TIME-RESOLVED OBSERVATION OF COHERENT PHONONS IN SUPERCONDUCTING YBA2CU3O7-DELTA THIN-FILMS

We report the observation of coherent optical phonons at 120 and 150 cm-1 in superconducting YBa2Cu3O7-delta (delta < 0.1) thin films. These phonons are excited by femtosecond laser pulses and observed by distinct oscillations of the reflectivity signal in real time. Below the transition temperature T(c) the amplitude of the Ba mode at 120 cm-1 increases drastically in close correlation to the density of optically excited quasiparticles. The dephasing times of the coherent Ba and Cu(2) vibrations exhibit sharp increases below T(c) excluding a "second" smaller gap below 150 cm-1.