Pair Structure and the Pairing Interaction in a Bilayer Hubbard Model

TL;DR

This paper investigates how the pairing symmetry in a bilayer Hubbard model changes with inter-layer coupling, revealing a transition from d-wave to s±-wave pairing driven by spin fluctuations, with implications for unconventional superconductivity.

Contribution

It demonstrates the dependence of pairing symmetry on inter-layer hopping and identifies the role of spin fluctuations in enhancing superconductivity in the bilayer Hubbard model.

Findings

Small inter-layer splitting favors d-wave pairing.

Large inter-layer splitting favors s±-wave pairing.

Inter-layer spin fluctuations drive s± pairing and increase T_c.

Abstract

The bilayer Hubbard model with an intra-layer hopping $t$ and an inter-layer hopping $t_\perp$ provides an interesting testing ground for several aspects of what has been called unconventional superconductivity. One can study the type of pair structures which arise when there are multiple Fermi surfaces. One can also examine the pairing for a system in which the structure of the spin-fluctuation spectral weight can be changed. Using a dynamic cluster quantum Monte Carlo approximation, we find that near half-filling, if the splitting between the bonding and anti-bonding bands $t_\perp/t$ is small, the gap has $B_{1g}$ ($d_{x^2-y^2}$-wave) symmetry but when the splitting becomes larger, $A_{1g}$ ($s^\pm$-wave) pairing is favored. We also find that in the $s^\pm$ pairing region, the pairing is driven by inter-layer spin fluctuations and that $T_c$ is enhanced.