New Quantum Criticality Discovered in Superconductivity

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Phase diagram of Ni doped CaKFe4As4.Understanding the link between magnetism and unconventional superconductivity remains a grand challenge in quantum materials. Significant progress gaining deeper insight into this challenge has been made via the first observation of a quantum criticality (QC) with a hedgehog spin-vortex crystal antiferromagnetic state. Two ordering states, antiferromagnetism and nematicity, have been observed in most iron-based superconductors. It is generally believed that either spin or nematic fluctuations work as glue for the Cooper pairs for superconductivity, but no definitive conclusion has been reached. The CaK(Fe1−xNix)4As4 superconductor exhibits an antiferromagnetic state with hedgehog spin-vortex crystal structure and without nematic order, providing the opportunity to investigate the relationship between spin fluctuations and superconductivity without any effects of nematic fluctuations. Here, using 75As nuclear magnetic resonance, Ames Laboratory researchers discovered that CaKFe4Asis located at a hedgehog spin-vortex crystal antiferromagnetic quantum critical point, which is avoided due to superconductivity. The magnetic QC without nematicity in CaK(Fe1−xNix)4As4 highlights the close connection of spin fluctuations and superconductivity in iron-based superconductors. The advantage of stoichiometric composition also makes CaKFe4As4 an ideal platform for further investigation of the relationship between magnetic QC and superconductivity without disorder effects which can lead to discovery of new superconducting compounds.

Q.-P. Ding, W. R. Meier, J. Cui , M. Xu, A. E. Bӧhmer, S. L. Bud’ko, P. C. Canfield, and Y. Furukawa, “Hedgehog Spin-vortex Crystal Antiferromagnetic Quantum Criticality in CaK(Fe1−xNix)4As4 Revealed by NMR”, Physical Review Letters, 2018, 121, 137204 (2018), DOI: 10.1103/PhysRevLett.121.137204