Evidence for crossover from a Bose-Einstein condensate to a BCS-like   superconductor with doping in YBa2Cu3O(7-x) 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 YBa2Cu3O(7-x) for (0.48 < x < 0.1). In the underdoped state (x > 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 x = 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. 
J.Demsar (1), B.Podobnik (1), J.Evetts (2), G.Wagner (2) and   D.Mihailovic (1) ((1)Jozef Stefan Institute, Ljubljana, Slove...