Insulator-pseudogap crossover in the Lieb lattice

TL;DR

This paper investigates the normal state of the attractive Hubbard model on the Lieb lattice, revealing an insulator at weak interactions and a pseudogap at stronger interactions, using advanced quantum many-body techniques.

Contribution

It provides the first detailed analysis of the insulator-pseudogap crossover in a flat band system with full quantum fluctuations via cluster dynamical mean-field theory.

Findings

Normal state is insulating at low interactions due to flat band localization.

A pseudogap develops at higher interactions, associated with flat band effects.

Quantum fluctuations are crucial for accurate susceptibility calculations.

Abstract

We study the attractive Hubbard model in 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 is an insulator. The insulating behavior stems from the localization properties of the flat band. A flat-band enhanced pseudogap with a depleted spectral function arises at larger interactions.