335 research outputs found
Dynamic spin response of a strongly interacting Fermi gas
We present an experimental investigation of the dynamic spin response of a
strongly interacting Fermi gas using Bragg spectroscopy. By varying the
detuning of the Bragg lasers, we show that it is possible to measure the
response in the spin and density channels separately. At low Bragg energies,
the spin response is suppressed due to pairing, whereas the density response is
enhanced. These experiments provide the first independent measurements of the
spin-parallel and spin-antiparallel dynamic and static structure factors and
open the way to a complete study of the structure factors at any momentum. At
high momentum the spin-antiparallel dynamic structure factor displays a
universal high frequency tail, proportional to , where is the probe energy.Comment: Replaced with final versio
Thermodynamics of an attractive 2D Fermi gas
Thermodynamic properties of matter are conveniently expressed as functional
relations between variables known as equations of state. Here we experimentally
determine the compressibility, density and pressure equations of state for an
attractive 2D Fermi gas in the normal phase as a function of temperature and
interaction strength. In 2D, interacting gases exhibit qualitatively different
features to those found in 3D. This is evident in the normalized density
equation of state, which peaks at intermediate densities corresponding to the
crossover from classical to quantum behaviour.Comment: Contains minor revision
Contact and sum-rules in a near-uniform Fermi gas at unitarity
We present an experimental study of the high-energy excitation spectra of
unitary Fermi gases. Using focussed beam Bragg spectroscopy, we locally probe
atoms in the central region of a harmonically trapped cloud where the density
is nearly uniform, enabling measurements of the dynamic structure factor for a
range of temperatures both below and above the superfluid transition. Applying
sum-rules to the measured Bragg spectra, we resolve the characteristic
behaviour of the universal contact parameter, , across the superfluid
transition. We also employ a recent theoretical result for the kinetic
(second-moment) sum-rule to obtain the internal energy of gases at unitarity.Comment: 5 pages, 4 figure
Crossover from 2D to 3D in a weakly interacting Fermi gas
We have studied the transition from two to three dimensions in a low
temperature weakly interacting Li Fermi gas. Below a critical atom number,
, only the lowest transverse vibrational state of a highly anisotropic
oblate trapping potential is occupied and the gas is two-dimensional. Above
the Fermi gas enters the quasi-2D regime where shell structure
associated with the filling of individual transverse oscillator states is
apparent. This dimensional crossover is demonstrated through measurements of
the cloud size and aspect ratio versus atom number.Comment: Replaced with published manuscrip
Quantum anomaly and 2D-3D crossover in strongly interacting Fermi gases
We present an experimental investigation of collective oscillations in
harmonically trapped Fermi gases through the crossover from two to three
dimensions. Specifically, we measure the frequency of the radial monopole or
breathing mode as a function of dimensionality in Fermi gases with tunable
interactions. The frequency of this mode is set by the adiabatic
compressibility and probes the thermodynamic equation of state. In 2D, a
dynamical scaling symmetry for atoms interacting via a {\delta}-potential
predicts the breathing mode to occur at exactly twice the harmonic confinement
frequency. However, a renormalized quantum treatment introduces a new length
scale which breaks this classical scale invariance resulting in a so-called
quantum anomaly. Our measurements deep in the 2D regime lie above the
scale-invariant prediction for a range of interaction strengths indicating the
breakdown of a {\delta}-potential model for atomic interactions. As the
dimensionality is tuned from 2D to 3D we see the breathing oscillation
frequency evolve smoothly towards the 3D limit.Comment: 5 pages, 3 figure
A laser based accelerator for ultracold atoms
We present first results on our implementation of a laser based accelerator
for ultracold atoms. Atoms cooled to a temperature of 420 nK are confined and
accelerated by means of laser tweezer beams and the atomic scattering is
directly observed in laser absorption imaging. The optical collider has been
characterized using Rb87 atoms in the |F=2,mF=2> state, but the scheme is not
restricted to atoms in any particular magnetic substates and can readily be
extended to other atomic species as well.Comment: (c) 2012 The Optical Society, 3 pages, 4 figures, 1 movie lin
Higgs Oscillations in a Unitary Fermi Superfluid
Symmetry-breaking phase transitions are central to our understanding of states of matter. When a continuous symmetry is spontaneously broken, new excitations appear that are tied to fluctuations of the order parameter. In superconductors and fermionic superfluids, the phase and amplitude can fluctuate independently, giving rise to two distinct collective branches. However, amplitude fluctuations are difficult to both generate and measure, as they do not couple directly to the density of fermions and have only been observed indirectly to date. Here, we excite amplitude oscillations in an atomic Fermi gas with resonant interactions by an interaction quench. Exploiting the sensitivity of Bragg spectroscopy to the amplitude of the order parameter, we measure the time-resolved response of the atom cloud, directly revealing amplitude oscillations at twice the frequency of the gap. The magnitude of the oscillatory response shows a strong temperature dependence, and the oscillations appear to decay faster than predicted by time-dependent Bardeen-Cooper-Schrieffer theory applied to our experimental setup.</p
Higgs oscillations in a unitary Fermi superfluid
Symmetry-breaking phase transitions are central to our understanding of
states of matter. When a continuous symmetry is spontaneously broken, new
excitations appear that are tied to fluctuations of the order parameter. In
superconductors and fermionic superfluids, the phase and amplitude can
fluctuate independently, giving rise to two distinct collective branches.
However, amplitude fluctuations are difficult to both generate and measure, as
they do not couple directly to the density of fermions and have only been
observed indirectly to date. Here, we excite amplitude oscillations in an
atomic Fermi gas with resonant interactions by an interaction quench.
Exploiting the sensitivity of Bragg spectroscopy to the amplitude of the order
parameter, we measure the time-resolved response of the atom cloud, directly
revealing amplitude oscillations at twice the frequency of the gap. The
magnitude of the oscillatory response shows a strong temperature dependence,
and the oscillations appear to decay faster than predicted by time-dependent
BCS theory applied to our experimental setup
Criteria for 2D kinematics in an interacting Fermi gas
Ultracold Fermi gases subject to tight transverse confinement offer a highly
controllable setting to study the two-dimensional (2D) BCS to
Berezinskii-Kosterlitz-Thouless superfluid crossover. Achieving the 2D regime
requires confining particles to their transverse ground state which presents
challenges in interacting systems. Here, we establish the conditions for an
interacting Fermi gas to behave kinematically 2D. Transverse excitations are
detected by measuring the transverse expansion rate which displays a sudden
increase when the atom number exceeds a critical value signifying a
density driven departure from 2D kinematics. For weak interactions is
set by the aspect ratio of the trap. Close to a Feshbach resonance, however,
the stronger interactions reduce and excitations appear at lower
density.Comment: Replaced with published version, includes supplementary informatio
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