NMR evidence of pairing fluctuations above T c and absence of spin magnetism in the time-reversal symmetry-breaking state of Ba 1 − x K x Fe 2 As 2
Abstract
Zero-field muon-spin-rotation ( μ SR ) experiments and the observation of a spontaneous Nernst effect indicate broken time-reversal symmetry (BTRS) at T c Z 2 , slightly above the superconducting transition temperature T c in Ba 1 − x K x Fe 2 As 2 , with x ∼ 0.8 . This was interpreted as evidence of a distinct state of matter where BTRS above T c 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 As 75 nuclear magnetic resonance (NMR) measurements of Ba 1 − x K x Fe 2 As 2 , with x = 0.766 , yielding T c Z 2 > T c . 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 / T 1 T below the characteristic temperature T * , well above T c . The specific heat yields somewhat lower values of T * , consistent with the microscopic theory of electron quadrupling. The observed monotonous suppression of T * and T c Z 2 in applied magnetic field, the overall temperature dependence of 1 / T 1 T , the NMR spectral characteristics, and the μ SR 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 *
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BibTeX
@article{bartl2026evidence,
title = {{NMR evidence of pairing fluctuations above T c and absence of spin magnetism in the time-reversal symmetry-breaking state of Ba 1 − x K x Fe 2 As 2}},
author = {Florian Bärtl and Nadia Stegani and Federico Caglieris and Ilya Shipulin and Yongwei Li and Ruidan Zhong and Quanxin Hu and Yu Zheng and Chi-Ming Yim and Sven Luther and Jochen Wosnitza and Rajib Sarkar and Hans-Henning Klauss and Julien Garaud and Albert Samoilenka and Anton Talkachov and Egor Babaev and Hannes Kühne and Vadim Grinenko},
journal = {Physical Review B},
year = {2026},
month = sep,
doi = {10.1103/928z-jgcx},
url = {https://hal.science/hal-05749838v1},
note = {CC BY},
}
RIS
TY - JOUR
TI - NMR evidence of pairing fluctuations above T c and absence of spin magnetism in the time-reversal symmetry-breaking state of Ba 1 − x K x Fe 2 As 2
AU - Florian Bärtl
AU - Nadia Stegani
AU - Federico Caglieris
AU - Ilya Shipulin
AU - Yongwei Li
AU - Ruidan Zhong
AU - Quanxin Hu
AU - Yu Zheng
AU - Chi-Ming Yim
AU - Sven Luther
AU - Jochen Wosnitza
AU - Rajib Sarkar
AU - Hans-Henning Klauss
AU - Julien Garaud
AU - Albert Samoilenka
AU - Anton Talkachov
AU - Egor Babaev
AU - Hannes Kühne
AU - Vadim Grinenko
PY - 2026
DA - 2026/09/04
JO - Physical Review B
DO - 10.1103/928z-jgcx
UR - https://hal.science/hal-05749838v1
AB - Zero-field muon-spin-rotation ( μ SR ) experiments and the observation of a spontaneous Nernst effect indicate broken time-reversal symmetry (BTRS) at T c Z 2 , slightly above the superconducting transition temperature T c in Ba 1 − x K x Fe 2 As 2 , with x ∼ 0.8 . This was interpreted as evidence of a distinct state of matter where BTRS above T c 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 As 75 nuclear magnetic resonance (NMR) measurements of Ba 1 − x K x Fe 2 As 2 , with x = 0.766 , yielding T c Z 2 > T c . 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 / T 1 T below the characteristic temperature T * , well above T c . The specific heat yields somewhat lower values of T * , consistent with the microscopic theory of electron quadrupling. The observed monotonous suppression of T * and T c Z 2 in applied magnetic field, the overall temperature dependence of 1 / T 1 T , the NMR spectral characteristics, and the μ SR 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 *
SN - 2469-9950
SN - 2469-9969
ER -