Evolution of nematic fluctuations in CaK(Fe${}_{1-x}$Ni${}_x$)${}_4$As${}_4$ with spin-vortex crystal magnetic order

Abstract

The CaK(Fe${}_{1-x}$Ni${}_x$)${}_4$As${}_4$ superconductors resemble the archetypal 122-type iron-based materials but have a crystal structure with distinctly lower symmetry. This family hosts one of the few examples of the so-called spin-vortex crystal magnetic order, a non-collinear magnetic configuration that preserves tetragonal symmetry, in contrast to the orthorhombic collinear stripe-type magnetic configuration common to the 122-type systems. Thus, nematic order is completely absent from its phase diagram. To investigate the evolution of nematic fluctuations in such a case, we present elastoresistance and elastic modulus measurements in CaK(Fe${}_{1-x}$Ni${}_x$)${}_4$As${}_4$ ($x=0-0.05$) combined with phenomenological modeling and density functional theory. We find clear experimental signatures of considerable nematic fluctuations, including softening of the Young's modulus $Y_{[110]}$ and a Curie-Weiss type divergence of the B2g elastoresistance coefficient in CaK(Fe${}_{0.951}$Ni${}_{0.049}$)${}_4$As${}_4$. Overall, nematic fluctuations within this series bear strong similarities to the hole-doped Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ series, including a substitution-induced sign change. Our theoretical analysis addresses the effect of the specific crystal symmetry of the 1144-type structure in determining its magnetic ground state and on the nematic fluctuations.