Fig. 4

Upper critical fields and phase diagram. a Angular dependence of the second magnetic critical field at 38.4 K (T _c ≈ 39 K), retrieved from transport experiments (black filled square symbol) in the Corbino disc measurement configuration, from theoretical model with s-wave and \(d_{x^2 - y^2}\)-wave symmetries (full blue line), from theoretical model with s _±-wave, \(d_{x^2 - y^2}\)-wave and d _xy -wave symmetries (full red and blue lines depending on dominant component). The mixing of the different symmetry components of the order parameters is indicated on the left and right side panels next to a schematic representation of each component of the order parameter as function of the internal momentum of the Cooper pairs. The indicated percentages correspond to the relative weights (r ₁, r ₂, and r ₃) of the wave function coefficients, \({\mathrm{\Phi }}\left( {\bf{k}} \right) = r_1{\mathrm{\Phi }}_s\left( {\bf{k}} \right) + r_2{\mathrm{\Phi }}_{d_{x^2 - y^2}}\left( {\bf{k}} \right) + r_3{\mathrm{\Phi }}_{d_{xy}}\left( {\bf{k}} \right)\). In this schematic representation red and green indicate positive and negative value of the components, respectively. b Phase diagram of hole-doped Ba_1−x K _x Fe₂As₂ iron-based superconductors. There is controversial evidence concerning the symmetry of the order parameter in the doping domain close to x = 1: refs. ^12,13,14 argue in favor of s _± and ref. ¹¹ (and references therein) in favor of \(d_{x^2 - y^2}\) pairing symmetry. The error bar for x = 0.5 corresponds to 15 samples with x ranging from 0.45 to 0.55. The red region demonstrates the normal state nematic fluctuations, which may originate from magnetic order, structural or charge/orbit order transition¹⁹. The superconducting (SC) nematic state is observed on the basis of the present IMR results (Figs. 2 and 3)