529 research outputs found
Dynamics of a tunable superfluid junction
We study the population dynamics of a Bose-Einstein condensate in a
double-well potential throughout the crossover from Josephson dynamics to
hydrodynamics. At barriers higher than the chemical potential, we observe slow
oscillations well described by a Josephson model. In the limit of low barriers,
the fundamental frequency agrees with a simple hydrodynamic model, but we also
observe a second, higher frequency. A full numerical simulation of the
Gross-Pitaevskii equation giving the frequencies and amplitudes of the observed
modes between these two limits is compared to the data and is used to
understand the origin of the higher mode. Implications for trapped matter-wave
interferometers are discussed.Comment: 8 pages, 7 figures; v3: Journal reference added, minor changes to
tex
Material properties of Islamic paper
In contrast to scientific research focussing on European paper, there is a significant gap in our knowledge of Islamic papermaking. This research surveys the evidence of techniques and materials typically used in Islamic papermaking, to deduce what might be considered as the most significant characteristics. A substantial collection of 228 Islamic papers (~18th–20th century) was characterized using chemical analytical methods: surface profilometry, gloss measurements, specular vs. diffuse reflectance ratio at 457 nm, scanning electron microscopy and infrared spectroscopy for identification of polishing, iodine test for identification of starch, Raspail test for identification of rosin, and fibre furnish analysis. Morphological analysis was performed to examine the presence of watermarks and sieve patterns. In addition, acidity and degree of polymerization of cellulose in paper were determined to explore the average material state of paper in the collection. Near infrared spectroscopic data of the collection were correlated to chemical properties with the aid of multivariate data analysis methods. Four different models were developed focusing on two main characteristics of Islamic paper: two for identification of polishing and starch, and two quantitative models to determine the acidity and degree of polymerization of cellulose in paper. While no single defining characteristic of Islamic paper was identified, 88 % of all papers in the studied collection either contain starch or are polished, or both. ~2 % of papers contain rosin. The majority of papers are neutral to mildly acidic, which is in contrast to their extensive degradation: ~69 % have DP < 1000. Polishing and starch appear to be associated with current values of pH and DP. The developed non-destructive characterization methodology could be applied to Islamic collections in libraries and archives to expand the database with the material properties of papers of known age and provenance and thus better understand geographic and temporal distributions of papermaking practices in Islamic countries
Transverse Demagnetization Dynamics of a Unitary Fermi Gas
Understanding the quantum dynamics of strongly interacting fermions is a
problem relevant to diverse forms of matter, including high-temperature
superconductors, neutron stars, and quark-gluon plasma. An appealing benchmark
is offered by cold atomic gases in the unitary limit of strong interactions.
Here we study the dynamics of a transversely magnetized unitary Fermi gas in an
inhomogeneous magnetic field. We observe the demagnetization of the gas, caused
by diffusive spin transport. At low temperatures, the diffusion constant
saturates to the conjectured quantum-mechanical lower bound ,
where is the particle mass. The development of pair correlations,
indicating the transformation of the initially non-interacting gas towards a
unitary spin mixture, is observed by measuring Tan's contact parameter.Comment: 8 pages, 6 figures. Accepted versio
Observation of the Leggett-Rice effect in a unitary Fermi gas
We observe that the diffusive spin current in a strongly interacting
degenerate Fermi gas of K precesses about the local magnetization. As
predicted by Leggett and Rice, precession is observed both in the Ramsey phase
of a spin-echo sequence, and in the nonlinearity of the magnetization decay. At
unitarity, we measure a Leggett-Rice parameter and a bare
transverse spin diffusivity for a normal-state
gas initialized with full polarization and at one fifth of the Fermi
temperature, where is the atomic mass. One might expect at
unitarity, where two-body scattering is purely dissipative. We observe as temperature is increased towards the Fermi temperature,
consistent with calculations that show the degenerate Fermi sea restores a
non-zero . Tuning the scattering length , we find that a sign change
in occurs in the range , where is
the Fermi momentum. We discuss how reveals the effective interaction
strength of the gas, such that the sign change in indicates a
switching of branch, between a repulsive and an attractive Fermi gas.Comment: 9 pages, 5 figures; Changed to the more conventional kF=(3 pi^2
n)^1/3, instead of the polarized definition we used in v
