and in related systems*

Quasiparticle interference has been shown to be a powerful tool to investigate bandstructure and superconducting gap opening in the pnictide superconductors. Here I will review efforts in the recently discovered family of 1144 materials, particularly in pure and Ni-doped CaKFe₄As₄. Pure CaKFe₄As₄ shows the highest T_c among stoichiometric pnictide materials (35 K). It is a two-gap, sign-changing superconductor [1, 2] located at optimal doping. Quasiparticle interference in the pure compound shows the opening of a superconducting gap in the hole bands around the zone center. The top of some bands is very close to the Fermi level, probably influencing the structure of Caroli de Gennes Matricon localized states within vortex cores. Ni doping reduces T_c and induces a magnetic transition where a unique hedgehog magnetic order has been proposed [3]. I will show atomic scale imaging and quasiparticle interference measurements and discuss those properties of the bandstructure and the vortex lattice that could be related to the decrease in the critical temperature. Finally, I will compare this with very recent results in the P-doped BaFe₂As₂ system.

[1] K. Cho, A. Fente et al., Phys. Rev. B 95, 100502(R) 2017.
[2] A. Fente et al., arXiv:1608.00605.
[3] W.R. Meier et al., arXiv:1706.01067.