Kramers non-magnetic superconductivity in LnNiAsO superconductors

We investigated a series of nickel-based oxyarsenides LnNiAsO (Ln = La, Ce, Pr, Nd, Sm) compounds. CeNiAsO undergoes two successive anti-ferromagnetic transitions at TN1 = 9.3 K and TN2 = 7.3 K; SmNiAsO becomes an anti-ferromagnet below TN 3.5 K; NdNiAsO keeps paramagnetic down to 2 K but orders anti-ferromagnetically below TN 1.3 K. Superconductivity was observed only in Kramers non-magnetic LaNiAsO and PrNiAsO with Tc = 2.7 K and 0.93 K, respectively. The superconductivity of PrNiAsO is further studied by upper critical field and specific heat measurements, which reveal that PrNiAsO is a weakly coupled Kramers non-magnetic superconductor. Our work confirms that the nickel-based oxyarsenide superconductors are substantially different in mechanism to iron-based ones, and are likely to be described by the conventional superconductivity theory.