Large nematic susceptibility in the double-QC4 magnetic phase of Ba1−xNaxFe2As2

The nematic susceptibility of Ba1−xNaxFe2As2 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 C4 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 C4 re-entrant region, the behavior is very similar to that of K-doped BaFe2As2. 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.