Electronic nematicity above the structural and superconducting transition in BaFe(AsP)

Kasahara, Shigeru*; Shi, H. J.*; Hashimoto, Kenichiro*; Tonegawa, Sho*; Mizukami, Yuta*; Shibauchi, Takasada*; Sugimoto, Kunihisa*; Fukuda, Tatsuo  ; Terashima, Takahito*; Nevidomskyy, A. H.*; Matsuda, Yuji*

The electron nematicity, which break rotational symmetry of the crystal lattice, has been recently observed in high- superconductors. In iron-pnictides, it has been associated with the tetragonal-to-orthorhombic structural transition at , and the observations has been limited in the orthorhombic phase or in the tetragonal phase under uniaxial strain. In this investigation we provide magnetic torque measurements, a thermodynamic evidence which clearly shows the development of nematicity well above and the persistence in the nonmagnetic superconducting regime. Combined with synchrotron X-ray measurements, our results identify two distinct temperatures-one at , signifying a true nematic transition, and the other at (), which we show to be not a true phase transition, but rather what we refer to as a "meta-nematic transition", in analogy to the well-known metamagnetic transition in the theory of magnetism.