Exciton-Exciton Interaction in Monolayer MoSe2 from Mutual Screening of Coulomb Binding

Excitons in two-dimensional (2D) semiconductors are particularly exciting, as reduced screening and dimensional confinement foster their pronounced many-body interactions. Optical pumping is typically used to create excitons so as to study their properties, but at the same time such pumping can also create unbound charge carriers. This makes experimental determination of the exciton-exciton interactions difficult. Most importantly, the two effects of band gap renormalization and Coulomb screening on the individual exciton resonance energy counteract each other. Here by comparing the influences of exciton and electron density on the exciton ground and excited states energies of monolayer MoSe2 using photoluminescence spectroscopy, we are able to distinctly identify the screening of Coulomb binding by the neutral excitons and by charge carriers. The energy difference between exciton ground state (A-1s) and excited state (A-2s) red-shifts by 5.5 meV when the neutral exciton density increases from 0 to 4 x 1011 cm-2, in contrast to the blue shifts with the increase of either electron or hole density. This energy difference change is attributed to the mutual screening of Coulomb binding of neutral excitons, a many-body effect that is over 5 times magnitude stronger than the conventional estimate of exciton-exciton interactions. From this mutual Coulomb screening we extract an exciton polarizability of alpha 2D exciton = 2.5 x 10-17 eV(m/V)2. Our finding uncovers a mechanism that dominates the repulsive part of many-body interaction between neutral excitons.