Quasiparticle relaxation dynamics in superconductors with different gap structures: Theory and experiments on YBa₂Cu₃O_7-δ

Photoexcited quasiparticle relaxation dynamics are investigated in a YBa₂Cu₃O_7-δ superconductor as a function of doping δ and temperature T using ultrafast time-resolved optical spectroscopy. A model calculation is presented that describes the temperature dependence of the photoinduced quasiparticle population n_pe, photoinduced transmission ΔT/T, and relaxation time τ for three different superconducting gaps: (i) a temperature-dependent collective gap such that Δ(T)→0 as T⃗T_c, (ii) a temperature-independent gap, which might arise for the case of a superconductor with preformed pairs, and (iii) an anisotropic (e.g., d-wave) gap with nodes. Comparison of the theory with data of photoinduced transmission |ΔT/T|, reflection |ΔR/R|, and quasiparticle recombination time τ in YBa₂Cu₃O_7-δ over a very wide range of doping (0.1<δ<0.48) is found to give good quantitative agreement with a temperature-dependent BCS-like isotropic gap near optimum doping (δ<0.1) and a temperature-independent isotropic gap in underdoped YBa₂Cu₃O_7-δ(0.15<δ<0.48). A pure d-wave gap was found to be inconsistent with the data.