Femtosecond spectroscopy of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$: Electron-phonon-interaction measurement and energy-gap observation

The femtosecond dynamics of reflection and transmission spectra of a ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ film have been studied in the wavelength range 620–680 nm at initial temperatures ${\mathit{T}}_0$ both above and below the critical temperature ${\mathit{T}}_{\mathit{c}}$. A difference has been found to exist between the spectral responses for ${\mathit{T}}_0$<${\mathit{T}}_{\mathit{c}}$ and ${\mathit{T}}_0$>${\mathit{T}}_{\mathit{c}}$, which can be attributed to the presence of an energy gap in the spectrum at ${\mathit{T}}_0$<${\mathit{T}}_{\mathit{c}}$. Based on the experimental data, the electron-phonon-interaction parameter λ〈${\mathrm{\omega }}^2$〉 has been estimated as (4±2)×${10}^2$ ${\mathrm{meV}}^2$.