488 research outputs found
Dynamical instability and dispersion management of an attractive condensate in an optical lattice
We investigate the stability of an attractive Bose-Einstein condensate in a
moving 1D optical lattice in the presence of transverse confinement. By means
of a Bogoliubov linear stability analysis we find that the system is
dynamically unstable for low quasimomenta and becomes stable near the band
edge, in a specular fashion with respect to the repulsive case. For low
interactions the instability occurs via long wavelength excitations that are
not sufficient for spoiling the condensate coherence, producing instead an
oscillating density pattern both in real and momentum space. This behaviour is
illustrated by simulations for the expansion of the condensate in a moving
lattice.Comment: 5 pages, 4 figure
Beyond Perrault's experiments: repeatability, didactics and complexity
Abstract. The naturalistic and philosophical studies conducted in the second half of the 17th century were crucial both for the birth of modern hydrological science and modern epistemology.
Thanks to quantitative observations and to the new experiment-based scientific approach, the Sun was about to be fully recognized as the engine of the hydrological cycle.
In this context of great vitality and rapid cultural changes, Pierre Perrault published his classical opus De l'origine des fontaines (On the origin of springs) in 1674.
The opus presents a discussion on the origin of springs and contains the report of a set of experiments of water flow through a soil column, which may be considered the first of modern hydrology. In assessing the importance of Perrault's opus, we will discuss his epistemological relevance by looking at the novelty of his approach, at the repeatability of the experiments, at the intriguing didactic aspects for the modern teaching of hydrology and at his attitude in facing the complexity of hydrological processes. Perrault places himself in the context of a novel experimental epistemology.
On the basis of our analyses he seems to be aware that the processes involved in the hydrological cycle and in soil hydrology are hardly reproducible by means of a controlled laboratory model.
This circumstance put the modern scientific approach to a severe test at its very beginning.
It is suggested that some of Perrault's epistemological and methodological reflections are precursors of the modern epistemology of complexity.
Thus even if Perrault's conclusions followed an ancient opinion, his work is not only seminal for hydrology, but also helps to enlighten some features of the scientific revolution of the 17th century
Association of ultracold double-species bosonic molecules
We report on the creation of heterospecies bosonic molecules, associated from
an ultracold Bose-Bose mixture of 41K and 87Rb, by using a resonantly modulated
magnetic field close to two Feshbach resonances. We measure the binding energy
of the weakly bound molecular states versus the Feshbach field and compare our
results to theoretical predictions. We observe the broadening and asymmetry of
the association spectrum due to thermal distribution of the atoms, and a
frequency shift occurring when the binding energy depends nonlinearly on the
Feshbach field. A simple model is developed to quantitatively describe the
association process. Our work marks an important step forward in the
experimental route towards Bose-Einstein condensates of dipolar molecules.Comment: 5 pages, 4 figure
Observing Coherence Effects in an Overdamped Quantum System
It is usually considered that the spectrum of an optical cavity coupled to an
atomic medium does not exhibit a normal-mode splitting unless the system
satisfies the strong coupling condition, meaning the Rabi frequency of the
coherent coupling exceeds the decay rates of atom and cavity excitations. Here
we show that this need not be the case, but depends on the way in which the
coupled system is probed. Measurements of the reflection of a probe laser from
the input mirror of an overdamped cavity reveal an avoided crossing in the
spectrum which is not observed when driving the atoms directly and measuring
the Purcell-enhanced cavity emission. We understand these observations by
noting a formal correspondence with electromagnetically-induced transparency of
a three-level atom in free space, where our cavity acts as the absorbing medium
and the coupled atoms play the role of the control field
Double species condensate with tunable interspecies interactions
We produce Bose-Einstein condensates of two different species, Rb and
K, in an optical dipole trap in proximity of interspecies Feshbach
resonances. We discover and characterize two Feshbach resonances, located
around 35 and 79 G, by observing the three-body losses and the elastic
cross-section. The narrower resonance is exploited to create a double species
condensate with tunable interactions. Our system opens the way to the
exploration of double species Mott insulators and, more in general, of the
quantum phase diagram of the two species Bose-Hubbard model.Comment: 4 pages, 4 figure
Collisional properties of sympathetically cooled K
We report the experimental evidence of the sympathetic cooling of K
with Rb down to 1 K, obtained in a novel tight confining magnetic
trap. This allowed us to perform the first direct measurement of the elastic
cross section of K below 50 K. The result obtained for the triplet
scattering length, Bohr radii, agrees with previous results
derived from photoassociation spectra and from Feshbach spectroscopy of
K.Comment: 7 pages, 4 figures, submitted to Phys. Rev.
