Large nematic susceptibility in the double-Q $C_4$ magnetic phase of Ba${}_{1-x}$Na${}_x$Fe${}_2$As${}_2$

Abstract

The nematic susceptibility of Ba${}_{1-x}$Na${}_x$Fe${}_2$As${}_2$ single crystals is studied by measuring the Young's modulus using a three-point-bending setup in a capacitance dilatometer over a wide doping range. Particular emphasis is placed on the behavior within the double-Q antiferromagnetic $C_4$ re-entrant phase. Here, we surprisingly still observe a sizable nematic susceptibility in spite of the well-developed magnetic order, consistent with recent theoretical calculations. Outside the $C_4$ re-entrant region, the behavior is very similar to that of K-doped BaFe${}_2$As${}_2$. A significant coupling of superconductivity to the shear modulus is observed and is discussed in terms of strong competition between the superconducting and magnetic phases.

5 pages, 5 figures