FEMTOSECOND DYNAMICS OF QUASI-PARTICLES IN YBA2CU3O7-DELTA SUPERCONDUCTOR FILMS

We have studied the transient electronic response of YBa2Cu3O7-delta epitaxialy grown HT(c) superconductor thin films in the femtosecond time domain, using transient photoinduced reflectivity (DELTA-R) with 60 fsec time resolution. For temperatures T > T(c) only a bolometric signal was observed with DELTA-R > 0. For T < T(c), DELTA-R < 0 with a temperature dependent rise time of order 300 fsec followed by a relaxation (of order 3 psec) into a state with DELTA-R > 0. The results for T < T(c) are explained in terms of quasi-particle (QP) electronic response giving DELTA-R < 0. Thus the femtosecond rise time is interpreted as avalanche multiplication of QP across the gap 2-DELTA and the subsequent picosecond relaxation as QP recombination. The QP optical response is explained within the two fluid model.