  Extensive inelastic neutron scattering measurements of phonons on a single
crystal of CaFe2As2 allowed us to establish a fairly complete picture of phonon
dispersions in the main symmetry directions. The phonon spectra were also
calculated by density functional theory (DFT) in the local density
approximation (LDA). There are serious discrepancies between calculations done
for the optimized structure and experiment, because the optimised structure is
not the ambient pressure structure but is very close to the collapsed structure
reached at p = 3.5 kbar. However, if the experimental crystal structure is used
the calculation gives correct frequencies of most phonons. The most important
new result is that linewidths/frequencies of certain modes are larger/softer
than predicted by DFT-LDA. We also observed strong temperature dependence of
some phonons near the structural phase transition near 173K. This behavior may
indicate anomalously strong electron phonon coupling and/or anharmonicity,
which may be important to the mechanism of superconductivity.
