Magneto-optic characterization of quaternion state in TMD bilayers with metallic screening

Van der Waals (vdW) heterostructures have served as a new platform for exploring novel higher-order excitonic bound states including trions, biexictions and even “hexcitons” and “oxcitons.” [1] In our recent experiments [2], we observed evidence for a new bound excitonic state in a bilayer structure: two free carriers bound to an exciton, forming a doubly charged “quaternion.” The key feature of this state is that the metallic screening helps to reduce the overall repulsion of like charges. We have conducted many controlled experiments with different designs to show that the quaternion state only exist when the full structure is in place, including the bilayer and the metallic layer under it.

We have measured the magnetic field dependence of the photoluminescence (PL) of these complexes. The magnetic field was applied in both out-of-plane directions from 0 to 14 Tesla, and the PL was measured in different circular polarized components of photoluminescence. The energy of the trions and excitons shift linearly with the magnetic field. In contrast, the energy of the quaternion shows a non-linear relationship with the magnetic field, which is also asymmetric with the direction of the magnetic field. The asymmetry is consistent with the fact that the quaternion state is asymmetric in the z-direction, as predicted by theory.

We are moving forward to put a top gate on these devices to allow continuous variation of the doping level. These metal-screened novel bound states are potentially charged bosons, which can even be a potential candidate for room-temperature and atmospheric-pressure superconductivity.