Figure 1: Left panel: Energy of 3 spin-up and 3 spin-down fermions in a two-dimensional harmonic trap interacting attractively. Calculations were performed with the FCI and the IM-SRG method. Both methods agree well for weak interactions while for stronger interactions the methods deviate. For even stronger ones IM-SRG calculations diverge. Also shown is the position of the Higgsminimum which signals the beginning of the experimentally interesting parameter regime.
Right panel: Energy of 4 bosons in a two-dimensional harmonic trap interacting attractively plotted over the two-body energy for different single-body basis cutoffs (max_energy). For strong interactions, no convergence can be achieved. This area is the one where the universal aspects of the bound state appear. For larger particle numbers, the convergence worsens.