3,674 research outputs found
Zener double exchange from local valence fluctuations in magnetite
Magnetite (FeO) is a mixed valent system where electronic
conductivity occurs on the B-site (octahedral) iron sublattice of the spinel
structure. Below K, a metal-insulator transition occurs which is
argued to arise from the charge ordering of 2+ and 3+ iron valences on the
B-sites (Verwey transition). Inelastic neutron scattering measurements show
that optical spin waves propagating on the B-site sublattice (80 meV) are
shifted upwards in energy above due to the occurrence of B-B
ferromagnetic double exchange in the mixed valent metallic phase. The double
exchange interaction affects only spin waves of symmetry, not all
modes, indicating that valence fluctuations are slow and the double exchange is
constrained by electron correlations above .Comment: 4 pages, 5 figure
Using Cold Atoms to Measure Neutrino Mass
We propose a beta decay experiment based on a sample of ultracold atomic
tritium. These initial conditions enable detection of the helium ion in
coincidence with the beta. We construct a two-dimensional fit incorporating
both the shape of the beta-spectrum and the direct reconstruction of the
neutrino mass peak. We present simulation results of the feasible limits on the
neutrino mass achievable in this new type of tritium beta-decay experiment.Comment: 10 pages, 5 figure
Optical interface created by laser-cooled atoms trapped in the evanescent field surrounding an optical nanofiber
Trapping and optically interfacing laser-cooled neutral atoms is an essential
requirement for their use in advanced quantum technologies. Here we
simultaneously realize both of these tasks with cesium atoms interacting with a
multi-color evanescent field surrounding an optical nanofiber. The atoms are
localized in a one-dimensional optical lattice about 200 nm above the nanofiber
surface and can be efficiently interrogated with a resonant light field sent
through the nanofiber. Our technique opens the route towards the direct
integration of laser-cooled atomic ensembles within fiber networks, an
important prerequisite for large scale quantum communication schemes. Moreover,
it is ideally suited to the realization of hybrid quantum systems that combine
atoms with, e.g., solid state quantum devices
Versatile compact atomic source for high resolution dual atom interferometry
We present a compact Rb atomic source for high precision dual atom
interferometers. The source is based on a double-stage magneto-optical trap
(MOT) design, consisting of a 2-dimensional (2D)-MOT for efficient loading of a
3D-MOT. The accumulated atoms are precisely launched in a horizontal moving
molasses. Our setup generates a high atomic flux ( atoms/s) with
precise and flexibly tunable atomic trajectories as required for high
resolution Sagnac atom interferometry. We characterize the performance of the
source with respect to the relevant parameters of the launched atoms, i.e.
temperature, absolute velocity and pointing, by utilizing time-of-flight
techniques and velocity selective Raman transitions.Comment: uses revtex4, 9 pages, 12 figures, submitted to Phys. Rev.
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Structural organization and interactions of transmembrane domains in tetraspanin proteins
BACKGROUND: Proteins of the tetraspanin family contain four transmembrane domains (TM1-4) linked by two extracellular loops and a short intracellular loop, and have short intracellular N- and C-termini. While structure and function analysis of the larger extracellular loop has been performed, the organization and role of transmembrane domains have not been systematically assessed. RESULTS: Among 28 human tetraspanin proteins, the TM1-3 sequences display a distinct heptad repeat motif (abcdefg)(n). In TM1, position a is occupied by structurally conserved bulky residues and position d contains highly conserved Asn and Gly residues. In TM2, position a is occupied by conserved small residues (Gly/Ala/Thr), and position d has a conserved Gly and two bulky aliphatic residues. In TM3, three a positions of the heptad repeat are filled by two leucines and a glutamate/glutamine residue, and two d positions are occupied by either Phe/Tyr or Val/Ile/Leu residues. No heptad motif is apparent in TM4 sequences. Mutations of conserved glycines in human CD9 (Gly25 and Gly32 in TM1; Gly67 and Gly74 in TM2) caused aggregation of mutant proteins inside the cell. Modeling of the TM1-TM2 interface in CD9, using a novel algorithm, predicts tight packing of conserved bulky residues against conserved Gly residues along the two helices. The homodimeric interface of CD9 was mapped, by disulfide cross-linking of single-cysteine mutants, to the vicinity of residues Leu14 and Phe17 in TM1 (positions g and c) and Gly77, Gly80 and Ala81 in TM2 (positions d, g and a, respectively). Mutations of a and d residues in both TM1 and TM2 (Gly25, Gly32, Gly67 and Gly74), involved in intramolecular TM1-TM2 interaction, also strongly diminished intermolecular interaction, as assessed by cross-linking of Cys80. CONCLUSION: Our results suggest that tetraspanin intra- and intermolecular interactions are mediated by conserved residues in adjacent, but distinct regions of TM1 and TM2. A key structural element that defines TM1-TM2 interaction in tetraspanins is the specific packing of bulky residues against small residues
Direct Observation of Sub-Poissonian Number Statistics in a Degenerate Bose Gas
We report the direct observation of sub-Poissonian number fluctuation for a
degenerate Bose gas confined in an optical trap. Reduction of number
fluctuations below the Poissonian limit is observed for average numbers that
range from 300 to 60 atoms.Comment: 5 pages, 4 figure
The Population of Dark Matter Subhaloes: Mass Functions and Average Mass Loss Rates
Using a cosmological N-Body simulation and a sample of re-simulated
cluster-like haloes, we study the mass loss rates of dark matter subhaloes, and
interpret the mass function of subhaloes at redshift zero in terms of the
evolution of the mass function of systems accreted by the main halo progenitor.
