Charged bosons made of fermions in bilayer structures near metallic surfaces

In 1996, V.I.Yudson proposed the existence of stable four-carrier complexes in bilayer semiconductors, which may be called "quaternions". If a bilayer semiconductor structure is placed parallel to a nearby metal layer, then under optical pumping an interlayer exciton can be created, which picks up two free same-layer electrons (or two holes). Due to image charge below the metal surface, much of the repulsive interaction in such a complex is canceled out, to make a stable charged boson state of four fermions. Such a quaternion complex is recently observed in PL spectroscopy experiments on doped bilayer TMD structures placed near the Nb surface at a distance controlled by extra hBN separation layers[1]. To explain these experiments, we develop the theory for a variant of the Yudson geometry, which is structurally a trion in one layer bound to a free like-charge carrier in the parallel layer. The calculations we discuss are consistent with experimental observations. A Bose-Einstein condensate of such quaternions would be a Schafroth superconductor ─ a collective multiparticle superfluid state of doubly charged bosons, a state not requiring Cooper pairing, to open up the door to a new kind of unconventional high-T superconductivity. – [1] Z.Sun, et al., Nano Lett. 21, 7669 (2021).