The role of nematic order for the mechanism of high-temperature superconductivity is highly debated. In most iron-based superconductors (IBSs) the tetragonal symmetry is broken already in the normal state, resulting in orthorhombic lattice distortions, static stripe magnetic order, or both. Superconductivity then emerges, at least at weak doping, already from the state with broken C4 rotational symmetry. One of the few stoichiometric IBSs, lithium iron arsenide superconducts below 18 K and does not display either structural or magnetic transition in the normal state. Here we demonstrate, using angle-resolved photoemission spectroscopy, that even the superconducting state in LiFeAs is also a nematic one. We observe spontaneous breaking of the rotational symmetry in the gap amplitude on all Fermi surfaces, as well as unidirectional distortion of the Fermi pockets. Remarkably, these deformations are hardly visible above superconducting Tc. Our results demonstrate the realization of the phenomenon of superconductivity-induced nematicity in IBSs, emphasizing the intimate relation between them. We suggest a theoretical explanation based on the emergence of a secondary instability inside the superconducting state, which leads to the nematic order and s-d mixing in the gap function.
Bibliographical noteFunding Information:
We are grateful to R. Fernandes, G. Behr, and C. Hess for fruitful discussions. We acknowledge the Diamond Light Source for time on Beamline I05 under proposals NT5008 and SI9689 and the BESSY II synchrotron. We are grateful to R. J. Cava, M. Ali, and Q. Gibson for providing us with crystals, which we used for the device accuracy estimation. Y.S.K. and S.V.B. are supported by DFG Grants No. BO1912/6-1 and No. SPP1458. A.V.C. is supported by the Office of Basic Energy Sciences, U.S. Department of Energy, under Award No. DE-SC0014402. I.M. was supported by RSF-DFG Grant No. 19-43-04129.
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