Direct observation of the coexistence of pseudogap and superconducting quasi-particles in Bi2212 by time-resolved optical spectroscopy

TL;DR

This study uses time-resolved optical spectroscopy to directly observe the coexistence of pseudogap and superconducting quasiparticles in underdoped Bi2212, revealing their distinct dynamics and coexistence below the critical temperature.

Contribution

It provides direct experimental evidence of pseudogap and superconducting quasiparticles coexisting in Bi2212, with their charge dynamics distinguished by probe energy and polarization.

Findings

Sign change in reflectivity at T_c separates PG and SC quasiparticle dynamics.

Coexistence of PG and SC quasiparticles observed below T_c.

Charge dynamics depend on probe energy and polarization.

Abstract

We report the ultra-fast optical response of quasi-particles (QPs) in both the pseudogap (PG) and superconducting (SC) states of underdoped (UD) Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ (Bi2212) single crystal measured with the time-resolved pump-probe technique. At a probe energy $\hbar\omega_{pr}$=1.55 eV, it is found that the reflectivity change $\Delta$R/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.