Transport properties of FeSe${}_{1-x}$S${}_{x}$ series. Quantum criticality and Van Hove scenario of superconductivity

The field and temperature dependencies of the longitudinal and Hall resistivity have been measured for high-quality FeSe${}_{1-x}$S${}_{x}$ (x up to 0.14) single crystals. Our data show the proximity of FeSe to the quantum critical point caused by inversion of the major carriers. The quasiclassical multicarrier analysis indicates a strong variation of the carriers concentrations under the isovalent substitution which reflects the overall instability of the electronic properties of the studied series. Besides, the multicarrier analysis reveals the presence of a tiny and highly mobile electron band in all the samples studied. We consider this band as a property of small pockets that appear when the Van Hove singularity is close to the Fermi level. 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 an increase in the distance between the Van Hove singularity and the Fermi level. At the same time, this also means that the position of the Van Hove singularity has no direct effect on the superconducting transition temperature in the studied series.