13,216 research outputs found
The Dicke model phase transition in the quantum motion of a Bose-Einstein condensate in an optical cavity
We show that the motion of a laser-driven Bose-Einstein condensate in a
high-finesse optical cavity realizes the spin-boson Dicke-model. The quantum
phase transition of the Dicke-model from the normal to the superradiant phase
corresponds to the self-organization of atoms from the homogeneous into a
periodically patterned distribution above a critical driving strength. The
fragility of the ground state due to photon measurement induced back action is
calculated.Comment: 5 pages, 2 figure
Dipole-dipole instability of atom clouds in a far-detuned optical dipole trap
The effect of the dipole-dipole interaction on the far-off-resonance optical
dipole trapping scheme is calculated by a mean-field approach. The trapping
laser field polarizes the atoms and the accompanying dipole-dipole energy shift
deepens the attractive potential minimum in a pancake-shaped cloud. At high
density the thermal motion cannot stabilize the gas against self-contraction
and an instability occurs. We calculate the boundary of the stable and unstable
equilibrium regions on a two-dimensional phase diagram of the atom number and
the ratio of the trap depth to the temperature. We discuss the limitations
imposed by the dipole-dipole instability on the parameters needed to reach
Bose-Einstein condensation in an optical dipole trap.Comment: 8 pages, 3 figure
Photonic tuning of quasi-particle decay in a superfluid
We show that the damping rate of elementary excitations of hybrid systems
close to a phase transition can undergo a remarkable resonance like enhancement
before mode softening takes place. In particular, we consider the friction of a
collective density wave in a homogeneous superfluid of weakly interacting
bosonic atoms coupled to the electromagnetic field of a single mode optical
resonator. Here the Beliaev damping can thus be controlled by an external laser
drive and be enhanced by several orders of magnitude
Dynamic interaction between structure and liquid propellants in a space shuttle vehicle model, part 1 Final report
Dynamic interaction between structure and liquid propellants in space shuttle vehicle model
Coupling between structure and liquids in a parallel stage space shuttle design
A study was conducted to determine the influence of liquid propellants on the dynamic loads for space shuttle vehicles. A parallel-stage configuration model was designed and tested to determine the influence of liquid propellants on coupled natural modes. A forty degree-of-freedom analytical model was also developed for predicting these modes. Currently available analytical models were used to represent the liquid contributions, even though coupled longitudinal and lateral motions are present in such a complex structure. Agreement between the results was found in the lower few modes
Cavity optomechanics with a trapped, interacting Bose-Einstein condensate
The dispersive interaction of a Bose-Einstein condensate with a single mode
of a high-finesse optical cavity realizes the radiation pressure coupling
Hamiltonian. In this system the role of the mechanical oscillator is played by
a single condensate excitation mode that is selected by the cavity mode
function. We study the effect of atomic s-wave collisions and show that it
merely renormalizes parameters of the usual optomechanical interaction.
Moreover, we show that even in the case of strong harmonic confinement---which
invalidates the use of Bloch states---a single excitation mode of the
Bose-Einstein condensate couples significantly to the light field, that is the
simplified picture of a single "mechanical" oscillator mode remains valid
The chemical structure of the very young starless core L1521E
L1521E is a dense starless core in Taurus that was found to have relatively
low molecular depletion by earlier studies, thus suggesting a recent formation.
We aim to characterize the chemical structure of L1521E and compare it to the
more evolved L1544 pre-stellar core. We have obtained 2.52.5
arcminute maps toward L1521E using the IRAM-30m telescope in transitions of
various species. We derived abundances for the species and compared them to
those obtained toward L1544. We estimated CO depletion factors. Similarly to
L1544, -CH and CHOH peak at different positions. Most species
peak toward the -CH peak. The CO depletion factor derived toward the
dust peak is 4.31.6, which is about a factor of three lower
than that toward L1544. The abundances of sulfur-bearing molecules are higher
toward L1521E than toward L1544 by factors of 2-20. The abundance of
methanol is similar toward the two cores. The higher abundances of
sulfur-bearing species toward L1521E than toward L1544 suggest that significant
sulfur depletion takes place during the dynamical evolution of dense cores,
from the starless to pre-stellar stage. The CO depletion factor measured toward
L1521E suggests that CO is more depleted than previously found. Similar
CHOH abundances between L1521E and L1544 hint that methanol is forming at
specific physical conditions in Taurus, characterized by densities of a few
10 cm and (H)10 cm, when CO
starts to catastrophically freeze-out, while water can still be significantly
photodissociated, so that the surfaces of dust grains become rich in solid CO
and CHOH, as already found toward L1544. Methanol can thus provide
selective crucial information about the transition region between dense cores
and the surrounding parent cloud.Comment: Accepted for publication in A&A, abstract abridge
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