# Feshbach resonance for Li: repulsive and attractive?

#### hungry_r2d2

Hello,

I am looking into the Feshbach resonance for Fermi gas (Lithium in particular). The following is unclear:

Is it correct to say that there is a repulsive interaction between the atoms in the regime, where the scattering length is positive?

I have found various sources saying so (e.g. arxiv.org/abs/0801.2500) and otherwise (e.g. arxiv.org/abs/0912.4205). The reason I am in doubt is that attractive square potential wells can produce both positive and negative scattering lengths. Also, atom-atom potentials always seem to be of the Lennard-Jones kind, i.e. they have an attractive tail.

Any input is welcome and thank you in advance!

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#### DrClaude

Mentor
In the context of Feshbach resonances, the inter-atomic potential used is always attractive.

The use of the "attractive" and "repulsive" applies here to the equivalent hard-sphere potential, which is where the scattering length enters. For example, an attractive potential which, for a certain collision energy, gives a scattering length $a>0$ will be said to be "repulsive."

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