We report phonon renormalization in bilayer graphene as a function of doping. The Raman $G$ peak stiffens and sharpens for both electron and hole doping as a result of the nonadiabatic Kohn anomaly at the $\Gamma$ point. The bilayer has two conduction and valence subbands, with splitting dependent on the interlayer coupling. This gives a change of slope in the variation of $G$ peak position with doping which allows a direct measurement of the interlayer coupling strength.

• Received 10 July 2008

DOI:https://doi.org/10.1103/PhysRevB.79.155417