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 {γ }_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{}^-5μ {{Ω }}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 ≤slant 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 {γ }_n in these nickel selenide superconductors may be related to the large density of states (DOS) at the Fermi surface.