Optical Absorption by Dirac Excitons in Single-Layer Transition-Metal   Dichalcogenides
  We develop an analytically solvable model able to qualitatively explain nonhydrogenic exciton spectra observed recently in two-dimensional (2d) semiconducting transition metal dichalcogenides. Our exciton Hamiltonian explicitly includes additional angular momentum associated with the pseudospin degree of freedom unavoidable in 2d semiconducting materials with honeycomb structure. We claim that this is the key ingredient for understanding the nonhydrogenic exciton spectra that was missing so far. 
Maxim Trushin, Mark Oliver Goerbig, Wolfgang Belzig
cond-mat.mes-hall