Superconductivity in a new layered nickel-selenide CsNi2Se2

The physical properties of CsNi${}_2$Se${}_2$ were characterized by electrical resistivity, magnetization and specific heat measurements. We found that the stoichiometric CsNi${}_2$Se${}_2$ compound is a superconductor with a transition temperature $\mathit{\text{T}}{}_c$=2.7K. A large Sommerfeld coefficient $\gamma$${}_n$ ($\sim$77.90 mJ/mol$\cdot$K${}^{-2}$), was obtained from the normal state electronic specific heat. However, the Kadowaki-Woods ratio of CsNi${}_2$Se${}_2$ was estimated to be about 0.041$\times$10${}^{-5}$ $\mu \mathrm{\Omega }$$\cdot$cm(mol$\cdot$K/mJ)${}^2$, indicating the absence of strong electron-electron correlations in this compound. In the superconducting state, we found that the zero-field electronic specific heat data, $C_{es}(T)$ (0.5K $\le$ T $<$ 2.6K), can be well fitted with a two-gap BCS model. The comparison with the results of the density functional theory (DFT) calculations suggested that the large $\gamma$${}_n$ in the nickel-selenide superconductors may be related to the large Density of States (DOS) at the fermi surface.