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Evidence for ferromagnetic instability in a repulsive Fermi gas of ultracold atoms

Alessio Recati, University of Trento

20.05.2016 at 09:00 

Whether a Fermi gas with short-range repulsion can exhibits a ferromagnetic instability- calledtinerant  ferromagnetism - is still an open question. In recent years a few attempts have been done to get an answer to the problem. The outcome is that starting from the paramagnetic homogeneous state it is impossible to explore the magnetic properties of the repulsive gas. The reason being the instability of the repulsive branch of a Fermi gas against pairing and molecule formation.
In this talk the results of a very recent experiment where we report the first signature of an increasing  susceptibility as well as evidences for ferromagnetic behavior in a sample of two- component mixtures of Li-6 gas are presented.
In order to avoid detrimental effects on the stability of the gas, we initially prepare the gas in a spin domain wall configuration and measure the subsequent real time evolution. We observe an increase of the gas susceptibility through a softening of the frequency of the spin dipole mode by increasing the scattering length. Furthermore, beyond a critical value of the scattering length we detect the emergence of a time window of vanishing spin diffusion, where the two magnetic domains remain immiscible. Notwithstanding the limited lifetime of the repulsive gas, we extract the critical values of repulsion and temperature for the new regime to exist. Our results are consistent with a ferromagnetic instability of the gas as predicted within Fermi Liquid theory.

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