A calorimetric investigation of RbFe sub(2)As sub(2) single crystals

We present properties of single crystals of the iron pnictide superconductor RbFe sub(2)As sub(2) grown using a self-flux method. A bulk superconducting transition at [Formulaomitted] [Formulaomitted]2.6K was consistently observed in resistivity, magnetic susceptibility, and specific heat measurements. While the resistivity behaves like a conventional metal at low T, the magnetic susceptibility shows an unusual broad maximum around 80K in the normal state. A large Sommerfeld coefficient, [Formulaomitted], was observed seemingly a common feature of heavily hole-doped "122" compounds. In the superconducting state, we analyzed our specific heat data based on the s- and d-wave two-gap models. The presence of the small gap with 2[Formulaomitted]/[Formulaomitted] was discussed to account for the large specific heat contribution remaining to low T far below [Formulaomitted]. The large [Formulaomitted] anisotropy ratio, [Formulaomitted] = [Formulaomitted]/[Formulaomitted] [Formulaomitted] 7 near [Formulaomitted] indicates that the large gap would occupy a highly two-dimensional Fermi surface.