Quasiparticle relaxation dynamics in superconductors with different gap structures: theory and experiments on YBa_{2}Cu_{3}O_{7-δ}

Abstract

Photoexcited quasiparticle relaxation dynamics are investigated in a $YBa_{2}Cu_{3}O_{7-δ}$ superconductor as a function of doping $δ$ 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 $Δ{\cal T}/{\cal T}$ and relaxation time $τ$ for three different superconducting gaps: (i) a temperature-dependent collective gap such that ${\bf Δ}(T)\to 0$ 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 $| Δ{\cal T}/{\cal T} |$, reflection $| Delta{\cal R}/{\cal R}|$ and quasiparticle recombination time $τ$ in $YBa_{2}Cu_{3}O_{7-δ}$ 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 $YBa_{2}Cu_{3}O_{7-δ}$ ($0.15<δ<0.48$). A pure d-wave gap was found to be inconsistent with the data.