Coherent Coulomb Intra- and Intervalley Many-Body Effects in Single-Layer Transition Metal Dichalcogenides.

The reduced Coulomb screening in single-layer (1L) transition metal dichalcogenides (TMDs) offers an ideal setting to explore excitonic many-body correlations. The interactions between excitons result in intra- and intervalley biexcitonic multiparticle states, whose contributions to the nonlinear optical response have remained elusive so far. Here, by using helicity-resolved transient absorption spectroscopy with sub-10 fs temporal resolution combined with a microscopic theory based on the excitonic Bloch equations we are able to unambiguously disentangle the contribution of two particle exciton and four particle biexciton correlations to the coherent optical response of 1L-WSe_{2} semiconductor. Upon resonant excitation of valley-polarized A exciton population we observe competing excitation-induced energy shift of the A exciton transition along with a coherent gain in the pumped valley and an instantaneous formation of an additional absorption peak in the unpumped valley, which we attribute to the effect of bound intervalley biexcitons. An excellent agreement between experimental results and calculations allows us to deepen understanding of many-body effects in 1L-TMDs, which is crucial for the development of excitonic and valleytronics devices.