Displacive Excitation of Coherent Phonons in YBa_2Cu_3 O_7: An Optical   Evidence for the Van-Hove Singularity
  The amplitude of displacive excitation of coherent phonons ({\it DECP}) in $YBa_2Cu_3O_7$ observed in time-resolved spectroscopy increases by a very large factor of 15 between 20 K (below $T_c$) and 100 K (above $T_c$). Microscopic modelling of phonon excitation reveals that these amplitude changes of the {\it $A_{1g}$} mode phonons are related to the existence of the {\it Van-Hove} singularity (VHS) below but very close to Fermi energy. PACS numbers: 78.47.+p, 74.25.Gz, 74.25.Jb, 71.15.Fv 
W.-S.Zeng and J.Kuhl