Study of pseudogap and superconducting quasiparticle dynamics in $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{r}}_2\mathrm{CaC}{\mathrm{u}}_2{\mathrm{O}}_{8+\hat{\mathrm{I}}´}$ by time-resolved optical reflectivity

Abstract

The relation between pseudogap (PG) and superconducting (SC) gap, whether PG is a precursor of SC or they coexist or compete, is a long-standing controversy in cuprate high-temperature supercondutors. Here, we report ultrafast time-resolved optical reflectivity investigation of the dynamic densities and relaxations of PG and SC quasiparticles (QPs) in the underdoped $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{r}}_2\mathrm{CaC}{\mathrm{u}}_2{\mathrm{O}}_{8+\hat{\mathrm{I}}´}$ ($T_c$ = 82 K) single crystals. We find evidence of two distinct PG components in the positive reflectivity changes in the PG state, characterized by relaxation timescales of $\ddot{I}{„}_{fast}$ $\approx$ 0.2 ps and $\ddot{I}{„}_{slow}$ $\approx$ 2 ps with abrupt changes in both amplitudes $A_{fast}$ and $A_{slow}$ at the PG-opening temperature $T^{\ast }$. The former presents no obvious change at $T_c$ and coexists with the SC QP. The latter's amplitude starts decreasing at the SC phase fluctuation $T_p$ and vanishes at $T_c$ followed by a negative amplitude signifying the emergence of the SC QP, therefore suggesting a competition with superconductivity.

5 pages, 4 figures