  The Hall effect is investigated in thin-film samples of iron-chalcogenide
superconductors in detail. The Hall coefficient (RH) of FeTe and Fe(Se1-xTex)
exhibits a similar positive value around 300 K, indicating that the
high-temperature normal state is dominated by hole-channel transport. FeTe
exhibits a sign reversal from positive to negative across the transition to the
low-temperature antiferromagnetic state, indicating the occurrence of drastic
reconstruction in the band structure. The mobility analysis using the carrier
density theoretically calculated reveals that the mobility of holes is strongly
suppressed to zero, and hence the electric transport looks to be dominated by
electrons. The Se substitution to Te suppresses the antiferromagnetic
long-range order and induces superconductivity instead. The similar mobility
analysis for Fe(Se0.4Te0.6) and Fe(Se0.5Te0.5) thin films shows that the
mobility of electrons increases with decreasing temperature even in the
paramagnetic state, and keeps sufficiently high values down to the
superconducting transition temperature. From the comparison between FeTe and
Fe(Se1-xTex), it is suggested that the coexistence of 'itinerant' carriers both
in electron and hole channels is indispensable for the occurrence of
superconductivity.
