Superconductivity with broken time-reversal symmetry inside a superconducting s-wave state

Grinenko, V. and Sarkar, R. and Kihou, K. and Lee, C.H. and Morozov, I. and Aswartham, S. and Büchner, B. and Chekhonin, P. and Skrotzki, W. and Nenkov, K. and Hühne, R. and Nielsch, K. and Drechsler, S.-L. and Vadimov, V.L. and Silaev, M.A. and Volkov, P.A. and Eremin, I. and Luetkens, H. and Klauss, H.-H.

Volume: 16 Pages: 789-794
DOI: 10.1038/s41567-020-0886-9
Published: 2020

In general, magnetism and superconductivity are antagonistic to each other. However, there are several families of superconductors in which superconductivity coexists with magnetism, and a few examples are known where the superconductivity itself induces spontaneous magnetism. The best known of these compounds are Sr2RuO4 and some non-centrosymmetric superconductors. Here, we report the finding of a narrow dome of an s+ is′ superconducting phase with apparent broken time-reversal symmetry (BTRS) inside the broad s-wave superconducting region of the centrosymmetric multiband superconductor Ba1 − xKxFe2As2 (0.7 ≲ x ≲ 0.85). We observe spontaneous magnetic fields inside this dome using the muon spin relaxation (μSR) technique. Furthermore, our detailed specific heat study reveals that the BTRS dome appears very close to a change in the topology of the Fermi surface. With this, we experimentally demonstrate the likely emergence of a novel quantum state due to topological changes of the electronic system. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.

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