Femtosecond quasiparticle relaxation dynamics and probe polarization anisotropy in YSrxBa2-xCu4O8 (x=0,0.4)

Femtosecond pump probe experiments are reported on quasiparticle relaxation and recombination in ${\mathrm{YSr}}_{x}{\mathrm{Ba}}_{2\ensuremath{-}x}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8}$ as a function of temperature and polarization. The data show a two-component relaxation similar to ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}},$ one component being associated with the superconducting transition, and the other with the pseudogap below ${T}^{*}.$ The relaxation time ${\ensuremath{\tau}}_{p}$ associated with the pseudogap is found to be T independent, while the relaxation time ${\ensuremath{\tau}}_{G}$ of the component observed only below ${T}_{c}$ exhibits a clear divergence near ${T}_{c}.$ A strong polarization anisotropy of the picosecond transient is observed below ${T}_{c}$ which is attributed to the anisotropy of the probe transition matrix elements.