Quasiparticle relaxation dynamics in cuprates and lifetimes of low-energy states: Femtosecond data from underdoped to overdoped YBCO and mercury compounds.

We show that low-energy spectral features in the cuprates can be separated into different components by the measurement of the recombination dynamics of different low-energy excitations in real-time using femtosecond laser sp ectroscopy. Quasiparticle (QP) recombination across the gap and intra-gap l ocalized state relaxation processes exhibit qualitatively different time- a nd temperature-dependences. The relaxation measurements also show the exist ence of two distinct coexisting energy gaps near optimum doping and in the overdoped region, one more or less temperature independent (which exists ab ove and below T-c) and one which closes at T-c in a mean-field like fashion . Thus systematic studies of QP recombination as a function of doping and t emperature suggest that the ground state of the cuprates is a mixed Boson-F ermion system with localised states present over the entire region of the p hase diagram.