Charged Bosonic Excitonic State in Bilayer Structures with Strong Metallic Screening*

    Two-dimensional monolayers of transition metal dichalcogenides (TMDs) have already shown the existence of many higher-order excitonic bound states such as trions and biexcitons.  Stacking multiple TMD layers allows even more complex and novel excitonic states.

    In our experiment, we observed a new excitonic state: Two free carriers bound to an exciton forms a  doubly charged “quaternion”. We measured a new spectral line at the predicted energy in a bilayer structure with an hBN spacer layer, TMD layers encapsulated in hBN, and placed on Nb metal pad. We have shown that this occurs in structures made of two different kinds of TMD (MoSe2 and WSe2) and proved that it occurs only when all required theoretical conditions for this complex are fulfilled, especially the presence of a metal layer. We are also working on the transport experiment of the quaternions by applying a strong electric field parallel to the sample and will report on these results.

    Since these four-carrier bound states are charged bosons, they may allow a new path to superconductivity without Cooper pairing, which might be a potential candidate for a room-temperature and atmospheric pressure superconductor.