Femto-second photoimpedance response and dynamic structural distortions in YBCO samples

Two novel experiments, one concerned with the fast optical response related to Cooper pair breaking using femto-second laser pulses in YBCO films and the second related to uncorrelated dynamic distortions as a function of oxygen concentration for YBCO single crystals by ion channeling, have provided information that may be relevant for understanding the mechanism of superconductivity in high Tc superconductors. In the first case a dramatic resonant photon energy dependence around 1.5 eV in the pair breaking rate has been observed, suggesting that an intermediate excitation may be playing an important role in the depairing process. In the second experiment, ion channeling investigations in YBa2Cu3O7-δ (δ = 0.05 to 0.8) crystals have clearly shown an evolution of the lattice from that of a Debye solid for δ = 0.8 to one exhibiting significant lattice coherence even at much higher temperatures than Tc, with discontinuities which track Tc. These two experimental results are discussed in the light of different mechanisms including the formation of Zhang-Rice singlet or other mechanisms that lead to a non local hole.