Femtosecond dynamics of quasi-particles in YBa2Cu3O7-δ superconductor films

We have studied the transient electronic response of YBa2Cu30y . 5 epitaxialy grown HTC superconductor thin films in the femtosecond time domain, using transient photoinduced reflectivity (∆R) with 60 fsec time resolution. For temperatures T>TC only a bolometric signal was observed with ∆R>0. For T<TC ∆R<0 with a temperature dependent rise time of order 300 fsec followed by a relaxation (of order 3 psec) into a state with ∆R>0. The results for T<TC are explained in terms of quasi-particle (QP) electronic response giving ∆R<0. Thus the femtosecond rise time is interpreted as avalanche multiplication of QP across the gap 2A and the subsequent picosecond relaxation as QP recombination. The QP optical response is explained within the two fluid model.