Study of pseudogap and superconducting quasiparticle dynamics in $\rm{Bi_2Sr_2CaCu_2O_{8+\delta}}$ by time-resolved optical reflectivity

TL;DR

This study uses ultrafast optical reflectivity to investigate the dynamics of pseudogap and superconducting quasiparticles in underdoped Bi-2212, revealing two distinct pseudogap components with different behaviors relative to the superconducting transition.

Contribution

It provides direct experimental evidence of two pseudogap components with different relaxation timescales and their relation to superconductivity in cuprate high-temperature superconductors.

Findings

Two distinct pseudogap components with different relaxation timescales.

One pseudogap component coexists with superconducting quasiparticles.

The other pseudogap component competes with superconductivity, vanishing at Tc.

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 $\rm{Bi_2Sr_2CaCu_2O_{8+\delta}}$ ($T_c$ = 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 $\tau_{fast}$ $\approx$ 0.2 ps and $\tau_{slow}$ $\approx$ 2 ps with abrupt changes in both amplitudes $A_{fast}$ and $A_{slow}$ at the PG-opening temperature $T^*$. 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.