Superconductivity enhanced by pair fluctuations between wide and narrow bands

TL;DR

This paper demonstrates that doublon-holon fluctuations in systems with half-filled narrow bands can significantly enhance superconductivity, especially in models with tunable wide and narrow bands, using advanced dynamical mean-field theory techniques.

Contribution

It introduces a novel mechanism where pair fluctuations between wide and narrow bands boost superconductivity, supported by a new anomalous worm sampling method.

Findings

Shrinking the narrow band increases the superconducting order parameter.

Coupling to the wide band sustains superconductivity in the flat band limit.

Interplay of interaction terms promotes pair fluctuations and superconductivity.

Abstract

Full or empty narrow bands near the Fermi level are known to enhance superconductivity by promoting scattering processes and spin fluctuations. Here, we demonstrate that doublon-holon fluctuations in systems with half-filled narrow bands can similarly boost the superconducting $T_c$. We study the half-filled attractive bilayer Hubbard model on the square lattice using dynamical mean-field theory. The band structure of the noninteracting system contains a wide band formed by bonding orbitals and a narrow band formed by antibonding orbitals, with bandwidths tunable by the inter-layer hopping. The shrinking of the narrow band can lead to a substantial increase in the superconducting order parameter and phase stiffness in the wide band. At the same time, the coupling to the wide band allows the narrow band to remain superconducting -- and to reach the largest order parameter -- in the flat band limit. We develop an anomalous worm sampling method to study superconductivity in the limit of vanishing effective hopping. By analyzing the histogram of the local eigenstates, we clarify how the interplay between different interaction terms in the bonding/antibonding basis promotes pair fluctuations and superconductivity.