Dipole ladders with large Hubbard interaction in a moire exciton lattice

Abstract

Strong dipole-dipole interactions between excitons in a moire superlattice create a manifestation of the Bose-Hubbard model with a ground state similar to a Mott insulator.Two-dimensional semiconductor moire superlattices have emerged as a powerful platform for engineering correlated electronic phenomena. On the other hand, optical excitation creates charge neutral interlayer excitons with an out-of-plane electric dipole. Strong onsite dipole-dipole interaction promises the formation of correlated bosonic states, akin to the Mott states of electrons, but has not yet been demonstrated. Here we report a large interaction between excitons occupying the same moire lattice site-characterized by the Hubbard U parameter-and consequent dipole ladders with spin- and electron-filling dependence in WSe2/WS2 moire superlattices. Photoluminescence measurements show successive peaks emerging with an energy separation of around 34 meV above the ground state as the exciton density is increased. This corresponds to the sequential injection of excitons into a single site with an energy cost to overcome the large exciton Hubbard U, forming a dipole ladder. Based on findings of local magnetic moments at two holes per moire cell, we show that excitons can also fill a second moire orbital, establishing the two-orbital nature of the moire potential landscape. Our results show that the Bose-Hubbard model with possible exciton crystal phases can be investigated in interacting opto-moire quantum matter.