Ultrafast quasiparticle relaxation dynamics in high quality epitaxial FeSe0.5Te0.5thin 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 T-c = 19.1 K by strain. Experimental results indicate the existence of a temperature dependence of the quasiparticle recombination time tau(R) in the superconducting state where an energy gap Delta(T) is present. A fitting procedure based on a BCS- like assumption for tau(R) gives Delta(0) = 4.1 +/- 0.4 meV for the superconducting energy gap extrapolated at T = 0, which corresponds to 2 Delta(0)/k(B)T(c) = 4.9 +/- 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 approximate to 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