Superconductivity in a new layered nickel selenide CsNi₂Se₂

The physical properties of CsNi₂Se₂ were characterized by electrical resistivity, magnetization and specific heat measurements. We found that the stoichiometric CsNi₂Se₂ compound undergoes a superconducting transition at T _c = 2.7 K. A large Sommerfeld coefficient ${\gamma }_n$ (∼77.90 mJ/mol K⁻²) was obtained from the normal state electronic specific heat. However, the Kadowaki–Woods ratio of CsNi₂Se₂ was estimated to be about 0.041 × 10${}^{-5}\mu \Omega$cm(mol K²/mJ)², indicating the absence of strong electron–electron correlations. In the superconducting state, we found that the zero-field electronic specific heat data, C _es(T) (0.5 K $⩽$ T < 2.7 K), can be fitted well with a two-gap BCS model, indicating the multi-gap feature of CsNi₂Se₂. The comparison with the density functional theory (DFT) calculations suggested that the large ${\gamma }_n$ in these nickel selenide superconductors may be related to the large density of states (DOS) at the Fermi surface. (paper)