Ultrafast quasiparticle relaxation dynamics in high quality epitaxial FeSe sub(0.5)Te sub(0.5) thin films

Femtosecond spectroscopy has been used to investigate the quasiparticle relaxation times in a nearly optimally doped chalcogenide superconducting FeSe sub(0.5)Te sub(0.5) thin film. It is characterized by an enhancement of the critical temperature T sub(c) = 19:1 K by strain. Experimental results indicate the existence of a temperature dependence of the quasiparticle recombination time [tau] sub(R) in the superconducting state where an energy gap Delta (T) is present. A fitting procedure based on a BCS-like assumption for [tau] sub(R) gives Delta sub(0) = 4.1 + or - 0.4 meV for the superconducting energy gap extrapolated at T + 0, which corresponds to 2 Delta sub(0)/k sub(B)T sub(c) = 4.9 = or - 0.5. Accordingly, an estimation of the characteristic phonon energy and the electron-phonon coupling constant [lambda] is reported and discussed. The obtained value [lambda] approximately 0.6 is in good agreement with other similar determinations obtained by different techniques and numerical approaches. The small value of [lambda] confirms that a phonon-mediated coupling cannot be the only mechanism leading to the formation of the superconducting state in FeSeTe films.