Evidence for crossover from a Bose-Einstein condensate to a BCS-like superconductor with doping in YBa2Cu3O7−δ from quasiparticle relaxation

Time resolved measurements of quasiparticle (QP) relaxation dynamics on the femtosecond timescale are reported as a function of temperature and doping in YBa2Cu3O7−δ for (0.48 < δ < 0.1). In the underdoped state (δ > 0.15), there is no evidence for any changes in the low-energy gap structure at Tc from either photoinduced QP absorption or QP relaxation time data. In combination with the sum rule, this implies the existence of pre-formed pairs up to a much higher temperature T . Near δ = 0.1 a rapid cross-over is observed, to a state where the QP recombination time diverges and the photoinduced QP density falls to zero at Tc, indicating the existence of a temperature-dependent superconducting gap which closes at Tc. Combining the universal correlations between Tc, the ratio of carrier density to effective mass ns/m ∗ and pseudogap behaviour in the underdoped region Uemura [1] suggested that the phase diagram of high-temperature superconducting cuprates could be described in terms of a crossover from Bose-Einstein (BE) to BCS-like condensation. According to this scenario, in the underdoped state hole pairs form at temperatures above Tc which subsequently form a phase-coherent superconducting condensate at Tc with no associated change of pairing amplitude at this temperature, while in the BCS-like state suggested to exist in the overdoped cuprates an energy gap, phase coherence and superconductivity would all