  Analogous to the charged electron-electron pair condensation in
superconductors, an excitonic insulator (EI) represents Fermi surface
instability due to spontaneous formation and condensation of charge-neutral
electron-hole pair (exciton). Unlike in superconductors, however, the
charge-neutral nature of exciton makes probing emergent EI phase via
macroscopic physical properties generally difficult. Here, we propose a van der
Waals coupled antiferromagnetic semiconductor GdGaI (GGI) as a new material
category leading to emergent multi-q magnet intertwined with spontaneous
exciton formation/condensation. Before excitonic band hybridization, a simple
picture for the parent electronic state consists of electron (Gd-derived 5d)
and hole (Ga-derived 4p) delocalized bands, together with Gd-derived 4f
localized antiferromagnets with S = 7/2 classical nature. Through intra Gd atom
4f-5d Hund's coupling, a notable finding is the emergent minimum length scale
(2a) Skyrmion-like spin texture resulting from spontaneous
condensation/formation of spin-polarized exciton with BCS-BEC crossover
phenomenology. This discovered platform is promising for realizing valuable
quantum matter on the nanoscale; our finding will provide significant insight
into designing the atomic scale topological magnetism out of itinerant systems.
