Fully gapped superconductivity in Ni-pnictide superconductors BaNi2As2 and SrNi2P2

We have performed low-temperature specific heat $C$ and thermal conductivity $\kappa$ measurements on the Ni-pnictide superconductors BaNi${}_2$As${}_2$ ($T_{\mathrm{c}}$=0.7 K and SrNi${}_2$P${}_2$ ($T_{\mathrm{c}}$=1.4 K). The temperature dependences $C(T)$ and $\kappa (T)$ of the two compounds are similar to the results of a number of s-wave superconductors. Furthermore, the concave field responses of the residual $\kappa$ for BaNi${}_2$As${}_2$ rules out the presence of nodes on the Fermi surfaces. We postulate that fully gapped superconductivity could be universal for Ni-pnictide superconductors. Specific heat data on Ba${}_{0.6}$La${}_{0.4}$Ni${}_2$As${}_2$ shows a mild suppression of $T_{\mathrm{c}}$ and $H_{\mathrm{c}2}$ relative to BaNi${}_2$As${}_2$.