Quasiparticle relaxation dynamics in superconductors with different gap structures: theory and experiments on YBa₂Cu₃O_(7-δ)

Photoexcited quasiparticle relaxation dynamics are investigated in a $YBa_{2}Cu_{3}O_{7-\delta}$ superconductor as a function of doping $\delta$ and temperature $T$ using ultrafast time-resolved optical spectroscopy. A model calculation is presented which describes the temperature dependence of the photoinduced quasiparticle population $n_{pe}$, photoinduced transmission $\Delta{\cal T}/{\cal T}$ and relaxation time $\tau $ for three different superconducting gaps: (i) a temperature-dependent collective gap such that ${\bf \Delta}(T)\to 0$ as $T\to T_{c}$, (ii) a temperature-independent gap, which might arise for the case of a superconductor with pre-formed pairs and (iii) an anisotropic (e.g. d-wave) gap with nodes. Comparison of the theory with data of photoinduced transmission $| \Delta{\cal T}/{\cal T} |$, reflection $| Delta{\cal R}/{\cal R}|$ and quasiparticle recombination time $\tau$ in $YBa_{2}Cu_{3}O_{7-\delta}$ over a very wide range of doping ($0.1 < \delta <0.48)$ is found to give good quantitative agreement with a temperature-dependent BCS-like isotropic gap near optimum doping ($\delta <0.1)$ and a temperature-independent isotropic gap in underdoped $YBa_{2}Cu_{3}O_{7-\delta}$ ($0.15<\delta <0.48$). A pure d-wave gap was found to be inconsistent with the data.