2,235 research outputs found
Observation of Feshbach resonances between two different atomic species
We have observed three Feshbach resonances in collisions between lithium-6
and sodium-23 atoms. The resonances were identified as narrow loss features
when the magnetic field was varied. The molecular states causing these
resonances have been identified, and additional lithium-sodium resonances are
predicted. These resonances will allow the study of degenerate Bose-Fermi
mixtures with adjustable interactions, and could be used to generate ultracold
heteronuclear molecules
Observation of Bose-Einstein Condensation of Molecules
We have observed Bose-Einstein condensation of molecules. When a spin mixture
of fermionic Li-6 atoms was evaporatively cooled in an optical dipole trap near
a Feshbach resonance, the atomic gas was converted into Li_2 molecules. Below
600 nK, a Bose-Einstein condensate of up to 900,000 molecules was identified by
the sudden onset of a bimodal density distribution. This condensate realizes
the limit of tightly bound fermion pairs in the crossover between BCS
superfluidity and Bose-Einstein condensation.Comment: 4 pages, 3 figure
Formation Time of a Fermion Pair Condensate
The formation time of a condensate of fermionic atom pairs close to a
Feshbach resonance was studied. This was done using a phase-shift method in
which the delayed response of the many-body system to a modulation of the
interaction strength was recorded. The observable was the fraction of condensed
molecules in the cloud after a rapid magnetic field ramp across the Feshbach
resonance. The measured response time was slow compared to the rapid ramp,
which provides final proof that the molecular condensates reflect the presence
of fermion pair condensates before the ramp.Comment: 5 pages, 4 figure
Condensation of Pairs of Fermionic Atoms Near a Feshbach Resonance
We have observed Bose-Einstein condensation of pairs of fermionic atoms in an
ultracold ^6Li gas at magnetic fields above a Feshbach resonance, where no
stable ^6Li_2 molecules would exist in vacuum. We accurately determined the
position of the resonance to be 822+-3 G. Molecular Bose-Einstein condensates
were detected after a fast magnetic field ramp, which transferred pairs of
atoms at close distances into bound molecules. Condensate fractions as high as
80% were obtained. The large condensate fractions are interpreted in terms of
pre-existing molecules which are quasi-stable even above the two-body Feshbach
resonance due to the presence of the degenerate Fermi gas.Comment: submitted to PRL. v3: clarifying revisions, added referenc
Finite Temperature DMRG Investigation of the Spin-Peierls Transition in CuGeO
We present a numerical study of thermodynamical properties of dimerized
frustrated Heisenberg chains down to extremely low temperatures with
applications to CuGeO. A variant of the finite temperature density matrix
renormalization group (DMRG) allows the study of the dimerized phase previously
unaccessible to ab initio calculations. We investigate static dimerized systems
as well as the instability of the quantum chain towards lattice dimerization.
The crossover from a quadratic response in the free energy to the distortion
field at finite temperature to nonanalytic behavior at zero temperature is
studied quantitatively. Various physical quantities are derived and compared
with experimental data for CuGeO such as magnetic dimerization, critical
temperature, susceptibility and entropy.Comment: LaTeX, 5 pages, 5 eps figures include
A two-species continuum model for aeolian sand transport
Starting from the physics on the grain scale, we develop a simple continuum
description of aeolian sand transport. Beyond popular mean-field models, but
without sacrificing their computational efficiency, it accounts for both
dominant grain populations, hopping (or "saltating") and creeping (or
"reptating") grains. The predicted stationary sand transport rate is in
excellent agreement with wind tunnel experiments simulating wind conditions
ranging from the onset of saltation to storms. Our closed set of equations thus
provides an analytically tractable, numerically precise, and computationally
efficient starting point for applications addressing a wealth of phenomena from
dune formation to dust emission.Comment: 23 pages, 9 figure
A clock network for geodesy and fundamental science
Leveraging the unrivaled performance of optical clocks in applications in
fundamental physics beyond the standard model, in geo-sciences, and in
astronomy requires comparing the frequency of distant optical clocks
truthfully. Meeting this requirement, we report on the first comparison and
agreement of fully independent optical clocks separated by 700 km being only
limited by the uncertainties of the clocks themselves. This is achieved by a
phase-coherent optical frequency transfer via a 1415 km long telecom fiber link
that enables substantially better precision than classical means of frequency
transfer. The fractional precision in comparing the optical clocks of three
parts in was reached after only 1000 s averaging time, which is
already 10 times better and more than four orders of magnitude faster than with
any other existing frequency transfer method. The capability of performing high
resolution international clock comparisons paves the way for a redefinition of
the unit of time and an all-optical dissemination of the SI-second.Comment: 14 pages, 3 figures, 1 tabl
Feshbach Resonances in Fermionic Lithium-6
Feshbach resonances in lithium-6 were experimentally studied and
theoretically analyzed. In addition to two previously known s-wave resonances,
we found three p-wave resonances. Four of these resonances are narrow and yield
a precise value of the singlet scattering length, but do not allow us to
accurately predict the location of the broad resonance near 83 mT. Its position
was previously measured in a molecule-dissociation experiment for which we,
here, discuss systematic shifts
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