Divergence of the quasiparticle lifetime with doping and evidence for pre-formed pairs below T* in YBa_2Cu3$O_{7-\delta}

TL;DR

This study uses femtosecond optical spectroscopy to investigate quasiparticle dynamics in YBa_2Cu_3O_{7-\delta}, revealing a pseudogap below T* and evidence of pre-formed pairs above T_c, with different behaviors across doping levels.

Contribution

It provides new time-resolved measurements showing the evolution of the pseudogap and pairing dynamics across doping levels in YBCO, highlighting the distinction between phase coherence and pair formation.

Findings

Pseudogap exists below T* in underdoped YBCO.

Quasiparticle relaxation time diverges with doping towards optimal doping.

No gap opens at T_c in underdoped YBCO, indicating phase coherence occurs without a gap.

Abstract

We report new time-resolved data of quasiparticle relaxation and Cooper pair recombination dynamics in YBa_2Cu_3O_{7-\delta}, measured as a function of temperature and doping \delta using femtosecond optical spectroscopy. The data show the existence of a normal state pseudo-gap for in-plane charge excitations below T* and an unusual divergence of the quasiparticle relaxation time in the superconducting state \tau_s with \delta towards optimum doping. In the underdoped state, no change in the amplitude of the induced transmission (which is proportional to the DOS at E_F), or relaxation time \tau_s is observed at T_c. From the sum rule, both observations signify that no gap opens at T_c in underdoped YBCO for \delta >0.15. T_c in this case only signifies the onset of phase coherence. The presented data thus suggest pair formation with an associated redistribution of the DOS starting at T* and the establishment of phase coherence at T_c consistent with Bose-Einstein condensation. In the optimally doped material, \delta ~ 0.1, on the other hand, both a divergence in lifetime and a change of the DOS occur at T_c, signifying the opening of a gap and the occurence of pairing takes place simultaneously.