Photoexcited carrier relaxation and localization in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_{1\mathrm{-}\mathit{y}}$${\mathrm{Y}}_{\mathit{y}}$${\mathrm{Cu}}_2$${\mathrm{O}}_8$ and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$: A study by femtosecond time-resolved spectroscopy

Ultrafast time-resolved optical absorption has been measured as a function of doping and temperature in ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_{1\mathrm{-}\mathit{y}}$ ${\mathrm{Y}}_{\mathit{y}}$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_8$ (0<y<1) and ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (0.06 <δ <1). A fast transient response is observed, which changes sign as a function of y in ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_{1\mathrm{-}\mathit{y}}$ ${\mathrm{Y}}_{\mathit{y}}$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_8$ and δ in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. Careful consideration of the possible mechanisms for the observed behavior leads us to suggest that two independent mechanisms are operating, whereby the two observed positive photoinduced absorption components are due to the electrodynamic response of photoinduced carriers, while the negative signal arises due to spectral hole burning. In the metallic samples a long-lived component is observed, which we suggest comes from photoinduced localized states intrinsic only to the superconducting species of the high-${\mathit{T}}_{\mathit{c}}$ materials. A very significant increase in the scattering rate at temperatures just above ${\mathit{T}}_{\mathit{c}}$ signifies coupling to charge and possibly spin fluctuations just above ${\mathit{T}}_{\mathit{c}}$. A large reduction in scattering rate is observed below ${\mathit{T}}_{\mathit{c}}$, similar to that observed previously in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.9}$ and ${\mathrm{Tl}}_2$ ${\mathrm{Ba}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$. © 1996 The American Physical Society.