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 Bi₂Sr₂CaCu₂O_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 YBa₂Cu₃O_7-δ thin film.