Features of excess conductivity and a possible pseudogap in FeSe superconductors

The temperature dependence of excess conductivity σ′(Т) has been studied in three polycrystalline samples of the FeSe0.94 superconductor, prepared by different technologies. The measured temperature dependences of the Δ*(T) parameter, which is associated with the pseudogap in cuprates, were analyzed using the local pair model. At high temperatures, all three samples exhibit a high narrow maximum along Δ*(T) at Ts1∼250 K, which is typical for magnetic superconductors. Below T ≈ 225 K, the dependences Δ*(T) become different. Over almost the entire temperature range below Ts1, the S2 sample, prepared by solid state reaction without impurities, exhibits a Δ*(T) that is typical for Fe-pnictides. An exception is the interval between the structural change temperature Ts = 85 K and Tc, where this Δ*(T) exhibits an atypical, broad maximum. An analysis of the obtained dependence suggests the discovery of a pseudogap in this FeSe0.94 sample, below Ts. Samples S1, containing 4 wt.%Ag, and S3, having a nominal composi...