Upper panel. The dashed line is the result of renormalization group (RG) analysis, i.e. Eq. (11), of the mean-field results, whereas the solid line uses the Gaussian theory as the starting point. The blue dots represent experimental data from ref. 21. The decrease of the critical temperature in the BCS and BEC limits is due to single-particle excitations and collective excitations contributing to superfluid density, respectively. This interplay results in a higher BKT critical temperature in the intermediate regime, i.e. when . It is important to note that experimental data may be affected by systematic errors, as analyzed in the main text. Lower panel. Comparison between the Kosterlitz-Thouless renormalization group (RG) equations (11) and the next-to-leading order RG equations (12). Here, in both cases the bare superfluid density is calculated within the Gaussian theory.