NONEQUILIBRIUM ELECTRONS, HOLES AND PHONONS IN YBA2CU3O7-X

We report an observation by picosecond Stokes/anti-Stokes Raman spectr oscopy of high non-equilibrium O(4) apical oxygen and Cu-O plane buckl ing phonon populations in insulating and metallic YBa2Cu3O7-x induced by high peak power pulsed laser excitation. Different excitation and p robe geometries were used to selectively measure the non-equilibrium p honon populations of the two phonons. We attribute the effects to stro ng coupling of photoexcited carriers to the lattice. A photoinduced me tal-insulator transition is observed in insulating YBa2Cu3O6.2 accompa nied with a qualitative change in nonequilibrium behaviour for the two phonons and an increase in carrier lifetime upon increasing doping. A polaronic model is used to discuss the observations.