Relaxation dynamics of optically excited quasi-particles in BSCCO

To investigate the multi-excitation processes on the low-lying electronic structure of high- T C superconductors, time-domain pump and probe measurement has been carried out on under- and over-doped Bi 2Sr 2CaCu 2O 8+ δ films. In the underdoped film, we found evidence for coexistence of two excitation states due to superconducting gap Δ s and pseudogap Δ p below T C. The observed relaxation processes of optically excited quasi-particles were well explained by two relaxation times of T-dependent τ s and almost T-independent τ p (0.7±0.3 ps below 150 K). The τ p existed up to T *∼200 K and disappeared above 225 K. On the other hand, only one relaxation time, τ s, was observed in the overdoped film at T=10 K. The relaxation time, τ s=4.2±0.3 ps (below 40 K), for the underdoped sample was shorter than τ s=5.9±0.2 ps (at T=10 K) for the overdoped one, and about two times longer than τ s=2.1±0.1 ps for YBa 2Cu 3O 7− δ thin film.