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

TL;DR

This study investigates the carrier type inversion in FeSe${}_{1-x}$S${}_{x}$ superconductors, proposing a 'doped semimetal' model that links electronic structure changes to superconducting properties and reveals a highly mobile electron band.

Contribution

It introduces a 'doped semimetal' scheme for iron-based superconductors, connecting electronic structure variations with superconducting phase behavior and carrier dynamics.

Findings

Carrier type inversion occurs near FeSe with sulfur substitution.

A tiny, highly mobile electron band is present in all samples.

Sulfur substitution reduces mobile electron concentration significantly.

Abstract

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.