Coexistence of s -wave superconductivity and phase separation in the half-filled extended Hubbard model with attractive interactions

Understanding competing instabilities in systems with correlated fermions remains one of the holy grails of modern condensed matter physics. Among the fermionic lattice models used to this effect, the extended Hubbard model occupies a prime place due to the potential relevance of its repulsive and attractive versions for both electronic materials and artificial systems. Using the recently introduced multichannel fluctuating field approach, we address the interplay of fluctuations in the charge density wave, $s$-wave superconducting, and phase separation channels in the attractive extended Hubbard model. Despite the fact that this model has been intensively studied for decades, our approach allows us to identify a phase that has not been analyzed before and which is characterized by the coexistence of collective $s$-wave superconducting and phase separation fluctuations. Our findings resonate with previous observations of interplaying phase separation and superconducting phases in electronic systems, most importantly in high-temperature superconductors.