1,448 research outputs found
A magnetic lens for cold atoms controlled by a rf field
We report on a new type of magnetic lens that focuses atomic clouds using a
static inhomogeneous magnetic field in combination with a radio-frequency
field. The experimental study is performed with a cloud of cold cesium atoms.
The rf field adiabatically deforms the magnetic potential of a coil and
therefore changes its focusing properties. The focal length can be tuned
precisely by changing the rf frequency value. Depending on the rf antenna
position relative to the DC magnetic profile, the focal length of the atomic
lens can be either decreased or increased by the rf field
Dipolar atomic spin ensembles in a double-well potential
We experimentally study the spin dynamics of mesoscopic ensembles of
ultracold magnetic spin-3 atoms located in two separated wells of an optical
dipole trap. We use a radio-frequency sweep to selectively flip the spin of the
atoms in one of the wells, which produces two separated spin domains of
opposite polarization. We observe that these engineered spin domains are
metastable with respect to the long-range magnetic dipolar interactions between
the two ensembles. The absence of inter-cloud dipolar spin-exchange processes
reveals a classical behavior, in contrast to previous results with atoms loaded
in an optical lattice. When we merge the two subsystems, we observe
spin-exchange dynamics due to contact interactions which enable the first
determination of the s-wave scattering length of 52Cr atoms in the S=0
molecular channel a_0=13.5^{+11}_{-10.5}a_B (where a_B is the Bohr radius).Comment: 9 pages, 7 figure
Resonant demagnetization of a dipolar BEC in a 3D optical lattice
We study dipolar relaxation of a chromium BEC loaded into a 3D optical
lattice. We observe dipolar relaxation resonances when the magnetic energy
released during the inelastic collision matches an excitation towards higher
energy bands. A spectroscopy of these resonances for two orientations of the
magnetic field provides a 3D band spectroscopy of the lattice. The narrowest
resonance is registered for the lowest excitation energy. Its line-shape is
sensitive to the on-site interaction energy. We use such sensitivity to probe
number squeezing in a Mott insulator, and we reveal the production of
three-body states with entangled spin and orbital degrees of freedom.Comment: 5 pages, 3 Figures, Supplemental Materia
Thermodynamics of a Bose Einstein condensate with free magnetization
We study thermodynamic properties of a gas of spin 3 52Cr atoms across Bose
Einstein condensation. Magnetization is free, due to dipole-dipole interactions
(DDIs). We show that the critical temperature for condensation is lowered at
extremely low magnetic fields, when the spin degree of freedom is thermally
activated. The depolarized gas condenses in only one spin component, unless the
magnetic field is set below a critical value, below which a non ferromagnetic
phase is favored. Finally we present a spin thermometry efficient even below
the degeneracy temperature.Comment: 4 pages, 4 figure
Accumulation and thermalization of cold atoms in a finite-depth magnetic trap
We experimentally and theoretically study the continuous accumulation of cold
atoms from a magneto-optical trap (MOT) into a finite depth trap, consisting in
a magnetic quadrupole trap dressed by a radiofrequency (RF) field. Chromium
atoms (52 isotope) in a MOT are continuously optically pumped by the MOT lasers
to metastable dark states. In presence of a RF field, the temperature of the
metastable atoms that remain magnetically trapped can be as low as 25 microK,
with a density of 10^17 atoms.m-3, resulting in an increase of the phase-space
density, still limited to 7.10^-6 by inelastic collisions. To investigate the
thermalization issues in the truncated trap, we measure the free evaporation
rate in the RF-truncated magnetic trap, and deduce the average elastic cross
section for atoms in the 5D4 metastable states, equal to 7.0 10^-16m2.Comment: 9 pages, 10 Figure
Proton-transfer pathways in the mitochondrial S. cerevisiae cytochrome c oxidase
