Optical fingerprint of dark 2p-states in transition metal   dichalcogenides
  We present a microscopic study on higher excitonic states in transition metal dichalcogenides in the presence of disorder. We show that the geometric phase cancels the degeneration of 2s and 2p states and that a significant disorder-induced coupling of bright and dark states offers a strategy to circumvent optical selection rules. As a prove, we show a direct fingerprint of dark 2p states in absorption spectra of WS$_2$. The predicted softening of optical selection rules through exciton-disorder coupling is of general nature and therefore applicable to related two-dimensional semiconductors. 
Gunnar Bergh\"auser, Andreas Knorr, Ermin Malic
cond-mat.mtrl-sci cond-mat.mes-hall