Bond-stretching-phonon anomalies in stripe-ordered La1.69Sr0.31NiO4

We report a neutron scattering study of bond-stretching phonons in La1.69Sr0.31NiO4, a doped antiferromagnet in which the added holes order in diagonal stripes at 45 to the Ni-O bonds. For the highest-energy longitudinal optical mode along the bonds, a softening of 20% is observed between the Brillouin zone center and the zone boundary. At 45 to the bonds, a splitting of the same magnitude is found across much of the zone. Surprisingly, the charge-ordering wave vector plays no apparent role in the anomalous dispersions. The implications for related anomalies in the cuprates are discussed.