Specific-heat study of superconducting and normal states in FeSe ^1-xTe ^x (0:6 < x < 1) single crystals: Strong-coupling superconductivity, strong electron-correlation, and inhomogeneity

The electronic specific heat of as-grown and annealed single-crystals of FeSe _1-xTe _x (0:6 ≤ x ≤ 1) has been investigated. It has been found that annealed single-crystals with x = 0:6-0:9 exhibit bulk superconductivity with a clear specific-heat jump at the superconducting (SC) transition temperature, T _c. Both Uδ ₀=kBT _c [Δ ₀: the SC gap at 0K estimated using the single-band BCS s-wave model] and ΔC=(γ _n =γ ₀)T _c [AC: the specific-heat jump at T _c, γ _n: the electronic specific-heat coefficient in the normal state, γ ₀: the residual electronic specific-heat coefficient at 0K in the SC state] are largest in the well-annealed single-crystal with x = 0:7, i.e., 4.29 and 2.76, respectively, indicating that the superconductivity is of the strong coupling. The thermodynamic critical field has also been estimated. yn has been found to be one order of magnitude larger than those estimated from the band calculations and increases with increasing x at x = 0:6{0:9, which is surmised to be due to the increase in the electronic effective mass, namely, the enhancement of the electron correlation. It has been found that there remains a finite value of γ ₀ in the SC state even in the well-annealed single-crystals with x = 0:8-0:9, suggesting an inhomogeneous electronic state in real space and/or momentum space.