Electronic specific heat of the iron chalcogenide superconductor Fe(Te$_{0.55}$Se$_{0.45})$

We report specific heat studies of superconducting Fe(Te$_{0.55}$Se$_{0.45})$[1]. We have obtained the electronic specific heat by subtracting the phonon contribution evaluated from the normalization of the phonon specific heat of a non-superconducting reference sample (Fe$_{0.9}$Cu$_{0.1})$(Te$_{0.55}$Se$_{0.45})$. Our results show that the superconducting ground state is accompanied by unpaired quasiparticles, as in FeAs superconductors, with \textit{$\gamma $}$_{0 }$\textit{$\sim $}2.3 mJ/mol K$^{2}$. The temperature dependence of the electronic specific heat $C_{es}(T)$/$T$ can be well fitted using either a single s-wave gap model with 2\textit{$\Delta $}= 5.2 k$_{B}T_{c}$ or a two-gap model with 2\textit{$\Delta $}$_{1}$/k$_{B}T_{c }$= 5.8 and 2\textit{$\Delta $}$_{2}$/k$_{B}T_{c}$ = 4.0; the two-gap model fitting is slightly better than the single gap fitting. Such large gaps, together with a large specific heat jump $\Delta C(T_{c})$/$T_{c}\sim $57.3 mJ/mol K$^{2}$, suggest a strong-coupling superconducting state. While $C_{es}(T)$/$T$ exhibits isotropic s-wave gap behavior, the magnetic field-induced change in the electronic specific heat $\Delta $\textit{$\gamma $}($H)$ exhibits sublinear field dependence, implying the superconducting pairing in iron chalcogenide superconductors also involves a multiple band effect, as seen in pnictide superconductors. [1] M. H. Fang, H. M. Pham, B. Qian, T. J. Liu, E. K. Vehstedt, Y. Liu, L. Spinu, and Z. Q. Mao, Superconductivity close to magnetic instability in Fe(Se$_{1-x}$Te$_{x})_{0.82}$, Phys. Rev. B 78, 224503 (2008).