  We comparatively investigated the transport properties for S- and
Te-substituted FeSe thin films under magnetic fields to clarify the origin of
the contrasting behavior of the superconducting transition temperature in S and
Te substitution. A classical two carrier analysis revealed that the carrier
densities of the films increased with increasing Te content, while no
significant change was observed for the S-substitution. This composition
dependence of the carrier density well corresponds to the $T_{\mathrm c}$
behavior. The clear correlation between $T_{\mathrm c}$ and the carrier
densities suggests that the structural transition affects the electronic
structure in a different manner between Fe(Se,S) and Fe(Se,Te) and that this
fact is the direct cause of the difference in the $T_{\mathrm c}$ behaviors at
the end point of the structural transition.
