Optical phonon mixing in bilayer graphene with a broken inversion symmetry

Pristine bilayer graphene is a centrosymmetric material in which parity is a conserved quantity. The high sensitivity of this atomic scale structure to external perturbations that break the inversion symmetry enables significant potentials for device applications. Raman spectroscopy is used here to probe the breakdown of parity conservation in a direct and quantitative manner via a phonon mixing phenomenon. The striking broken-symmetry effects display anticrossing coupling between two opposite parity long-wavelength optical phonons. The spectral intensity transfer between the two observed Raman peaks offers quantitative measurements of the evolution of the phonon wave function and demonstrates a manifestation of broken inversion symmetry in graphene layers.