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We have carried out nuclear magnetic resonance measurements on the recent discovered new iron-based superconductor CaKFe₄As₄ (T_c = 35 K). Based on NMR data, we will discuss the details of superconducting properties and magnetic fluctuations of CaKFe₄As₄ [1], which will be compared with those in other Fe-based SC such as 122 systems. In addition, we will report the first observation of the hedgehog spin vortex crystal order in the antiferromagnetic state in Ni-doped CaK(Fe_1-xNi_x)₄As₄ (T_n = 52 K and T_c= 9 K for x=0.05) [2,3]. Furthermore, the Ni doping x dependence of magnetic fluctuations will be presented and we will discuss a possible quantum critical behavior in in Ni-doped CaK(Fe_1-xNi_x)₄As₄.

This work was done in collaboration with Q.-P. Ding, J. Cui, P. Wiecki, K. Rana, W. R. Meier, S. L. Bud’ko, A. E. Böhmer, P. C. Canfield, Y. Lee, V. Borisov, R. Valentí, P. Orth, and R. Fernandes.


[1] J. Cui, et al., Phys. Rev. B 96, 104512 (2017).
[2] W. R. Meier , et al., arXive:1706.01067.
[3] Q.-P. Ding et al., submitted to PRL