Fig. 1

Schematic view of the experiment and flavour of the major findings within the generic phase diagram of Ba_1−xK_xFe₂As₂: The optimally doped (x = 0.4 and T_c = 38.5 K) sample is illuminated with polarized light while external uniaxial compressive stress (p) is applied on the lateral face of the specimen. We adopt an experimental protocol consisting in the measurement of the optical reflectivity signal (R) during a p-loop after a zero-p-cooling (ZPC) procedure (see Methods as well as Supplementary Information).^24,25 We define the b-axis as the direction along which compressive p is applied and the a-axis as its orthogonal direction. Light is then polarized parallel to either axis (in the convention of our experimental set-up the polarization 0 is parallel to the b-axis and 90 to the a-axis). Below T_c (inset a) we clearly observe an anisotropy in the detected R-signal between the two axes when p is progressively applied, so that the ratio R_a/R_b deviates from 1 (i.e., isotropic limit). The ratios R_a/R_b are read from Fig. 2c at 0, at saturation (i.e., thick dotted line at 1.2 bar) and at 0 released p. R_a/R_b disappears upon releasing p back to zero (‘(r)’ denotes released p). At T_c (inset b) the onset of the optical anisotropy is only incipient and above T_c (inset c) there is no signature of the optical anisotropy upon sweeping p. We note that for compositions between \(x \simeq 0.25\) and 0.3, Ba_1−xK_xFe₂As₂ suffers a reentrant C4 antiferromagnetic phase transition at temperatures below the C2 antiferromagnetic/nematic transition.³² This aspect as well as the still on-going debate on the symmetry of the superconducting order parameter¹ are neglected in this schematic representation of the phase diagram