In cytochrome c oxidase (CytcO) reduction of O2 to water is linked to uptake of eight protons from the negative side of the membrane: four are substrate protons used to form water and four are pumped across the membrane. In bacterial oxidases, the substrate protons are taken up through the K and the D proton pathways, while the pumped protons are transferred through the D pathway. On the basis of studies with CytcO isolated from bovine heart mitochondria, it was suggested that in mitochondrial CytcOs the pumped protons are transferred though a third proton pathway, the H pathway, rather than through the D pathway. Here, we studied these reactions in S. cerevisiae CytcO, which serves as a model of the mammalian counterpart. We analyzed the effect of mutations in the D (Asn99Asp and Ile67Asn) and H pathways (Ser382Ala and Ser458Ala) and investigated the kinetics of electron and proton transfer during the reaction of the reduced CytcO with O2. No effects were observed with the H pathway variants while in the D pathway variants the functional effects were similar to those observed with the R. sphaeroides CytcO. The data indicate that the S. cerevisiae CytcO uses the D pathway for proton uptake and presumably also for proton pumping
A common coupling mechanism for A-type heme-copper oxidases from bacteria to mitochondria
Mitochondria metabolise almost all of the oxygen that we consume, reducing it to water by cytochrome c oxidase (CcO). CcO maximises energy capture into the protonmotive force by pumping protons across the mitochondrial inner membrane. Forty years after the H+/e- stoichiometry was established, a consensus has yet to be reached on the route taken by pumped protons to traverse CcO’s hydrophobic core and on whether bacterial and mitochondrial CcOs operate via the same coupling mechanism. To resolve this, we exploited the unique amenability to mitochondrial DNA mutagenesis of the yeast S. cerevisiae to introduce single point mutations in the hydrophilic pathways of CcO to test function. From ADP/O ratio measurements on preparations of intact mitochondria, we definitely established that the D-channel, and not the H-channel, is the proton pump of the yeast mitochondrial enzyme, supporting an identical coupling mechanism in all forms of the enzyme
Radio-frequency induced ground state degeneracy in a Chromium Bose-Einstein condensate
We study the effect of strong radio-frequency (rf) fields on a chromium
Bose-Einstein condensate (BEC), in a regime where the rf frequency is much
larger than the Larmor frequency. We use the modification of the Land\'{e}
factor by the rf field to bring all Zeeman states to degeneracy, despite the
presence of a static magnetic field of up to 100 mG. This is demonstrated by
analyzing the trajectories of the atoms under the influence of dressed magnetic
potentials in the strong field regime. We investigate the problem of
adiabaticity of the rf dressing process, and relate it to how close the dressed
states are to degeneracy. Finally, we measure the lifetime of the rf dressed
BECs, and identify a new rf-assisted two-body loss process induced by
dipole-dipole interactions.Comment: 4 pages, 4 figure
Brownian Dynamics Simulation of Polydisperse Hard Spheres
Standard algorithms for the numerical integration of the Langevin equation
require that interactions are slowly varying during to the integration
timestep. This in not the case for hard-body systems, where there is no
clearcut between the correlation time of the noise and the timescale of the
interactions. Starting from a short time approximation of the Smoluchowsky
equation, we introduce an algorithm for the simulation of the overdamped
Brownian dynamics of polydisperse hard-spheres in absence of hydrodynamics
interactions and briefly discuss the extension to the case of external drifts
Comparison of redox and ligand binding behaviour of yeast and bovine cytochrome c oxidases using FTIR spectroscopy
Redox and CO photolysis FTIR spectra of yeast cytochrome c oxidase WT and mutants are compared to those from bovine and P. denitrificans CcOs in order to establish common functional features. All display changes that can be assigned to their E242 (bovine numbering) equivalent and to weakly H-bonded water molecules. The additional redox-sensitive band reported at 1736 cm-1 in bovine CcO and previously assigned to D51 is absent from yeast CcO and couldn’t be restored by introduction of a D residue at the equivalent position of the yeast protein. Redox spectra of yeast CcO also show much smaller changes in the amide I region, which may relate to structural differences in the region around D51 and the subunit I/II interface
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