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 T-N1 = 9.3K and T-N2 = 7.3K; SmNiAsO becomes an anti-ferromagnet below T-N similar or equal to 3.5K; NdNiAsO keeps paramagnetic down to 2K but orders anti-ferromagnetically below T-N similar or equal to 1.3K. Superconductivity was observed only in Kramers non-magnetic LaNiAsO and PrNiAsO with T-c = 2.7K 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.