Femtosecond photoimpedance response and dynamic structural distortions in YBCO samples

摘要

Abstract: Two novel experiments, one concerned with the fastoptical response related to Cooper pair breaking usingfemto-second laser pulses in YBCO films and the secondrelated to uncorrelated dynamic distortions as afunction of oxygen concentration for YBCO singlecrystals by ion channeling, have provided informationthat may be relevant for understanding the mechanism ofsuperconductivity in high T$-c$/ superconductors. Inthe first case a dramatic resonant photon energydependence around 1.5 eV in the pair breaking rate hasbeen observed, suggesting that an intermediateexcitation may be playing an important role in thedepairing process. In the second experiment, ionchanneling investigations in YBa$-2$/Cu$-3$/O$-7$MIN$delta$/ ($delta equals 0.05 to 0.8)crystals have clearly shown an evolution of the latticefrom that of a Debye solid for $delta equals 0.8 to oneexhibiting significant lattice coherence even at muchhigher temperatures than T$-c$/, with discontinuitieswhich track T$-c$/. These two experimental results arediscussed in the light of different mechanismsincluding the formation of Zhang-Rice singlet or othermechanisms that lead to a non local hole. !14