Majority carrier type inversion in FeSe family and "doped semimetal" scheme in iron-based superconductors

The field and temperature dependencies of the longitudinal and Hall resistivity have been studied for high-quality FeSe${}_{1-x}$S${}_{x}$ (x up to 0.14) single crystals. Quasiclassical analysis of the obtained data indicates a strong variation of the electron and hole concentrations under the studied isovalent substitution and proximity of FeSe to the point of the majority carrier-type inversion. On this basis, we propose a `doped semimetal' scheme for the superconducting phase diagram of the FeSe family, which can be applied to other iron-based superconductors. In this scheme, the two local maxima of the superconducting temperature can be associated with the Van Hove singularities of a simplified semi-metallic electronic structure. The multicarrier analysis of the experimental data also reveals the presence of a tiny and highly mobile electron band for all the samples studied. Sulfur substitution in the studied range leads to a decrease in the number of mobile electrons by more than ten times, from about 3\% to about 0.2\%. This behavior may indicate a successive change of the Fermi level position relative to singular points of the electronic structure which is consistent with the `doped semimetal' scheme. The scattering time for mobile carriers does not depend on impurities, which allows us to consider this group as a possible source of unusual acoustic properties of FeSe.