Possible insulator-pseudogap crossover in the attractive Hubbard model on the Lieb lattice

We study the attractive Hubbard model on the Lieb lattice to understand the normal state above the superconducting critical temperature in a flat band system. We use cluster dynamical mean-field theory to compute the two-particle susceptibilities with full quantum fluctuations included in the cluster. At interaction strengths lower than the hopping amplitude, we find that the normal state on the flat band shows insulating behavior stemming from the localization properties of the flat band. A flat-band enhanced pseudogap with a depleted spectral function arises at larger interactions.