Theoretical prediction of the quadruplon in a monolayer semiconductor

Understanding multiparticle entities in a many-body system is not only of fundamental importance in condensed matter physics but also critical for many technological applications. So far, multiparticle entities in semiconductors have been studied with corresponding irreducible clusters up to the second order only. In this Letter, we theoretically predict the general existence of a different four-particle entity involving two electrons and two holes using the four-body Bethe-Salpeter equation (4B-BSE). The 4B-BSE was applied to calculate the helicity-resolved absorption for a monolayer $\mathrm{MoT}{\mathrm{e}}_{2}$. Surprisingly, we found a rich series of spectral peaks within an energy span of \ensuremath{\sim}40 meV below the exciton. To understand the origin of the spectral peaks, the Feynman diagrams of the 4B-BSE are recast into the cluster expansion formalism, allowing us to study the individual effects of various clusters. We found that the irreducible clusters of up to the third order and their factorized combinations cannot explain the spectral features. Importantly, we found that the fourth-order irreducible correlation, corresponding to a quadron or quadruplon, is necessary and sufficient to explain the different features. The prediction of quadruplon not only enriches our understanding of many-body correlations but also provides a different light-matter interaction mechanism for optoelectronic applications.