A time-reversal invariant vortex in topological superconductors and gravitational $\mathbb{Z}_2$ topology

Abstract: We study a topological superconductor in the presence of a time-reversal invariant vortex. The eigenmodes of the Bogoliubov-de-Genne (BdG) Hamiltonian show a $\mathbb{Z}_2$ topology: the time-reversal invariant vortex with odd winding number supports a pair of helical Majorana zero-modes at the vortex and the edge, while there is no such zero-modes when the winding number is even. We find that this $\mathbb{Z}_2$ structure can be interpreted as an emergent gravitational effect. Identifying the gap function as spatial components of the vielbein in 2 +1-dimensional gravity theory, we can explicitly convert the BdG equation into the Dirac equation coupled to a nontrivial gravitational background. We find that the gravitational curvature is induced at the vortex core, with its total flux quantized in integer multiples of $\pi$, reflecting the $\mathbb{Z}_2$ topological structure. Although the curvature vanishes everywhere except at the vortex core, the fermionic spectrum remains sensitive to the total curvature flux, owing to the gravitational Aharonov-Bohm effect.