Electronic Nematicity above the Structural and Superconducting Transition in BaFe2(As1−xP X )2

Electronic nematicity, a unidirectional self-organized state that breaks the rotational symmetry of the underlying lattice, has been observed in an iron-based superconductor, BaFe2(As1−xP x )2, over a wide range of phosphorus concentration, resulting in a phase diagram similar to the pseudogap phase diagram of the copper oxides. Electron nematicity, a unidirectional self-organized state that breaks the rotational symmetry of the underlying lattice, has been observed in the iron pnictide and copper oxide high-temperature superconductors, but whether it can exist above the structural transition temperature (Ts) without an external driving force was not known. Kasahara et al. report magnetic torque measurements in the iron pnictide superconductor BaFe2(As1–xPx)2 showing that the nematicity develops well above Ts and persists to the nonmagnetic superconducting regime, resulting in a phase diagram similar to the pseudogap phase diagram of the copper oxides. The authors identify two distinct temperatures — one at T*, signifying a true nematic transition, and the other at Ts (less than T*) — which they show is not a true phase transition, but rather a 'meta-nematic' transition. Electronic nematicity, a unidirectional self-organized state that breaks the rotational symmetry of the underlying lattice1,2, has been observed in the iron pnictide3,4,5,6,7 and copper oxide8,9,10,11 high-temperature superconductors. Whether nematicity plays an equally important role in these two systems is highly controversial. In iron pnictides, the nematicity has usually been associated with the tetragonal-to-orthorhombic structural transition at temperature Ts. Although recent experiments3,4,5,6,7 have provided hints of nematicity, they were performed either in the low-temperature orthorhombic phase3,5 or in the tetragonal phase under uniaxial strain4,6,7, both of which break the 90° rotational C4 symmetry. Therefore, the question remains open whether the nematicity can exist above Ts without an external driving force. Here we report magnetic torque measurements of the isovalent-doping system BaFe2(As1−xP x )2, showing that the nematicity develops well above Ts and, moreover, persists to the non-magnetic superconducting regime, resulting in a phase diagram similar to the pseudogap phase diagram of the copper oxides8,12. By combining these results with synchrotron X-ray measurements, we identify two distinct temperatures—one at T*, signifying a true nematic transition, and the other at Ts (