Defect-induced perturbations of atomic monolayers on solid surfaces
We study long-range morphological changes in atomic monolayers on solid
substrates induced by different types of defects; e.g., by monoatomic steps in
the surface, or by the tip of an atomic force microscope (AFM), placed at some
distance above the substrate. Representing the monolayer in terms of a suitably
extended Frenkel-Kontorova-type model, we calculate the defect-induced density
profiles for several possible geometries. In case of an AFM tip, we also
determine the extra force exerted on the tip due to the tip-induced
de-homogenization of the monolayer.Comment: 4 pages, 2 figure
Kinetics of Anchoring of Polymer Chains on Substrates with Chemically Active Sites
We consider dynamics of an isolated polymer chain with a chemically active
end-bead on a 2D solid substrate containing immobile, randomly placed
chemically active sites (traps). For a particular situation when the end-bead
can be irreversibly trapped by any of these sites, which results in a complete
anchoring of the whole chain, we calculate the time evolution of the
probability that the initially non-anchored chain remains mobile
until time . We find that for relatively short chains follows at
intermediate times a standard-form 2D Smoluchowski-type decay law , which crosses over at very large times to the
fluctuation-induced dependence , associated with
fluctuations in the spatial distribution of traps. We show next that for long
chains the kinetic behavior is quite different; here the intermediate-time
decay is of the form , which is the
Smoluchowski-type law associated with subdiffusive motion of the end-bead,
while the long-time fluctuation-induced decay is described by the dependence
, stemming out of the interplay between
fluctuations in traps distribution and internal relaxations of the chain.Comment: Latex file, 19 pages, one ps figure, to appear in PR
Quantum transport in ultracold atoms
Ultracold atoms confined by engineered magnetic or optical potentials are
ideal systems for studying phenomena otherwise difficult to realize or probe in
the solid state because their atomic interaction strength, number of species,
density, and geometry can be independently controlled. This review focuses on
quantum transport phenomena in atomic gases that mirror and oftentimes either
better elucidate or show fundamental differences with those observed in
mesoscopic and nanoscopic systems. We discuss significant progress in
performing transport experiments in atomic gases, contrast similarities and
differences between transport in cold atoms and in condensed matter systems,
and survey inspiring theoretical predictions that are difficult to verify in
conventional setups. These results further demonstrate the versatility offered
by atomic systems in the study of nonequilibrium phenomena and their promise
for novel applications.Comment: 24 pages, 7 figures. A revie
Theory and applications of atomic and ionic polarizabilities
Atomic polarization phenomena impinge upon a number of areas and processes in
physics. The dielectric constant and refractive index of any gas are examples
of macroscopic properties that are largely determined by the dipole
polarizability. When it comes to microscopic phenomena, the existence of
alkaline-earth anions and the recently discovered ability of positrons to bind
to many atoms are predominantly due to the polarization interaction. An
imperfect knowledge of atomic polarizabilities is presently looming as the
largest source of uncertainty in the new generation of optical frequency
standards. Accurate polarizabilities for the group I and II atoms and ions of
the periodic table have recently become available by a variety of techniques.
These include refined many-body perturbation theory and coupled-cluster
calculations sometimes combined with precise experimental data for selected
transitions, microwave spectroscopy of Rydberg atoms and ions, refractive index
measurements in microwave cavities, ab initio calculations of atomic structures
using explicitly correlated wave functions, interferometry with atom beams, and
velocity changes of laser cooled atoms induced by an electric field. This
review examines existing theoretical methods of determining atomic and ionic
polarizabilities, and discusses their relevance to various applications with
particular emphasis on cold-atom physics and the metrology of atomic frequency
standards.Comment: Review paper, 44 page
A Search for Jet Handedness in Hadronic Decays
We have searched for signatures of polarization in hadronic jets from decays using the ``jet handedness'' method. The polar angle
asymmetry induced by the high SLC electron-beam polarization was used to
separate quark jets from antiquark jets, expected to be left- and
right-polarized, respectively. We find no evidence for jet handedness in our
global sample or in a sample of light quark jets and we set upper limits at the
95% C.L. of 0.063 and 0.099 respectively on the magnitude of the analyzing
power of the method proposed by Efremov {\it et al.}Comment: Revtex, 8 pages, 2 figure
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