Evidence for ferromagnetic instability in a repulsive Fermi gas of ultracold atoms
INO-CNR and LENS - University of Florence (Italy)
The fine control of interactions and optical trapping potentials in ultracold atomic ensembles provides unique opportunities to explore strongly correlated fermion phenomena, such as superfluidity and magnetism. In our experiment, we produce lithium-6 degenerate Fermi gases in the vicinity of a broad Feshbach resonance  and we subsequently superimpose to the samples a thin repulsive optical barrier. This technique enabled the study of the Josephson dynamics of fermionic superfluids flowing through an insulating barrier, spanning a wide range of interaction strengths across the BEC-BCS crossover . More recently, by preparing two adjacent and fully spin-polarised domains at rest, we were able to study the magnetic properties of a repulsively interacting Fermi gas on the many-body upper energy branch. I'll report on the investigation of collective spin modes in this system and of the stability of the initial fully-polarized state. Diffusion measurements of the two approaching spin clouds reveal the temporary suppression of spin conductance above a critical interaction strength, closely connected to the concomitant observation of a spin-dipole mode softening, which points to the occurrence of a ferromagnetic instability.
 Burchianti, A. et al., Efficient all-optical production of large 6Li quantum gases using D1 gray-molasses cooling. Phys. Rev. A 90, 043408 (2014)
 Valtolina, G. et al., Josephson effect in fermionic superfluids across the BEC-BCS crossover. Science 350, 1505 (2015)
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