Sharp optical-phonon softening near optimal doping in La2-xBaxCuO4+δ observed via inelastic x-ray scattering

We report a direct observation of a sharp Kohn-type anomaly in the doubly degenerate copper-oxygen bond-stretching phonon mode occurring at q=(0.3,0,0) in La2-xBaxCuO4+delta, with x=0.14 +/- 0.01, thanks to the high Q resolution of inelastic x-ray scattering. This anomaly is clearly seen when the inelastic signal is analyzed using a single mode but is also consistent with a two-mode hypothesis, possibly due to a splitting of the degenerate modes due to symmetry-breaking stripes. Our observation shows that the effect persists at the stripe-propagation vector in a superconducting system close to optimal doping.