Electron-boson coupling constant of oxygen-deficient Y_0.7Ca_0.3Ba₂Cu₃O_7-δ measured by ultrafast spectroscopy

The ultrafast dynamics in an (001) oxygen-deficient Y _0.7Ca _0.3Ba ₂Cu ₃O _7-δ thin film were measured by using the ultrafast spectroscopy. According to the model of time-dependent distributions of electrons in the case of electron-phonon relaxation, the electron-boson coupling constant λ could be directly obtained from the relaxation time of the transient reflectivity change (ΔR/R). For the overdoped case with T _c = 61.3 K and the underdoped case with T _c = 65.5 K, the relaxation time of ΔR/R is around 4.32 ps and 3.66 ps, respectively, which correspond to λ ∼ 0.06. However, the λ become larger (∼0.15) in the optimal doped region with T _c = 82.1 K while the relaxation time of ΔR/R shrinks to ∼1.51 ps. These results indicate that the λ is strongly hole-doping dependent, which may govern the T _c of cuprate superconductors.