Ultrafast quasiparticle relaxation dynamics in high quality epitaxial FeSe0.5Te0.5 thin films

Femtosecond spectroscopy has been used to investigate the quasiparticle relaxation times in a nearly optimally doped chalcogenide superconducting FeSe0.5Te0.5 thin film. It is characterized by an enhancement of the critical temperature Tc = 19.1 K by strain. Experimental results indicate the existence of a temperature dependence of the quasiparticle recombination time ?R in the superconducting state where an energy gap ?(T) is present. A fitting procedure based on a BCS-like assumption for ?R gives ?0 = 4.1 ± 0.4 meV for the superconducting energy gap extrapolated at T = 0, which corresponds to 2?0/kBTc = 4.9 ± 0.5. Accordingly, an estimation of the characteristic phonon energy and the electron-phonon coupling constant ? is reported and discussed. The obtained value ? ? 0.6 is in good agreement with other similar determinations obtained by different techniques and numerical approaches. The small value of ? confirms that a phonon-mediated coupling cannot be the only mechanism leading to the formation of the superconducting state in FeSeTe films. © 2013 IOP Publishing Ltd.