Evidence of s-wave pairing symmetry in layered superconductor Li${}_{0.68}$NbO${}_2$ from the specific heat measurement

A high quality superconducting Li${}_{0.68}$NbO${}_2$ polycrystalline sample was synthesized by deintercalation of Li ions from Li${}_{0.93}$NbO${}_2$. The field dependent resistivity and specific heat were measured down to 0.5 K. The upper critical field $H_{c2}(T)$ is deduced from the resistivity data and $H_{c2}(0)$ is estimated to be $\sim 2.98$ T. A notable specific heat jump is observed at the superconducting transition temperature $T_c\sim 5.0$ K at zero field. Below $T_c$, the electronic specific heat shows a thermal activated behavior and agrees well with the theoretical result of the BCS s-wave superconductors. It indicates that the superconducting pairing in Li${}_{0.68}$NbO${}_2$ has s-wave symmetry.