Charged Bosons Made of Fermions in Bilayer Structures with Strong Metallic Screening

Two-dimensional monolayer structures of transition metal dichalogenides (TMDs) have been shown to allow many higher-order excitonic bound states, including trions (charged excitons), biexcitons (excitonic molecules), and charged biexcitons. We report here experimental evidence and the theoretical basis for a new bound excitonic complex, consisting two free carriers bound to an exciton in a bilayer structure. Our experimental measurements on structures made using two different materials show a new spectral line at the predicted energy with two different TMD materials (MoSe2 and WSe2) with both n- and p-doping if and only if all the required theoretical conditions for this complex are fulfilled, in particular, only in the presence of a parallel metal layer that significantly screens the repulsive interaction between the like-charge carriers. Because these four-carrier bound states are charged bosons, they could eventually be the basis for a new path to superconductivity without Cooper pairing.

Automated summary

Research has shown that two-dimensional monolayer structures of transition metal dichalogenides can accommodate various high-order excitonic bound states, such as trions, biexcitons, and charged biexcitons. Experimental evidence and theoretical basis have revealed a new bound excitonic complex in a bilayer structure, consisting of two free carriers bound to an exciton. These four-carrier bound states, being charged bosons, could potentially be a new pathway to superconductivity.