van der Waals heterostructures*

In Raman measurements on WSe₂/WS₂ van der Waals bilayers, two new satellite peaks are observed on either side of the well-known WSe₂ A_1g/E_2g peak.  We make three observations about these satellite peaks: 1) They are observed in samples with a small twist angle deviation from aligned 0^o (3R) or 60^o (2H) stacking and are not observed in samples with a large twist angle. 2) The frequencies of the satellite peaks are identical for 60^o+θ and 0^o+θ samples, where θ is the same in each sample. 3) The frequency of the satellite peaks shifts with small variations in twist angle. Using density functional theory calculations and geometric arguments, we show that all three of these observations can be explained if we identify these new satellite peaks as zone-folded optical phonon modes from the WSe₂ layer.