To investigate the multi-excitation processes on the low-lying electronic structure of high-TC superconductors, time-domain pump and probe measurement has been carried out on under- and over-doped Bi2Sr2CaCu2O8+δ films. In the underdoped film, we found evidence for coexistence of two excitation states due to superconducting gap Δs and pseudogap Δp below TC. The observed relaxation processes of optically excited quasi-particles were well explained by two relaxation times of T-dependent τs and almost T-independent τp (0.7 ± 0.3 ps below 150 K). The τp existed up to T* ∼ 200 K and disappeared above 225 K. On the other hand, only one relaxation time, τs, was observed in the overdoped film at T = 10 K. The relaxation time, τs=4.2 ± 0.3 ps (below 40 K), for the underdoped sample was shorter than τs = 5.9 ± 0.2 ps (at T = 10 K) for the overdoped one, and about two times longer than τs = 2.1 ± 0.1 ps for YBa2Cu3O7-δ thin film.
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