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Withboth spin and valley degrees of freedom, the low-lying excitonicspectra of photoexcited transition-metal dichalcogenide monolayers(TMDC-MLs) are featured by rich fine structures, comprising the intravalleybright exciton states as well as various intra- and intervalley darkones. The latter states can be classified as those of the spin- andmomentum-forbidden dark excitons according to the violated opticalselection rules. Because of their optical invisibility, these twotypes of the dark states are in principle hardly observed and evendistinguished in conventional spectroscopies although their impactson the optical and dynamical properties of TMDC-MLs have been wellnoticed. In this Letter, we present a theoretical and computationalinvestigation of the exciton fine structures and the temperature-dependentphotoluminescence spectra of strained tungsten diselenide monolayers(WSe2-MLs) where the intravalley spin-forbidden dark excitonlies in the lowest exciton states and other momentum-forbidden statesare in the higher energies that are tunable by external stress. Thenumerical computations are carried out by solving the Bethe-Salpeterequation for an exciton in a WSe2-ML under the stress-controlin the tight-binding scheme established from the first principle computationin the density functional theory. According to the numerical computationand supportive model analysis, we reveal the distinctive signaturesof the spin- and momentum-forbidden exciton states of strained WSe2-MLs in the temperature-dependent photoluminescences and presentthe guiding principle to infer the relative energetic locations ofthe two types of dark excitons.

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