  Understanding the formation of Cooper pairs and the resulting thermodynamic
properties of a low-dimensional Fermi gas is an important area of research,
elucidating our understanding of high temperature superconductors. In lower
dimensions quantum fluctuations are expected to play an increasingly important
role and the reliability of strong-coupling theories becomes questionable.
Here, we present a comparison of recent thermodynamic measurements and
theoretical predictions from different many-body $T$-matrix theories for a
two-dimensional strongly interacting Fermi gas in the normal state. We find
that the fully self-consistent $T$-matrix theory provides the best description
of the experimental data over a wide range of temperatures and interatomic
interactions. Our comparison reveals the crucial role played by the
interactions between Cooper pairs and suggests that the future development of a
quantitative strong-coupling theory for two-dimensional Fermi superfluids must
explicitly take into account the diagrams that are responsible for pair-pair
interactions.
