Exciton dynamics and high-temperature excitonic superfluidity in S-doped graphyne.

S-doped graphyne (S-GY) is a recently synthesized two-dimensional graphyne-based carbon allotrope that provides a promising platform for exciton engineering and coherent many-body phases. Here, we investigate the quasiparticle electronic structure, optical response, and exciton dynamics of monolayer S-GY using the G0W0 approximation and the Bethe-Salpeter equation (BSE). Quasiparticle corrections increase the fundamental band gap from 0.88 eV (PBE) to 1.95 eV, while slightly reducing the carrier effective masses. The BSE optical response reveals strongly bound excitons, with the lowest bright exciton exhibiting a binding energy of 0.72 eV, as well as a nearly degenerate dark exciton within the thermal energy scale. Analysis of exciton wavefunctions in reciprocal space confirms a hydrogenic Rydberg series with well-defined angular-momentum character, and radiative lifetimes in the nanosecond range at room temperature, comparable to those in transition-metal dichalcogenide monolayers. Finally, we construct the excitonic phase diagram and estimate a crossover density of ∼6.8 × 1012 cm-2, below which the exciton gas behaves as a dilute Bose system, and the Berezinskii-Kosterlitz-Thouless (BKT) superfluid phase becomes accessible. We estimate a maximum BKT transition temperature of ∼143 K in the freestanding limit for the 1s exciton, indicating that monolayer S-GY may provide favorable conditions for high-temperature excitonic superfluidity in graphyne-based materials.