Majority carrier type inversion in the FeSe family and a ‘doped semimetal’ scheme in iron-based superconductors

The field and temperature dependencies of the longitudinal and Hall components of resistivity have been studied for high-quality FeSe1–xSx (x up to 0.14) single crystals. A quasiclassical analysis of the experimental data indicates a strong variation of electron and hole concentrations under the studied isovalent substitution and a clear trend towards 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 probably 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. A multicarrier analysis of the experimental data also reveals the presence of a tiny and highly mobile electron band for all the samples studied. Substitution for sulfur in the studied range leads to one order decreasing in the density of highly mobile electrons, from 3% to 0.2% of the total carrier concentration. The mobility of these carriers does not depend on impurities, which may indicate an enhanced electron–phonon interaction and allows us to consider the highly mobile carriers as a possible source of unusual acoustic properties of FeSe.