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To study the effect of repulsive interactions on the equation of state of a Fermi we have measured the isothermal compressibility of the gas as a function of interaction strength [1].  The strength of repulsive interactions for 6Li atoms can be arbitrarily tuned by varying the magnetic field around a broad Feshbach resonance centered at 832G. The compressibility is obtained from equilibrium in-trap atomic density profiles, and agrees well with first-order perturbation theory at weak interactions. This is the first observation of the mean-field term in a thermodynamic quantity for repulsively interacting fermions. For stronger interaction, the atoms rapidly decay into pairs making it impossible to characterize the equilibrium density distribution of the atomic gas [1].

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