Limitation of the Lee-Huang-Yang interaction in forming a self-bound state in Bose-Einstein condensates

The perturbative Lee-Huang-Yang (LHY) interaction proportional to $n^{3\mathrm{/}2}$, where $n$ is the density, creates an infinitely repulsive potential at the center of a Bose-Einstein condensate (BEC) with net attraction, which stops the collapse to form a self-bound state in a dipolar BEC and in a binary BEC. However, recent microscopic calculations of the non-perturbative beyond-mean-field (BMF) interaction indicate that the LHY interaction %with the $n^{3\mathrm{/}2}$ term is valid only for very small values of gas parameter $x$. We show that a realistic non-perturbative BMF interaction can stop collapse and form a self-bound state only in a weakly attractive binary BEC with small $x$ values ($x⪅0.01$), whereas the perturbative LHY interaction stops collapse for all attractions. We demonstrate these aspects using an analytic BMF interaction with appropriate weak-coupling LHY and strong coupling limits.