Single-quasiparticle stability and quasiparticle-pair decay in YBa2Cu3O6.5 -: art. no. 014504

We report results and analysis of time-resolved photoinduced reflectivity experiments on the cuprate superconductor YBa2Cu3O6.5. The sample, which has T-c=45 K, was characterized by a high degree of purity and Ortho II ordering. The change in reflectivity DeltaR was induced and probed using pulses of 100 femtosecond duration and photon energy 1.55 eV from a Ti:Sapphire laser. We provide a detailed picture of the decay rate gamma of DeltaR as a function of temperature T and pump intensity I. At low T, gamma decreases linearly with decreasing I, extrapolating to nearly zero in the limit that I tends to zero. At higher temperature gamma has the same linear dependence, but with nonzero limit as I-->0. In the interpretation of these results we assume that DeltaR is proportional to the nonequilibrium quasiparticle density created by the laser. From an analysis of the gamma vs I we estimate beta, the coefficient of proportionality relating the quasiparticle decay rate to the density. The intercept of gamma vs I yields the thermal equilibrium quasiparticle decay rate. In a discussion section, we argue that the quasiparticles induced by the laser occupy primarily states near the antinodal regions of the Brillouin zone. We explain the divergence of the lifetime of these particles as T and I both tend to zero as a consequence of momentum and energy conservation in electron-electron scattering. Next, we discuss the significance of the measured value of beta, which is approximate to0.1 cm(2) s(-1) . We point out that the natural unit for beta in a two-dimensional superconductor is h/m(*), and define a dimensionless constant C such that betaequivalent toCh/m(*). If the decay process is one in which quasiparticles return to the condensate with emission of a phonon, then C is a measure of the electron-phonon interaction. Alternatively, expressing the marginal Fermi liquid scattering in the normal state in terms of an effective beta implies C=1/pi, which is in excellent agreement with the experimentally determined value in the superconducting state.