Quantum-geometry-induced intrinsic optical anomaly in multiorbital superconductors

TL;DR

This paper reveals how quantum geometric properties of multiorbital superconductors cause an intrinsic optical anomaly, including suppressed conductivity at certain frequencies and potential low-frequency conductivity due to interband pairing.

Contribution

It uncovers the role of quantum geometry in the optical response of multiorbital superconductors, linking Berry connection to observable optical anomalies.

Findings

Optical conductivity is suppressed at frequencies matching band separation.

Interband Cooper pairing can induce low-frequency conductivity.

The anomaly may have been observed in previous iron-based superconductor experiments.

Abstract

Bloch electrons in multiorbital systems carry nontrivial quantum geometric information characteristic of their orbital composition as a function of their wave vector. When such electrons form Cooper pairs, the resultant superconducting state naturally inherits aspects of the quantum geometry. In this paper, we study how this geometric character is revealed in the intrinsic optical response of the superconducting state. In particular, due to the superconducting gap opening, interband optical transitions involving states around the Fermi level are forbidden. This generally leads to an anomalous suppression of the optical conductivity at frequencies matching the band separation -- which could be significantly higher than the superconducting gap energy. We discuss how the predicted anomaly may have already emerged in two earlier measurements on an iron-based superconductor. When interband Cooper pairing is present, intraband optical transitions may be allowed and finite conductivity emerges at low frequencies right above the gap edge. These conductivities depend crucially on the off-diagonal elements of the velocity matrix in the band representation, i.e. the interband velocity -- which is related to the non-Abelian Berry connection of the Bloch states.