Flat-band mechanism for strongly bound dark excitons in two-dimensional magnetic semiconductors

Two-dimensional (2D) materials provide an essential platform for the formation and manipulation of excitons, in which 2D magnetic materials often host weakly dispersive flat bands arising from localized d orbitals. Using CrSBr and CoCl₂, we show that such flat bands support dark Frenkel excitons governed by spin- or parity-forbidden transitions. These excitons are highly localized in real space, energetically lower than bright excitons, and exhibit millisecond-scale radiative lifetimes and room-temperature Bose–Einstein condensation thresholds. This mechanism highlights the role of flat-band topology and selection rules in engineering dark exciton states. Our findings reveal a general route to long-lived dark excitons in magnetic 2D systems, with implications for excitonic quantum phases and optoelectronic applications.