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

Abstract

We report new time-resolved data of quasiparticle relaxation and Cooper pair recombination dynamics in YBa_2Cu_3O_{7-δ}, measured as a function of temperature and doping δusing 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 τ_s with δtowards 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 τ_s is observed at T_c. From the sum rule, both observations signify that no gap opens at T_c in underdoped YBCO for δ>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, δ~ 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.