Evolution of the Gap Structure from Underdoped to Optimally Doped YBa₂Cu₃O_7−δ from Femtosecond Optical Spectroscopy

Abstract

The evolution of the gap structure with doping in YBa₂Cu₃O_7−δ is investigated with real-time optical time-resolved spectroscopy measurements of quasi-particle (QP) recombination dynamics. In the underdoped phase the existence of a T-independent gap for charge excitations is found. Its presence becomes apparent when kT^*∼Δ, where T^* (>T_c), in good agreement with many other experimental techniques. Approaching optimum doping another T-dependent gap becomes apparent which opens at T_c. Careful examination of the data near optimum doping reveals that the T-independent and T-dependent gaps coexist and have approximately the same value. Both gaps were found to be nodeless, whereas a d wave gap was found to be incompatible with the data. In the underdoped phase, where no change in DOS is observed at T_c we conclude that phase coherence is established at T_c with no change in pairing amplitude at this temperature.