From Dark to Bright: Temperature Effects in SnO₂ and WSe₂

The optical and excitonic properties of electronic materials are central to their functionality, with symmetry playing a key role in governing optical absorption through dipole selection rules. These rules determine whether the lowest electron-hole transitions are optically allowed or forbidden. In bulk SnO₂, a known host of dark states¹, we use first-principles simulations within the density functional theory framework to study how phonon-induced symmetry breaking can relax dipole selection rules. By solving the Bethe-Salpeter equation (BSE), we investigate the impact of temperature on excitonic effects, revealing brightening of dark states, a redshift in the absorption onset, and modifications in absorption coefficients.

Extending this approach, we examine monolayer WSe₂, a two-dimensional transition metal dichalcogenide with intrinsic ground-state dark excitons². Due to reduced dielectric screening, WSe₂ exhibits strong Coulomb interactions and tightly bound excitons even at room temperature, making it an ideal platform for studying temperature effects on excitonic behavior. Unlike SnO₂, the dark states in WSe₂ are spin- and momentum-forbidden, arising from spin-valley coupling and symmetry breaking induced by strong spin-orbit coupling. Using BSE calculations, we explore how thermal activation influences the dark excitons and the absorption onset. Our study offers insights into temperature-dependent optical phenomena across dimensionalities and material classes.