Fully gapped superconductivity in SrNi₂P₂

We investigated the superconducting gap structure of SrNi$_2$P$_{2}$ ($T_c$=1.4 K) via low-temperature magneto-thermal conductivity $\kappa(T,H)$ measurements. Zero field thermal conductivity $\kappa$ decreases exponentially $\kappa \propto$ exp($-aT_c/T$) with $a$=1.5, in accord with the BCS theory, and rolls over to a phonon-like $\kappa\propto T^3$ behavior at low temperature, similar to a number of conventional s-wave superconductors. In addition, we observed a "concave" field dependence of the residual linear term $\kappa_0(H)/T$. These facts strongly rule out the presence of nodes in the superconducting energy gap of SrNi$_2$P$_{2}$. Together with a fully gapped Fermi surface in the superconducting state of BaNi$_2$As$_{2}$ ($T_c$=0.6-0.7 K), demonstrated in our recent work, these results lead us to postulate that fully gapped superconductivity is a universal feature of Ni-based pnictide superconductors.