Quantum geometric superfluid weight in multiband superconductors: A microscopic interpretation

Even in the noninteracting limit, electrons on different Bloch bands of a multiband system do not move as if they are oblivious to the presence of one another. Instead, they move in concert by virtue of a non-Abelian interband Berry connection. While the impact of this quantum geometric attribute manifests most famously through the Hall response of topological bands, the geometric effects in superconductors have attracted significant recent attention. In particular, much has been discussed about the quantum-metric-induced superfluid weight (SW) in flatband superconductors. In this study, we revisit the geometric SW in generic multiband superconductors and trace its origin to a series of microscopic processes. We separately derive the SW of models containing only intraband Cooper pairing and those involving interband pairing. Two classes of processes enabled by the so-called interband velocity (or the closely related interband Berry connection) are identified: One resembles the transfer of Cooper pairs between different bands, and the other corresponds to virtual single-electron back-and-forth tunneling between the bands. The former contribution manifests as effective Josephson coupling between the multiband superconducting order parameters, while the latter constitutes the only source of SW for a superconducting flatband well isolated from other bands. We further numerically evaluate the SW of a simple two-band superconductor with trivial band topology, showcasing how the geometric contribution is sensitive to the details of the multiband pairing configuration. In particular, we highlight an intriguing scenario of negative SW, which may pave the way for the formation of novel pair density wave order. Our study provides deeper and more intuitive insight into the origin and nature of the SW induced by the quantum geometry of paired Bloch states.