Quantum-vortex excitons beyond band topology

Optically exciting a typical low-dimensional semiconductor produces a nodeless s-type exciton as its lowest-energy bound state. Here, we explain how Coulomb phase matching stabilises a quantum-vortex exciton as the lowest bound state in flat Chern bands. Using a prototypical Yin--Yang kagome lattice, we demonstrate that increasing the spin--orbit coupling drives a transition of the lowest exciton from a quantum-vortex state to a zero-winding state, while leaving the band Chern numbers unchanged. We trace this behaviour to a gauge-invariant combination of exciton phase differences and Bloch-overlap phases that can reduce the Coulomb energy. By comparing states with identical wavefunction amplitudes, we isolate this phase contribution and show that it is sufficient to drive the reversal in exciton ordering. We classify exciton topology by the momentum-space wavefunction winding relative to the conduction--valence Chern-number difference,connecting the competing states to circular-polarisation selection rules. We then show that the resulting reordering can be tracked directly in the optical absorption spectrum, through the transition from a dark vortex exciton to a bright zero-winding exciton. These findings establish a microscopic link between exciton topology, Coulomb binding, and optical response that extends beyond the information contained in the underlying electronic band Chern numbers.