Observation of pseudogaplike feature in LiFeAs by ultrafast optical spectroscopy

We utilize ultrafast optical spectroscopy to study the quasiparticle relaxation in stoichiometric LiFeAs crystals. According to our temperature-dependent studies, we have observed three electronic phases in LiFeAs. Below the superconducting (SC) temperature Tc ($=$ 15 K), the relaxation time of quasiparticles due to the SC gaps is far longer than 50 ps in our experimental conditions. In addition to SC gaps, we have also found a gaplike feature in SC state. This gaplike feature is evident up to 40 K, which is above the SC temperature. Because this is similar to pseudogap in curate superconductors, we call this new electronic phase as pseudogaplike feature. The quasiparticle relaxation time due to pseudogaplike feature is in the range of 1-2 ps. We suggest the pseudogaplike feature in LiFeAs is induced by magnetic fluctuations.