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−xPx)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 (<T*), which we show not to be a true phase transition, but rather what we refer to as a ‘meta-nematic transition’, in analogy to the well-known meta-magnetic transition in the theory of magnetism.
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We thank J. G. Analytis, A. Q. R. Baron, E. Bascones, A. Carrington, A. V. Chubukov, R. M. Fernandes, I. Fischer, H. Ikeda, H. Kontani, R. Okazaki and J. Schmalian for discussions. This research was supported by a Grant-in-Aid for the Global COE programme ‘The Next Generation of Physics, Spun from Universality and Emergence’ from MEXT of Japan, and by the KAKENHI programme from JSPS. A.H.N. and Y.M. acknowledge the hospitality of the Aspen Center for Physics. The synchrotron radiation experiments were performed at the BL02B1 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI).
This file contains Supplementary Text and Data, Supplementary Figures 1-6 and additional references.
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Structural phase transition, antiferromagnetism and two superconducting domes in LaFeAsO1-xFx (0 < x ≤ 0.75)
Science China Physics, Mechanics & Astronomy (2018)