Features of excess conductivity and a possible pseudogap in FeSe superconductors

Abstract

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 composition but containing nonsuperconducting hexagonal phase inclusions, both prepared by partial melting, show identical Δ*(T), but different from S2. They have a number of features that correlate with temperatures at which there are also features along M(T), and the Hall coefficient RH(T) changes signs several times with decreasing T, which indicates that there is change in the type of charge carriers in FeSe. The Δ*(T) dependence of the S3 sample below Ts has almost no maximum, since the nonsuperconducting impurities of the hexagonal phase in S3 prevent the formation of paired fermions near Tc. As a result, S3 also has the minimum local pair density <n↑n↓> = 0.26, determined by comparing Δ*(TG)/Δmax near Tc using the Peters–Bauer theory, whereas the dependence Δ*(T) does not follow the theory. S1 has the maximum <n ↑ n ↓> = 0.47, supposedly due to the influence of Ag impurities. In S2, which is pure, <n ↑ n ↓> ≈ 0.3, which is the same as that of YBa2Cu3O7−δ, and both dependences Δ*(Т) for S1 and S2 follow the theory over a wide temperature range.