Giant phonon anomalies in the bond-stretching branches in Ba_0.6K_0.4BiO_3

The dispersion of the longitudinal bond-stretching (BS) branches in superconducting Ba0.6K0.4BiO3 (BKBO) studied by inelastic neutron scattering reveals giant phonon anomalies. In the [110] and [111] directions these branches even split close to the middle of the zone, in contradiction to any harmonic lattice dynamical model with cubic symmetry. The additional low energy branch in the [111] direction ends at the R-point with a frequency near 10.5THz representing a doping induced renormalization of a breathing type vibration by about 40%, the strongest observed so far in any superconductor.