8,924 research outputs found
Subradiance in a Large Cloud of Cold Atoms
Since Dicke's seminal paper on coherence in spontaneous radiation by atomic
ensembles, superradiance has been extensively studied. Subradiance, on the
contrary, has remained elusive, mainly because subradiant states are weakly
coupled to the environment and are very sensitive to nonradiative decoherence
processes.Here we report the experimental observation of subradiance in an
extended and dilute cold-atom sample containing a large number of particles. We
use a far detuned laser to avoid multiple scattering and observe the temporal
decay after a sudden switch-off of the laser beam. After the fast decay of most
of the fluorescence, we detect a very slow decay, with time constants as long
as 100 times the natural lifetime of the excited state of individual atoms.
This subradiant time constant scales linearly with the cooperativity parameter,
corresponding to the on-resonance optical depth of the sample, and is
independent of the laser detuning, as expected from a coupled-dipole model
Superradiance in a Large and Dilute Cloud of Cold Atoms in the Linear-Optics Regime
Superradiance has been extensively studied in the 1970s and 1980s in the
regime of superfluores-cence, where a large number of atoms are initially
excited. Cooperative scattering in the linear-optics regime, or "single-photon
superradiance" , has been investigated much more recently, and superra-diant
decay has also been predicted, even for a spherical sample of large extent and
low density, where the distance between atoms is much larger than the
wavelength. Here, we demonstrate this effect experimentally by directly
measuring the decay rate of the off-axis fluorescence of a large and dilute
cloud of cold rubidium atoms after the sudden switch-off of a low-intensity
laser driving the atomic transition. We show that, at large detuning, the decay
rate increases with the on-resonance optical depth. In contrast to forward
scattering, the superradiant decay of off-axis fluorescence is suppressed near
resonance due to attenuation and multiple-scattering effects
Nuclear energy density functional from chiral pion-nucleon dynamics: Isovector terms
We extend a recent calculation of the nuclear energy density functional in
the framework of chiral perturbation theory by computing the isovector surface
and spin-orbit terms: (\vec \nabla \rho_p- \vec \nabla \rho_n)^2 G_d(\rho)+
(\vec \nabla \rho_p- \vec \nabla \rho_n)\cdot(\vec J_p-\vec J_n)
G_{so(\rho)+(\vec J_p-\vec J_n)^2 G_J(\rho) pertaining to different proton and
neutron densities. Our calculation treats systematically the effects from
-exchange, iterated -exchange, and irreducible -exchange with
intermediate -isobar excitations, including Pauli-blocking corrections
up to three-loop order. Using an improved density-matrix expansion, we obtain
results for the strength functions , and
which are considerably larger than those of phenomenological Skyrme forces.
These (parameter-free) predictions for the strength of the isovector surface
and spin-orbit terms as provided by the long-range pion-exchange dynamics in
the nuclear medium should be examined in nuclear structure calculations at
large neutron excess.Comment: 12 pages, 5 figure
Nanostructuring Graphene by Dense Electronic Excitation
The ability to manufacture tailored graphene nanostructures is a key factor
to fully exploit its enormous technological potential. We have investigated
nanostructures created in graphene by swift heavy ion induced folding. For our
experiments, single layers of graphene exfoliated on various substrates and
freestanding graphene have been irradiated and analyzed by atomic force and
high resolution transmission electron microscopy as well as Raman spectroscopy.
We show that the dense electronic excitation in the wake of the traversing ion
yields characteristic nanostructures each of which may be fabricated by
choosing the proper irradiation conditions. These nanostructures include unique
morphologies such as closed bilayer edges with a given chirality or nanopores
within supported as well as freestanding graphene. The length and orientation
of the nanopore, and thus of the associated closed bilayer edge, may be simply
controlled by the direction of the incoming ion beam. In freestanding graphene,
swift heavy ion irradiation induces extremely small openings, offering the
possibility to perforate graphene membranes in a controlled way.Comment: 16 pages, 5 figures, submitted to Nanotechnolog
The Absorption of Chlorinated Hydrocarbon Insecticides by Frog Skin
The absorption of lindane, heptachlor, aldrin, dieldrin and p, p\u27 DDT by frog skin has been studied. Various experiments indicate that water-soluble fractions of these insecticides are selectively absorbed by the skin. The relative quantities of insecticides absorbed appear to be partially related to their water solubilities. Selected studies using isolated frog skin indicate that the passage of insecticides across the skin does not alter the electrical potential difference across the skin. The observed results are discussed with respect to the accumulation of these chemicals in the tissues of the frogs
Temporal intensity correlation of light scattered by a hot atomic vapor
We present temporal intensity correlation measurements of light scattered by
a hot atomic vapor. Clear evidence of photon bunching is shown at very short
time-scales (nanoseconds) imposed by the Doppler broadening of the hot vapor.
Moreover, we demonstrate that relevant information about the scattering
process, such as the ratio of single to multiple scattering, can be deduced
from the measured intensity correlation function. These measurements confirm
the interest of temporal intensity correlation to access non-trivial spectral
features, with potential applications in astrophysics
Comment about constraints on nanometer-range modifications to gravity from low-energy neutron experiments
A topic of present interest is the application of experimentally observed
quantum mechanical levels of ultra-cold neutrons in the earth's gravitational
field for searching short-range modifications to gravity. A constraint on new
forces in the nanometer-range published by Nesvizhevsky and Protasov follows
from inadequate modelling of the interaction potential of a neutron with a
mirror wall. Limits by many orders of magnitude better were already derived
long ago from the consistency of experiments on the neutron-electron
interaction.Comment: three page
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