⁷⁵As Nuclear Spin–Spin Relaxation Measurement in Iron-Pnictide Superconductor Ba0.66K0.34Fe₂As₂

We performed 75As nuclear magnetic resonance (NMR) measurements on the single crystal of the hole-doped iron-pnictide superconductor Ba0.66K0.34Fe2As2 to study the low-energy spin dynamics. Nuclear spin-echo decay rate 1/T2 measurements reveal that the Lorentzian decay component 1/T2L exhibits the anisotropy of magnetic fluctuations due to a contribution of the nuclear spin–lattice relaxation rate 1/T1, indicating the striped correlations between Fe spins. We succeeded in observing the longitudinal fluctuations near zero frequency, which develop toward a superconducting temperature Tc below ∼100 K. Spin fluctuations are robust against frequency because both the longitudinal fluctuations near zero frequency and the transverse fluctuations at NMR angular frequency as 1/T1 increase toward Tc. The Gaussian decay component 1/T2G is independent of temperature T due to the direct nuclear spin–spin couplings. With decreasing temperature, both 1/T1T in the present NMR experiment and the nematic susceptibility previously reported in the shear-modulus measurement increase toward Tc, suggesting a coupling between spin and orbital fluctuations.