Estimate of turbulent fluxes with eddy-covariance technique in a complex topography: A case study in the Italian Alps
A sensitivity analysis to different eddy—covariance data processing algorithms is presented for a dataset collected in an
Alpine environment with complex topography. In Summer 2012 a micrometeorological station was installed at Cividate
Camuno (274 m a.s.l., Oglio river basin, Central Italian Alps), in a flat and rectangular grass-covered lawn. The grass was
0.6 m tall during most of the field campaign. The station is equipped with traditional devices, four multiplexed TDR
probes, and an eddy--covariance apparatus sampling at 20 Hz (Gill WindMaster Sonic Anemometer and Licor Li7500 Gas
Analyzer), at about 3 m above the ground. The local winds regime is strongly affected by the morphology of the valley,
and the topography is complex also due to the heterogeneity of the surrounding-areas land—cover. Using EddyPro
software, the sensitivity of the turbulent fluxes estimate was assessed addressing three major issues of the data processing
procedure, i.e. the choice of the computational averaging period, of the axis rotation method and of the data detrending
criterion. Once identified three test periods of consecutive days without rainfall, the fluxes of momentum, sensible heat
and latent heat were computed at the averaging period of 30, 60 and 120 min respectively. At each averaging period, both
the triple rotation method, the double rotation method and the planar fit method were applied. Particularly the latter was
applied both fitting a unique plane for all the wind directions and fitting multiple planes, one for each sector of the wind
rose. Regarding the detrending criteria, data were processed with a block average and a linear detrend, the latter with
time constant of 5, 30, 60 and 120 min respectively. Therefore, for each test period about 50 estimates of the fluxes were
provided. As a result the obtained fluxes were compared. Even if with different flux quality, their pattern is quite stable
with regard to the applied estimate procedures, but with sensitively different average values
Scanning electron microscopy of Rydberg-excited Bose-Einstein condensates
We report on the realization of high resolution electron microscopy of
Rydberg-excited ultracold atomic samples. The implementation of an ultraviolet
laser system allows us to excite the atom, with a single-photon transition, to
Rydberg states. By using the electron microscopy technique during the Rydberg
excitation of the atoms, we observe a giant enhancement in the production of
ions. This is due to -changing collisions, which broaden the Rydberg level
and therefore increase the excitation rate of Rydberg atoms. Our results pave
the way for the high resolution spatial detection of Rydberg atoms in an atomic
sample
Observation of Stable Jones-Roberts Solitons in Bose-Einstein Condensates
We experimentally generate two-dimensional Jones-Roberts solitons in a
three-dimensional atomic Bose-Einstein condensate by imprinting a triangular
phase pattern. By monitoring their dynamics we observe that this kind of
solitary waves are resistant to both dynamic (snaking) and thermodynamic
instabilities, that usually are known to strongly limit the lifetime of dark
plane solitons in dimensions higher than one. We additionally find signatures
of a possible dipole-like interaction between them. Our results confirm that
Jones-Roberts solitons are stable solutions of the non-linear Schr\"odinger
equation in higher dimensions and promote these excitations for applications
beyond matter wave physics, like energy and information transport in noisy and
inhomogeneous environments
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