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NMR evidence of pairing fluctuations above Tc and absence of spin magnetism in the time-reversal symmetry-breaking state of Ba1−xKxFe2As2

Florian Bärtl1,2,*, Nadia Stegani3,4, Federico Caglieris4, Ilya Shipulin2, Yongwei Li5, Ruidan Zhong5, Quanxin Hu5, Yu Zheng5, Chi-Ming Yim5 et al.

Sven Luther1, Jochen Wosnitza1,2, Rajib Sarkar2, Hans-Henning Klauss2, Julien Garaud6, Albert Samoilenka7,8, Anton Talkachov7, Egor Babaev7,9,†, Hannes Kühne1, and Vadim Grinenko5,‡

  • *Contact author: f.baertl@hzdr.de
  • †Contact author: babaev@kth.se
  • ‡Contact author: vadim.grinenko@sjtu.edu.cn
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Phys. Rev. B 114, 134504 – Published 4 September, 2026

DOI: https://doi.org/10.1103/928z-jgcx

Abstract

Zero-field muon-spin-rotation (μSR) experiments and the observation of a spontaneous Nernst effect indicate broken time-reversal symmetry (BTRS) at TcZ2, slightly above the superconducting transition temperature Tc in Ba1−xKxFe2As2, with x∼0.8. This was interpreted as evidence of a distinct state of matter where BTRS above Tc arises from noncondensed Cooper pairs, which maintain long-range interband correlations. However, an inhomogeneous superconducting state competing with magnetism as the origin of the observed behavior cannot be completely ruled out to date. Here, we present detailed As75 nuclear magnetic resonance (NMR) measurements of Ba1−xKxFe2As2, with x=0.766, yielding TcZ2>Tc. The NMR data demonstrate that the superconductivity is homogeneous throughout the entire sample volume despite the occurrence of nanoclustering of Ba/K atoms. We observe a suppression of the spin-lattice relaxation rate 1/T1T below the characteristic temperature T*, well above Tc. The specific heat yields somewhat lower values of T*, consistent with the microscopic theory of electron quadrupling. The observed monotonous suppression of T* and TcZ2 in applied magnetic field, the overall temperature dependence of 1/T1T, the NMR spectral characteristics, and the μSR Knight shift rule out spin magnetism and the proximity to a magnetic instability. We conclude that our data indicate the appearance of pairing fluctuations below T* and that the breaking of time-reversal symmetry represents an unconventional state which is unrelated to spin magnetism.

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Phys. Rev. B 114, 134504– Published 4 September, 2026
  • Received 20 January 2025
  • Revised 20 May 2026
  • Accepted 3 August 2026
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