Specific heat study of the BiS₂-based superconductor

There have been a few reports on specific heat measurements in BiS₂-based superconductors. Electronic specific heat coefficient γ and Debye temperature Θ_D are not obtained accurately from specific heat measurements, because the measured temperature is relatively high. Furthermore, polycrystalline samples show a distinct upturn in C_p/T below 2.0 K; superconducting (SC) gap symmetry cannot be discussed. It is highly desirable to measure the heat capacity using a single crystal to reveal the SC state. This review article summarizes studies on specific heat of the BiS₂-based superconductors. Particularly, the constructional details of our sensitive calorimeter designed for the BiS₂-based superconductors with very small heat capacity are reviewed. The addendum value reaches approximately 0.8 nJ/K² at 1 K, which enables the measurement of the heat capacity of the small single crystal (∼1 mg) of the BiS₂-based superconductors. The SC-gap symmetry of LaO_0.5F_0.5BiSSe is elucidated through specific heat measurements using our homemade calorimeter. (author)