8,825 research outputs found
Impact-induced acceleration by obstacles
We explore a surprising phenomenon in which an obstruction accelerates,
rather than decelerates, a moving flexible object. It has been claimed that the
right kind of discrete chain falling onto a table falls \emph{faster} than a
free-falling body. We confirm and quantify this effect, reveal its complicated
dependence on angle of incidence, and identify multiple operative mechanisms.
Prior theories for direct impact onto flat surfaces, which involve a single
constitutive parameter, match our data well if we account for a characteristic
delay length that must impinge before the onset of excess acceleration. Our
measurements provide a robust determination of this parameter. This supports
the possibility of modeling such discrete structures as continuous bodies with
a complicated constitutive law of impact that includes angle of incidence as an
input.Comment: small changes and corrections, added reference
Onset of Interlayer Phase Coherence in a Bilayer Two-Dimensional Electron System: Effect of Layer Density Imbalance
Tunneling and Coulomb drag are sensitive probes of spontaneous interlayer
phase coherence in bilayer two-dimensional electron systems at total Landau
level filling factor . We find that the phase boundary between the
interlayer phase coherent state and the weakly-coupled compressible phase moves
to larger layer separations as the electron density distribution in the bilayer
is imbalanced. The critical layer separation increases quadratically with layer
density difference.Comment: 4 pages, 3 figure
Charge Imbalance and Bilayer 2D Electron Systems at
We use interlayer tunneling to study bilayer 2D electron systems at over a wide range of charge density imbalance, ,
between the two layers. We find that the strongly enhanced tunneling associated
with the coherent excitonic phase at small layer separation can
survive at least up to an imbalance of = 0.5, i.e
= (3/4, 1/4). Phase transitions between the excitonic state and
bilayer states which lack significant interlayer correlations can be induced in
three different ways: by increasing the effective interlayer spacing ,
the temperature , or the charge imbalance, . We observe that
close to the phase boundary the coherent phase can be absent at
= 0, present at intermediate , but then absent again
at large , thus indicating an intricate phase competition between
it and incoherent quasi-independent layer states. At zero imbalance, the
critical shifts linearly with temperature, while at = 1/3
the critical is only weakly dependent on . At = 1/3 we
report the first observation of a direct phase transition between the coherent
excitonic bilayer integer quantum Hall phase and the pair of single
layer fractional quantized Hall states at = 2/3 and .Comment: 13 pages, 8 postscript figures. Final published versio
Measuring the condensate fraction of rapidly rotating trapped boson systems: off-diagonal order from the density
We demonstrate a direct connection between the density profile of a system of
ultra-cold trapped bosonic particles in the rapid-rotation limit and its
condensate fraction. This connection can be used to probe the crossover from
condensed vortex-lattice states to uncondensed quantum fluid states that occurs
in rapidly rotating boson systems as the particle density decreases or the
rotation frequency increases. We illustrate our proposal with a series of
examples, including ones based on models of realistic finite trap systems, and
comment on its application to freely expanding boson density profile
measurements.Comment: 4 pages, 3 figures, version accepted for publication in Phys. Rev.
Let
Optically trapped atom interferometry using the clock transition of large Rb-87 Bose-Einstein condensates
We present a Ramsey-type atom interferometer operating with an optically
trapped sample of 10^6 Bose-condensed Rb-87 atoms. The optical trap allows us
to couple the |F =1, mF =0>\rightarrow |F =2, mF =0> clock states using a
single photon 6.8GHz microwave transition, while state selective readout is
achieved with absorption imaging. Interference fringes with contrast
approaching 100% are observed for short evolution times. We analyse the process
of absorption imaging and show that it is possible to observe atom number
variance directly, with a signal-to-noise ratio ten times better than the
atomic projection noise limit on 10^6 condensate atoms. We discuss the
technical and fundamental noise sources that limit our current system, and
outline the improvements that can be made. Our results indicate that, with
further experimental refinements, it will be possible to produce and measure
the output of a sub-shot-noise limited, large atom number BEC-based
interferometer.
In an addendum to the original paper, we attribute our inability to observe
quantum projection noise to the stability of our microwave oscillator and
background magnetic field. Numerical simulations of the Gross-Pitaevskii
equations for our system show that dephasing due to spatial dynamics driven by
interparticle interactions account for much of the observed decay in fringe
visibility at long interrogation times. The simulations show good agreement
with the experimental data when additional technical decoherence is accounted
for, and suggest that the clock states are indeed immiscible. With smaller
samples of 5 \times 10^4 atoms, we observe a coherence time of {\tau} =
(1.0+0.5-0.3) s.Comment: 22 pages, 6 figures Addendum: 11 pages, 6 figure
Spin Transition in Strongly Correlated Bilayer Two Dimensional Electron Systems
Using a combination of heat pulse and nuclear magnetic resonance techniques
we demonstrate that the phase boundary separating the interlayer phase coherent
quantum Hall effect at in bilayer electron gases from the weakly
coupled compressible phase depends upon the spin polarization of the nuclei in
the host semiconductor crystal. Our results strongly suggest that, contrary to
the usual assumption, the transition is attended by a change in the electronic
spin polarization.Comment: 4 pages, 3 postscript figur
Pattern of Reaction Diffusion Front in Laminar Flows
Autocatalytic reaction between reacted and unreacted species may propagate as
solitary waves, namely at a constant front velocity and with a stationary
concentration profile, resulting from a balance between molecular diffusion and
chemical reaction. The effect of advective flow on the autocatalytic reaction
between iodate and arsenous acid in cylindrical tubes and Hele-Shaw cells is
analyzed experimentally and numerically using lattice BGK simulations. We do
observe the existence of solitary waves with concentration profiles exhibiting
a cusp and we delineate the eikonal and mixing regimes recently predicted.Comment: 4 pages, 3 figures. This paper report on experiments and simulations
in different geometries which test the theory of Boyd Edwards on flow
advection of chemical reaction front which just appears in PRL (PRL Vol
89,104501, sept2002
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