Doping-dependent phase diagram of Ca-doped YBCO observed by femtosecond spectroscopy

The quasiparticle relaxation dynamics in a single (001) Ca-doped YBCO (Y _0.7Ca _0.3Ba ₂Cu ₃O _7-δ) superconducting thin film was probed by using the femtosecond timeresolved spectroscopy. The (001) Y _0.7Ca _0.3Ba ₂Cu ₃O _7-δ thin film was prepared on a (100)STO substrate by pulsed laser deposition. Through controlling the oxygen pressure and the annealing temperature within quartz tube, the doping level (0.08 < p < 0.22) in the phase diagram could be widely tuned from the overdoped region to the underdoped region with varying the hole concentration (p) in a single sample. In the overdoped region, two different components (positive/negative) were obviously identified in the transient reflectivity curves (ΔR/R). The negative component in ΔR/R emerges at T < T _c and quickly defeated by the development of the positive component. The dramatic change in the positive component of ΔR/R arises well below T _c. However, this anomaly change does not appear in the underdoped region. Only the positive ΔR/R was clearly observed at whole temperatures. These results indicate that the scenario of high-T _c superconductivity in the overdoped region would be different from the underdoped region.