Quantum geometry of singlet superconductors

Abstract

We elaborate that $s$-wave and $d$-wave superconductors described by mean field theories possess a nontrivial quantum geometry. From the overlap of two quasihole states at slightly different momenta, one can define a quantum metric that measures the distance in the curved momentum space. The momentum-integration of the quantum metric represents an average distance that we call the fidelity number, which may be further expressed as a fidelity marker defined locally on every lattice site. For $s$-wave superconductors, we unveil that the quantum metric generally influences the electromagnetic responses at finite wave length, such as the infrared absorption and paramagnetic current. In addition, the dielectric response is directly proportional to the fidelity number, which is found to be determined by the coherence length and suppressed by disorder. For $d$-wave superconductors, we demonstrate the singular behavior of the quantum metric near the nodal points, and a metric-curvature correspondence between the azimuthal quantum metric and the non-Abelian Berry connection that integrates to a topological charge of the nodal points.