Dipole Ladders with Giant Hubbard U in a Moiré Exciton Lattice

Two-dimensional semiconductor moiré superlattices have emerged as a powerful platform for engineering correlated electronic phenomena. Moreover, 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 its observation remains elusive. Here, we report a giant exciton Hubbard U and consequent dipole ladders with spin- and electron-filling dependence in WSe₂/WS₂ moiré superlattices. By measuring interlayer exciton photoluminescence as a function of excitation intensity, we identify successive new peaks emerge with an energy separation of ~34 meV above the ground state. This corresponds to the sequential injection of excitons into a single site with an energy cost to overcome the remarkably large exciton Hubbard U, forming a dipole ladder. By measuring time-resolved photoluminescence, we also find that the interlayer exciton decay dynamics display a sequential cascaded decay down the dipole ladder. Based on findings of local magnetic moments at two holes per moiré cell, we show that excitons can also fill a second moiré orbital, establishing the two-orbital nature of the moiré potential landscape. Our results pave the way for investigating the Bose-Hubbard model with possible exciton crystal phases in interacting opto-moiré quantum matter.