When expressed in terms of the ratio between the mass of the subhalo at the
time of accretion and the present day host mass the unevolved subhalo mass
function is found to be universal. However, the subhalo mass function at
redshift zero clearly depends on , in that more massive host haloes host
more subhaloes. To relate the unevolved and evolved subhalo mass functions, we
measure the subhalo mass loss rate as a function of host mass and redshift. We
find that the average, specific mass loss rate of dark matter subhaloes depends
mainly on redshift. These results suggest a pleasingly simple picture for the
evolution and mass dependence of the evolved subhalo mass function. Less
massive host haloes accrete their subhaloes earlier, which are thus subjected
to mass loss for a longer time. In addition, their subhaloes are typically
accreted by denser hosts, which causes an additional boost of the mass loss
rate. To test the self-consistency of this picture, we use a merger trees
constructed using the extended Press-Schechter formalism, and evolve the
subhalo populations using the average mass loss rates obtained from our
simulations, finding the subhalo mass functions to be in good agreement with
the simulations. [abridged]Comment: 12 pages, 12 figures; submitted to MNRA
Using Absorption Imaging to Study Ion Dynamics in an Ultracold Neutral Plasma
We report optical absorption imaging of ultracold neutral plasmas.Images are
used to measure the ion absorption spectrum, which is Doppler-broadened.
Through the spectral width, we monitor ion equilibration in the first 250ns
after plasma formation. The equilibration leaves ions on the border between the
weakly coupled gaseous and strongly coupled liquid states. On a longer
timescale of microseconds, we observe radial acceleration of ions resulting
from pressure exerted by the trapped electron gas.Comment: 4 pages, 4 figure
Static vs. dynamical mean field theory of Mott antiferromagnets
Studying the antiferromagnetic phase of the Hubbard model by dynamical mean
field theory, we observe striking differences with static (Hartree-Fock) mean
field: The Slater band is strongly renormalized and spectral weight is
transferred to spin-polaron side bands. Already for intermediate values of the
interaction the overall bandwidth is larger than in Hartree-Fock, and the
gap is considerably smaller. Such differences survive any renormalization of
. Our photoemission experiments for Cr-doped VO show spectra
qualitatively well described by dynamical mean field theory.Comment: 6 pages, 5 figures - one figure added and further details about
quasiparticle dispersio
The Low Redshift survey at Calar Alto (LoRCA)
The Baryon Acoustic Oscillation (BAO) feature in the power spectrum of
galaxies provides a standard ruler to measure the accelerated expansion of the
Universe. To extract all available information about dark energy, it is
necessary to measure a standard ruler in the local, z<0.2, universe where dark
energy dominates most the energy density of the Universe. Though the volume
available in the local universe is limited, it is just big enough to measure
accurately the long 100 Mpc/h wave-mode of the BAO. Using cosmological N-body
simulations and approximate methods based on Lagrangian perturbation theory, we
construct a suite of a thousand light-cones to evaluate the precision at which
one can measure the BAO standard ruler in the local universe. We find that
using the most massive galaxies on the full sky (34,000 sq. deg.), i.e. a
K(2MASS)<14 magnitude-limited sample, one can measure the BAO scale up to a
precision of 4\% and 1.2\% using reconstruction). We also find that such a
survey would help to detect the dynamics of dark energy.Therefore, we propose a
3-year long observational project, named the Low Redshift survey at Calar Alto
(LoRCA), to observe spectroscopically about 200,000 galaxies in the northern
sky to contribute to the construction of aforementioned galaxy sample. The
suite of light-cones is made available to the public.Comment: 15 pages. Accepted in MNRAS. Please visit our website:
http://lorca-survey.ft.uam.es
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