270 research outputs found
Impact of disorder on the 5/2 fractional quantum Hall state
We compare the energy gap of the \nu=5/2 fractional quantum Hall effect state
obtained in conventional high mobility modulation doped quantum well samples
with those obtained in high quality GaAs transistors (heterojunction insulated
gate field-effect transistors). We are able to identify the different roles
that long range and short range disorders play in the 5/2 state and observe
that the long range potential fluctuations are more detrimental to the strength
of the 5/2 state than short-range potential disorder.Comment: PRL 106, 206806 (2011
Dipolar Relaxation in an ultra-cold Gas of magnetically trapped chromium atoms
We have investigated both theoretically and experimentally dipolar relaxation
in a gas of magnetically trapped chromium atoms. We have found that the large
magnetic moment of 6 results in an event rate coefficient for dipolar
relaxation processes of up to cms at a magnetic
field of 44 G. We present a theoretical model based on pure dipolar coupling,
which predicts dipolar relaxation rates in agreement with our experimental
observations. This very general approach can be applied to a large variety of
dipolar gases.Comment: 9 pages, 9 figure
Atomic Bose Gas with Negative Scattering Length
We derive the equation of state of a dilute atomic Bose gas with an
interatomic interaction that has a negative scattering length and argue that
two continuous phase transitions, occuring in the gas due to quantum degeneracy
effects, are preempted by a first-order gas-liquid or gas-solid transition
depending on the details of the interaction potential. We also discuss the
consequences of this result for future experiments with magnetically trapped
spin-polarized atomic gasses such as lithium and cesium.Comment: 16 PAGES, REVTEX 3.0, ACCEPTED FOR PUBLICATION IN PHYS. REV.
Quantum Limits of Stochastic Cooling of a Bosonic Gas
The quantum limits of stochastic cooling of trapped atoms are studied. The
energy subtraction due to the applied feedback is shown to contain an
additional noise term due to atom-number fluctuations in the feedback region.
This novel effect is shown to dominate the cooling efficiency near the
condensation point. Furthermore, we show first results that indicate that
Bose--Einstein condensation could be reached via stochastic cooling.Comment: 5 pages, 3 figures, to appear in Phys. Rev.
Dipolar collisions of polar molecules in the quantum regime
Ultracold polar molecules offer the possibility of exploring quantum gases
with interparticle interactions that are strong, long-range, and spatially
anisotropic. This is in stark contrast to the dilute gases of ultracold atoms,
which have isotropic and extremely short-range, or "contact", interactions. The
large electric dipole moment of polar molecules can be tuned with an external
electric field; this provides unique opportunities such as control of ultracold
chemical reactions, quantum information processing, and the realization of
novel quantum many-body systems. In spite of intense experimental efforts aimed
at observing the influence of dipoles on ultracold molecules, only recently
have sufficiently high densities been achieved. Here, we report the observation
of dipolar collisions in an ultracold molecular gas prepared close to quantum
degeneracy. For modest values of an applied electric field, we observe a
dramatic increase in the loss rate of fermionic KRb molecules due to ultrcold
chemical reactions. We find that the loss rate has a steep power-law dependence
on the induced electric dipole moment, and we show that this dependence can be
understood with a relatively simple model based on quantum threshold laws for
scattering of fermionic polar molecules. We directly observe the spatial
anisotropy of the dipolar interaction as manifested in measurements of the
thermodynamics of the dipolar gas. These results demonstrate how the long-range
dipolar interaction can be used for electric-field control of chemical reaction
rates in an ultracold polar molecule gas. The large loss rates in an applied
electric field suggest that creating a long-lived ensemble of ultracold polar
molecules may require confinement in a two-dimensional trap geometry to
suppress the influence of the attractive dipolar interactions
Exciting, Cooling And Vortex Trapping In A Bose-Condensed Gas
A straight forward numerical technique, based on the Gross-Pitaevskii
equation, is used to generate a self-consistent description of
thermally-excited states of a dilute boson gas. The process of evaporative
cooling is then modelled by following the time evolution of the system using
the same equation. It is shown that the subsequent rethermalisation of the
thermally-excited state produces a cooler coherent condensate. Other results
presented show that trapping vortex states with the ground state may be
possible in a two-dimensional experimental environment.Comment: 9 pages, 7 figures. It's worth the wait! To be published in Physical
Review A, 1st February 199
High frequency sound in superfluid 3He-B
We present measurements of the absolute phase velocity of transverse and
longitudinal sound in superfluid 3He-B at low temperature, extending from the
imaginary squashing mode to near pair-breaking. Changes in the transverse phase
velocity near pair-breaking have been explained in terms of an order parameter
collective mode that arises from f-wave pairing interactions, the so-called
J=4- mode. Using these measurements, we establish lower bounds on the energy
gap in the B-phase. Measurement of attenuation of longitudinal sound at low
temperature and energies far above the pair-breaking threshold, are in
agreement with the lower bounds set on pair-breaking. Finally, we discuss our
estimations for the strength of the f-wave pairing interactions and the Fermi
liquid parameter, F4s.Comment: 15 pages, 8 figures, accepted to J. Low Temp. Phy
Hidden spin-current conservation in 2d Fermi liquids
We report the existence of regimes of the two dimensional Fermi liquid that
show unusual conservation of the spin current and may be tuned by varying some
parameter like the density of fermions. We show that for reasonable models of
the effective interaction the spin current may be conserved in general in 2d,
not only for a particular regime. Low temperature spin waves propagate
distinctively in these regimes and entirely new ``spin-acoustic'' modes are
predicted for scattering-dominated temperature ranges. These new
high-temperature propagating spin waves provide a clear signature for the
experimental search of such regimes.Comment: 4 pages, no figures, revised version, accepted for pub. in the PR
Policy Brief: UNSCR 1325: The Challenges of Framing Womenâs Rights as a Security Matter
While UN Security Council Resolution (UNSCR) 1325 has certainly increased awareness among international actors about womenâs and gender issues in armed conflict, opened new spaces for dialogue and partnerships from global to local levels, and even created opportunities for new resources for womenâs rights, successes remain limited and notably inconsistent. To understand some of these shortcomings and think creatively about how to move the women, peace and security agenda forward, it is essential to understand the conceptual assumptions underscoring UNSCR 1325
Cold Collision Frequency Shift of the 1S-2S Transition in Hydrogen
We have observed the cold collision frequency shift of the 1S-2S transition
in trapped spin-polarized atomic hydrogen. We find , where is the sample density. From this
we derive the 1S-2S s-wave triplet scattering length, nm,
which is in fair agreement with a recent calculation. The shift provides a
valuable probe of the distribution of densities in a trapped sample.Comment: Accepted for publication in PRL, 9 pages, 4 PostScript figures,
ReVTeX. Updated connection of our measurement to theoretical wor
- âŠ