Local Plaquette Physics as Key Ingredient of High-Temperature Superconductivity in Cuprates

TL;DR

This paper demonstrates that the critical behavior of a four-site plaquette in the Hubbard model induces d-wave superconductivity, highlighting the importance of local plaquette physics in understanding high-temperature superconductivity in cuprates.

Contribution

It introduces a superperturbation theory based on the dual fermion approach with a critical plaquette as the reference, capturing essential d-wave fluctuations and superconductivity mechanisms.

Findings

Critical plaquette induces d-wave pairing instability.

Next-nearest-neighbour hopping $t'$ enhances bipolaron formation.

Superconductivity persists over a broad parameter range.

Abstract

A major pathway towards understanding complex systems is given by exactly solvable reference systems that contain the essential physics of the system. For the $t-t'-U$ Hubbard model, the four-site plaquette is known to have a quantum critical point in the $U-\mu$ space where states with electron occupations $N=2, 3, 4$ per plaquette are degenerate [Phys. Rev. B {\bf 94}, 125133 (2016)]. We show that such a critical point in the lattice causes an instability in the particle-particle singlet d-wave channel and manifests some of the essential elements of the cuprate superconductivity. For this purpose we design an efficient superperturbation theory -- based on the dual fermion approach -- with the critical plaquette as the reference system. Thus, the perturbation theory already contains the relevant d-wave fluctuations from the beginning via the two-particle correlations of the plaquette. We find that d-wave superconductivity remains a leading instability channel under reasonably broad range of parameters. The next-nearest-neighbour hopping $t'$ is shown to play a crucial role in a formation of strongly bound electronic bipolarons whose coherence at lower temperature results in superconductivity. The physics of the pseudogap within the developed picture is also discussed.