DISPLACIVE EXCITATION OF COHERENT PHONONS IN YBA2CU3O7

We suggest a microscopic model that provides a consistent explanation of the recent femtosecond pump-probe experiments on YBa2Cu3O7, includi ng the fact that the reflectivity change in the superconducting state is much larger than that in the normal state. In this model, not only the oscillatory part of the reflectivity, but also the total reflectiv ity change, is due to displacive excitation of coherent phonons. The m icroscopic reason for this excitation is that superconductivity induce s small displacements in the equilibrium positions of the ions, since the pairing energy depends on the density of states at the Fermi level , which changes with the ionic positions. When superconductivity is de stroyed by the femtosecond laser pulse, the ions are pulled back to th eir normal equilibrium positions, thus exciting coherent phonons. The relative size of the oscillatory contribution to the reflectivity depe nds upon the ratio of the phonon period to the time scale of the pair breaking, and when this ratio is small, the oscillations are suppresse d, as observed in the experiment. Ab initio calculations confirm this model. The model also provides an explanation for why the magnitude of the 150 cm-1 mode below T(c) may be much smaller than that of the 120 cm-1 mode.