Spin dynamics studied by ⁷⁷Se nuclear spin-echo decay measurements in FeSe

We carried out ⁷⁷Se nuclear magnetic resonance measurement in the iron-chalcogenide superconductor FeSe, which has only orbital order without magnetic order. From the measured ⁷⁷Se nuclear transverse relaxation rate 1/T₂, we evaluate the longitudinal fluctuations near zero frequency u_zz(0), which develop on cooling below the structural transition temperature of T_s,nem = 90 K. The contrasting behavior between u_zz(0) and the transverse fluctuations at an NMR angular frequency as a nuclear spin-lattice relaxation rate 1/T₁(ω₀) indicates the sensitive change of spin fluctuations against frequency and the manifestation of ultra-slow spin dynamics in FeSe, the latter of which will be discussed in terms of the fluctuating domain walls and magnetic frustration effect. (author)