Optically controllable spin polarization in two-dimensional altermagnets via robust spin-momentum locking excitons

Spin-momentum locking (SML) excitons in two-dimensional semiconductors are appealing to programmable optical control of spin-polarized carriers in ultrafast spintronics. To address the current thirsty for long-lived excitons with zero-external-field stability and room-temperature spin polarization, we predict the existence of intrinsically SML excitons in altermagnetic ${\mathrm{V}}_{2}{X}_{2}\mathrm{O}$ ($X=\mathrm{S}$, Se) driven by giant nonrelativistic spin splittings ($>1.2$ eV). First-principles calculations reveal SML excitons with binding energies exceeding 1400 meV in monolayers and 430 meV in their van der Waals heterobilayers, along with stacking-dependent optical selection rules for tunable interlayer excitons. These remarkable physical properties, combined with their long radiative lifetimes, strongly suggest the feasibility of SML excitons with robust spin polarization at room temperature. Our work introduces a paradigm for SML exciton physics via altermagnetism, opening possibilities for all-optical manipulation in advanced opto-spintronics.