustable trap frequencies, and , in the axial and radial
directions respectively. This has been achieved by applying an axial modulation
to a Time-averaged Orbiting Potential (TOP) trap. The frequency ratio of the
trap, , can be decreased continuously from the original
TOP trap value of 2.83 down to 1.6. We have transferred a Bose-Einstein
condensate (BEC) into this trap and obtained very good agreement between its
observed anisotropic expansion and the hydrodynamic predictions. Our method can
be extended to obtain a spherical trapping potential, which has a geometry of
particular theoretical interest.Comment: 4 pages, 3 figure
Calculation of mode coupling for quadrupole excitations in a Bose-Einstein condensate
In this paper we give a theoretical description of resonant coupling between
two collective excitations of a Bose condensed gas (BEC) on, or close, to a
second harmonic resonance. Using analytic expressions for the quasi-particle
wavefunctions we show that the coupling between quadrupole modes is strong,
leading to a coupling time of a few milliseconds (for a TOP trap with radial
frequency 100 Hz and 10^4 atoms). Using the hydrodynamic approximation, we
derive analytic expression for the coupling matrix element. These can be used
with an effective Hamiltonian (that we also derive) to describe the dynamics of
the coupling process and the associated squeezing effects.Comment: 12 pages, 3 figure
Quadrupole collective modes in trapped finite-temperature Bose-Einstein condensates
Finite temperature simulations are used to study quadrupole excitations of a
trapped Bose-Einstein condensate. We focus specifically on the m=0 mode, where
a long-standing theoretical problem has been to account for an anomalous
variation of the mode frequency with temperature. We explain this behavior in
terms of the excitation of two separate modes, corresponding to coupled motion
of the condensate and thermal cloud. The relative amplitudes of the modes
depends sensitively on the temperature and on the frequency of the harmonic
drive used to excite them. Good agreement with experiment is found for
appropriate drive frequencies.Comment: 4 pages, 3 figure
Temperature Dependence of Damping and Frequency Shifts of the Scissors Mode of a trapped Bose-Einstein Condensate
We have studied the properties of the scissors mode of a trapped
Bose-Einstein condensate of Rb atoms at finite temperature. We measured
a significant shift in the frequency of the mode below the hydrodynamic limit
and a strong dependence of the damping rate as the temperature increased. We
compared our damping rate results to recent theoretical calculations for other
observed collective modes finding a fair agreement. From the frequency
measurements we deduce the moment of inertia of the gas and show that it is
quenched below the transition point, because of the superfluid nature of the
condensed gas.Comment: 5 pages, 4 figure
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