Pauli-limit violation in lanthanide infinite-layer nickelate superconductors

Abstract

Superconductivity can be destroyed by a magnetic field with an upper bound known as the Pauli-limit in spin-singlet superconductors. Almost all the discovered superconductors are spin-singlet, with the highest transition temperature $T_c$ at ambient pressure achieved in the cuprate family. The closest cuprate analogue is the recently discovered infinite-layer nickelate, which hosts substantial structural and electronic similarity to the cuprate. A previous magnetotransport study on Nd${}_{0.775}$Sr${}_{0.225}$NiO${}_2$ has observed an isotropic Pauli-limited upper critical field. Here, we report a large violation (>2 times) of Pauli-limit in every crystallographic directions in La${}_{1-x}$(Ca/Sr)${}_x$NiO${}_2$ regardless of the doping $x$. Such a large violation of the Pauli-limit in all directions in La${}_{1-x}$(Ca/Sr)${}_x$NiO${}_2$ is unexpected and unlikely accounted by a Fulde Ferrell-Larkin-Ovchinnikov (FFLO)-state, strong spin-orbit-coupling, strong-coupling or a large pseudogap. On the other hand, in agreement with the previous report, we observe a Pauli-limiting critical field in Nd${}_{1-x}$Sr${}_x$NiO${}_2$ and the superconducting anisotropy decreases as doping increases, suggesting a spin-singlet pairing. Therefore, superconductivity in La${}_{1-x}$(Ca/Sr)${}_x$NiO${}_2$ could be driven by a non-spin-singlet Cooper pairing mechanism with an attractive high-$T_c$ at 10 K, an order of magnitude higher than the known spin triplet superconductors, favourably extending the application of spin-triplet superconductivity in topological matter, non-dissipative spintronics, and quantum computing.

21 pages, 4 main figures, 2 extended data