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

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 *