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

Time-resolved measurements of quasiparticle (QP) relaxation dynamics on the femtosecond timescale are reported as a function of temperature and doping in YBa(2)Cu(3)O(7-delta) for (0.48 < delta < 0.1). In the underdoped state (delta > 0.15), there is no evidence for any changes in the low-energy gap structure at T(c) 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 delta = 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 T(c), indicating the existence of a temperature-dependent superconducting gap which closes at T(c).