Symmetry Assignment of the Direct Band Gap of T1C1 and Evidence for Exciton Intervalley Scattering by Stress Optical Experiments

The doublet structure observed for the first exciton transition in T1C1 is investigated with the aid of uniaxial stress. It is shown experimentally that the direct energy gap belongs to point $X=\left(\frac{\pi }{a}\right)\left(001\right)\mathrm{}$ of the Brillouin zone. The exciton structure following from this identification is discussed, and a new interpretation of the doublet splitting based on $X$-point degeneracy is suggested. First conclusive evidence is given for intervalley scattering of direct excitons created by transitions outside the center of the Brillouin zone.