Hidden fermionic excitation in the superconductivity of the strongly attractive Hubbard model

TL;DR

This study investigates the self-energy structure in the superconducting state of the attractive Hubbard model, revealing a hidden fermionic excitation that influences the pseudogap and superconductivity, with implications for strongly correlated systems.

Contribution

It uncovers a hidden fermionic excitation in the attractive Hubbard model's superconducting phase, linking it to pseudogap phenomena and comparing it with the repulsive case.

Findings

Self-energy exhibits a pole structure near the critical temperature.

Hidden fermion persists in the normal state, causing a pseudogap.

Hidden fermions are a common feature in strongly correlated superconductors.

Abstract

We scrutinize the real-frequency structure of the self-energy in the superconducting state of the attractive Hubbard model within the dynamical mean-field theory. Within the strong-coupling superconducting phase which has been understood in terms of the Bose-Einstein condensation in the literature, we find two qualitatively different regions crossing over each other. In one region close to zero temperature, the self-energy depends on the frequency only weakly at low energy. On the other hand, in the region close to the critical temperature, the self-energy shows a pole structure. The latter region becomes more dominant as the interaction becomes stronger. We reveal that the self-energy pole in the latter region is generated by a coupling to a hidden fermionic excitation. The hidden fermion persists in the normal state, where it yields a pseudogap. We compare these properties with those of the repulsive Hubbard model relevant for high-temperature cuprate superconductors, showing that hidden fermions are a key common ingredient in strongly correlated superconductivity.