Spectrum masking approach to study moiré excitons in van der Waals heterostructures

Moiré patterns in transition metal dichalcogenide (TMD) heterostructures provide an exceptional platform to explore correlated excitonic phenomena. The emergence of modulated Wannier and charge-transfer excitons are prominent only in large-area moiré superlattices comprising several thousands of atoms in the unit cell. To address the challenge of studying large superlattices, the pristine unit-cell matrix projection method was recently developed, reducing the cost of Bethe–Salpeter Equation (BSE) calculations by several orders of magnitude. However, another challenge to study these systems is the extensive band folding which brings multiple TMD valleys from the pristine unit-cell Brillouin zones into the tiny moiré Brillouin zone, generating a wide energy window of intrinsically momentum-indirect (dark) transitions mixed with bright transitions. Treating these transitions hence requires exceedingly large moiré BSE matrices, escalating the computation cost. To overcome this limitation, we develop a general spectrum masking approach that systematically masks only the optically active transitions in the Brillouin zone relevant for bright excitons and significantly speeds up the calculation of moiré absorption spectra.