Optically bright $p$-excitons indicating strong Coulomb coupling in transition-metal dichalcogenides

Abstract

It is shown that the strong Coulomb coupling in intrinsic suspended semiconducting transition metal dichalcogenides can exceed the critical value needed for an excitonic ground state. The dipole-allowed optical excitations then correspond to intra-excitonic transitions such that the optically bright excitonic transitions near the Dirac points have a $p$-like symmetry whereas the $s$-like states are dipole forbidden. The large intrinsic coupling strength seems to be a generic property of the semiconducting transition metal dichalcogenides and strong Coulomb-coupling signatures in the form of the optical selection rules can be observed even in samples grown on typical substrates like SiO$_2$. For the examples of WS$_2$ and WSe$_2$, excellent agreement of the computed excitonic resonance energies with recent experiments is demonstrated.