  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.
