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 $YB{a}_{2}C{u}_3{O}_{7-\hat{I}´}$ superconductor as a function of doping $\hat{I}´$ 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 $\hat{I}”\mathcal{T}/\mathcal{T}$ and relaxation time $\ddot{I}„$ for three different superconducting gaps: (i) a temperature-dependent collective gap such that $\hat{\mathbf{I}}”(T)\to 0$ 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 $|\hat{I}”\mathcal{T}/\mathcal{T}|$, reflection $|Delta\mathcal{R}/\mathcal{R}|$ and quasiparticle recombination time $\ddot{I}„$ in $YB{a}_{2}C{u}_3{O}_{7-\hat{I}´}$ over a very wide range of doping ($0.1<\hat{I}´<0.48)$ is found to give good quantitative agreement with a temperature-dependent BCS-like isotropic gap near optimum doping ($\hat{I}´<0.1)$ and a temperature-independent isotropic gap in underdoped $YB{a}_{2}C{u}_3{O}_{7-\hat{I}´}$ ($0.15<\hat{I}´<0.48$). A pure d-wave gap was found to be inconsistent with the data.

12 pages, 7 ps figures, to be published in Phys. Rev. B