Twist-Induced New Phonon Scattering Pathways in Bilayer Graphene Probed by Helicity-Resolved Raman Spectroscopy

The electron density of state near saddle points in an electronic band structure is greatly enhanced, forming the wellknown Van Hove singularity (VHS), which leads to strengthened electron-photon coupling. However, the VHS-related electron- phonon interaction is rarely studied. Here we report the first observation of intra-mini valley phonon scattering pathways near the saddle points in twisted bilayer graphene (tBLG) through helicity-resolved Raman spectroscopy. Two new second-order Raman modes near the G peak are observed and assigned to combination modes of in-plane optical (TO, LO) and out-of-plane acoustic (ZA) mode, at similar to 1580 cm(-1) (TOZA) and similar to 1595 cm(-1) (LOZA), respectively. The twist-angle-dependence of their intensities and frequencies can be explained by double resonance Raman processes considering the fine electronic band structure of saddle points. Our findings provide a deeper understanding of the electronic band structure at saddle points of tBLG and enrich the VHS-related physics from an electron-phonon interaction point of view.