Single-quasiparticle stability and quasiparticle-pair decay in YBa2Cu3O6.5 (Ortho II)

TL;DR

This study investigates quasiparticle dynamics in YBa2Cu3O6.5 using time-resolved reflectivity, revealing how decay rates depend on temperature and excitation intensity, and providing insights into quasiparticle interactions near the antinodal regions.

Contribution

It provides the first detailed analysis of quasiparticle decay rates and their dependence on temperature and laser intensity in YBa2Cu3O6.5, highlighting the role of electron-electron and electron-phonon interactions.

Findings

Decay rate gamma decreases linearly with laser intensity at low T.

Extrapolated zero-intensity decay rate indicates intrinsic quasiparticle lifetime.

Quasiparticles primarily occupy antinodal states near the Brillouin zone.

Abstract

We report results and analysis of time-resolved photoinduced reflectivity experiments on the cuprate superconductor YBa2Cu3O6.5. The sample, which has Tc=45 K, was characterized by a high degree of purity and Ortho II ordering. The change in reflectivity (Delta R) 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 Delta R 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 temperatures gamma has the same linear dependence, but with nonzero limit as I goes to zero. In the interpretation of these results we assume that Delta R 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 as T and I both tend to zero as a consequence of momentum and energy conservation in electron-electron scattering. We discuss the significance of the measured value of beta (approximately 0.1 cm2/s) in terms of the electron-phonon interactions.