Intervalley excitons in a minimal supercell moir\'e crystal

It is known that a van der Waals bilayer forms a commensurate crystal only at a discrete set of twisted angles. Whenever unit cells are commensurate in real space, their corresponding Brillouin zone (BZ) are also commensurate in momentum space. This alignment facilitates coherent tunneling and leads to conductance peaks in 21.8◦ and 38.2◦ graphene bilayers even at room temperature. In this work, we study light-matter interaction in large twist angle WSe2 bilayers. We observe surprisingly strong intervalley exciton transitions assisted by phonons in 21.8◦ WSe2 bilayer while these resonances are completely absent in a 38.2◦ bilayers. This exciton-phonon coupling also leads to a large energy shift between the intervalley exciton resonances observed in reflectivity and photoluminescence spectra. The different optical spectra in these two commensurate WSe2 bilayer originate from interlayer coupling either facilitated or suppressed by parallel or anti-parallel spins.

We gratefully acknowledge funding from the NSF MRSEC DMR-2308817 and NSF ECCS- 2130552 (Z. Liu), Department of Energy, Basic Energy Science program via grant DESC0019398 (X, Liu and K. Hao), and the Welch Foundation chair F-0014 for sample preparation.