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APS March Meeting 2011

## Volume 56, Number 1

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Monday–Friday, March 21–25, 2011;
Dallas, Texas

### Abstract: X6.00005 : Universal Spin Transport in Strongly Interacting Fermi Gases

4:54 PM–5:30 PM

Preview Abstract

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Author:

Ultracold gases of fermionic atoms have emerged as a unique
platform to study strongly interacting fermion systems.
Here we study spin transport in a two-state mixture of fermionic
atoms near a Feshbach resonance.
Starting with two separate spin domains in an atom trap, we
observe the subsequent evolution of the spin mixture towards the
eventual ground state, a superfluid of fermion pairs. Initially,
the gas clouds of unlike spins almost perfectly bounce off each
other, despite densities a million times thinner than air. Only
over several seconds, about 100 000 collision times, the spins
slowly diffuse into each other and, below a critical temperature,
form a superfluid of fermion pairs. We determine the transport
properties in this gas as a function of interaction strength and
temperature.
In particular, we find the spin diffusion coefficient in the
strongly interacting, degenerate regime to take on the universal
value for a ``perfect fluid'', $D \simeq \hbar/m = \frac{(100 \mu
\rm m)^2}{{\rm s}}$, where $m$ is the mass of the $^6$Li atoms.
At high temperatures, we find the universal law $D = \alpha
\frac{\hbar}{m} \left(T/T_F\right)^{3/2}$ with a constant
$\alpha$. The ratio of spin conductivity and spin diffusion
coefficient yields the spin susceptibility in these gases,
showing the Curie law at high temperatures and a departure from
the compressibility at low temperatures, that we interpret as a
signature for entering the Fermi liquid regime.
Our transport experiments near and far equilibrium have
implications on other strongly interacting Fermi systems,
suggesting a fundamental lower limit to the spin diffusion
coefficient, in the absence of localization, on the order of
$\hbar/m$ - a conjecture already made by Onsager. Our spin
susceptibility measurements appear to exclude a ferromagnetic
ground state on the repulsive side of the Feshbach resonance.

To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2011.MAR.X6.5