Thermodynamic study of an incommensurate organic superconductor (MDT-TSF)(AuI₂)_0.436

Low-temperature heat capacity measurements are performed on the organic superconductor (MDT-TSF)(AuI₂)_0.436; its critical temperature determined by the transport measurement is 4.5 K. A broad but reproducible thermal anomaly related to the superconducting transition is observed during precise measurements of the heat capacity under magnetic fields of up to 6 T. An important finding is that almost 60% of the normal electron density of states remains even in the superconducting ground state. This phenomenon and the intrinsic broadening of the thermal anomaly are common to another organic superconductor κ-(BEDT-TTF)₄Hg _2.89Br₈, in which an incommensurate relationship exists between anion and donor lattices. However, it is emphasized that (MDT-TSF)(AuI₂)_0.436 exhibits the superconducting transition at a relatively high temperature despite the small electronic heat capacity coefficient γ and non-dimerized molecular arrangement in the donor layers.