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Ultracold atoms teach us about life on a neutron star

Fermi gases in the so-called unitary regime—where the diverging interactions between atoms make their thermodynamics universal—are an excellent test bed for an array of strongly interacting matter systems. Two places to find a unitary Fermi gas are in the crust of a neutron start, and in basement of McLennan.

Physics graduate students Alma Bardon, Nathan Cheng, Graham Edge, Daniel Fine, and Chris Luciuk,  along with postdoctoral fellows Scott Beattie and Stefan Trotzky and summer student Will Cairncross, create and probe unitary gases using lasers and magnetic fields. The transport characteristics in this regime are particularly intriguing, and a discrepancy between two- and three-dimensional transport coefficients has been observed.

Published in the 16 May issue of Science is a study of the demagnetization dynamics of a three-dimensional Fermi gas. The gas was initially polarized along a single direction and was noninteracting. An applied magnetic field gradient then caused a spin spiral to form; as the gas relaxed from this state, the team extracted the diffusion coefficient and observed the buildup of interactions between the atoms.

"It's amazing to see a dynamical transformation from an ideal gas to a strongly correlated system in one millisecond", says Prof Joseph Thywissen, who led the effort. "Remarkable in our case was that diffusion appears to have a lower limit. One usually thinks of a speed limit as an upper bound (for instance when driving on the freeway), but in our case the speed of spin transport has a lower bound. Einstein's law of diffusion predicts that diffusivity should decrease with temperature, but as we cooled the gas towards absolute zero temperature, the diffusivity saturated above an interesting bound. This observation provides both an answer and a puzzle, since the physics behind the bound is not understood."

The work was a collaboration between the Thywissen group and theorists Edward Taylor (McMaster University) and Shizhong Zhang (Hong Kong University).

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