Divergence of the quasiparticle lifetime with doping and evidence for pre-formed pairs below T* in YBa2Cu3O7-delta direct measurements by femtosecondtime-resolved spectroscopy

We report new time-resolved data of quasiparticle relaxation and Cooper pai r recombination dynamics in YBa2Cu3O7-delta, measured as a function of temp erature and doping delta using femtosecond optical spectroscopy. The data s how the existence of a normal stare pseudo-gap for in-plane charge excitati ons below T* and an unusual divergence of the quasiparticle relaxation time in the superconducting stare tau(s) with delta towards optimum doping. In the underdoped state, no change in the amplitude of the induced transmissio n (which is proportional to the DOS at E-F) Or relaxation time tau(s) is ob served at T-c. From the sum rule, both observations signify that no gap ope ns at T-c in underdoped YBCO for delta > 0.15. T-c in this case only signif ies the onset of phase coherence. The presented data thus suggest pair form ation with an associated redistribution of the DOS starting at T* and the e stablishment of phase coherence at T-c consistent with Bose-Einstein conden sation. In the optimally doped material, delta approximate to 0.1, on the o ther hand, both a divergence in lifetime and a change of the DOS occur at T -c, signifying the opening of a gap and the occurrence of pairing takes pla ce simultaneously. (C) 1998 Elsevier Science Ltd. All rights reserved.