Extended Data Fig. 3: The cut in the phonon BZ investigated in our experiments.

a, In the electronic momentum space, we show the first BZ of the top (blue) and bottom (red) graphene layers at a generic twist angle. The inelastic tunnelling processes examined in our experiments involve electrons scattering between the small Fermi surface of the top bottom layer, around the K_B point (blue circle), to the Fermi surface of the top layer around the K_T point (red circle), while emitting a phonon with momentum \({q}_{{\rm{M}}}={2K}_{{\rm{D}}}\sin \left(\frac{\theta }{2}\right)\). Electrons can also tunnel from the other valley at K′_B of the bottom layer to the valley at the K_T point in the top layer and emit a phonon with momentum \({\widetilde{q}}_{{\rm{M}}}={2K}_{{\rm{D}}}\sin \left(\frac{60-\theta }{2}\right)\). b, In the phononic momentum space, q_M cuts through the phononic BZ along a specific arc that goes from the Γ point to the K point (orange). For \({\widetilde{q}}_{{\rm{M}}}\), the trajectory is reversed.