  We theoretically investigate the isothermal compressibility $\kappa_{T}$ in
the normal state of an ultracold Fermi gas. Including pairing fluctuations, as
well as preformed-pair formations, within the framework of the self-consistent
$T$-matrix approximation (SCTMA), we evaluate the temperature dependence of
this thermodynamic quantity over the entire BCS (Bardeen-Cooper-Schrieffer)-BEC
(Bose-Einstein condensation) crossover region. While $\kappa_T$ in the
weak-coupling BCS regime is dominated by Fermi atoms near the Fermi surface,
correlations between tightly bound Cooper-pair molecules are found to play
crucial roles in the strong-coupling BEC regime. In the latter region, besides
a two-body molecular interaction, a three-body one is shown to sizably affect
$\kappa_T$ near the superfluid phase transition temperature. Our results
indicate that the strong-coupling BEC regime of an ultracold Fermi gas would
provide a unique opportunity to study multi-body correlations between
Cooper-pair molecules.

Quantum Gases