Direct Observation of the Coexistence of the Pseudogap and Superconducting Quasiparticles in Bi_2Sr_2CaCu_2O_(8+y) by Time-Resolved Optical Spectroscopy

摘要

We report the ultrafast optical response of quasiparticles (QPs) in both the pseudogap (PG) and superconducting (SC) states of an underdoped Bi_2Sr_2CaCu_2O_(8+y) (Bi2212) single crystal measured with the time-resolved pump-probe technique. At a probe energy hω_(pt) = 1.55 eV, it is found that the reflectivity change ΔR/R changes its sign at exactly T_c, which allows the direct separation of the charge dynamics of PG and SC QPs. Further systematic investigations indicate that the transient signals associated with PG and SC QPs depend on the probe beam energy and polarization. By tuning them below T_c, two distinct components can be detected simultaneously, providing evidence for the coexistence of PG and SC QPs.