Three-dimensional spin susceptibility in ; Out-of-plane modulation revealed by neutron spectroscopy and theoretical modeling

Murai, Naoki  ; Suzuki, Katsuhiro*; Nakajima, Masamichi*; Kofu, Maiko*   ; Kawamura, Seiko   ; Inamura, Yasuhiro  ; Kajimoto, Ryoichi  

We investigated the three-dimensional nature of spin fluctuations in the iron-based superconductor by combining time-of-flight inelastic neutron scattering with DFT-based theoretical modeling. The neutron measurements revealed a clear out-of-plane modulation of the low-energy magnetic intensity, with maxima at odd positions, indicating robust antiferromagnetic correlations along the axis. With increasing energy, this modulation gradually disappears, signaling a crossover from three-dimensional to nearly two-dimensional spin dynamics. Calculations based on a realistic three-dimensional electronic structure reproduce both the low-energy modulation and its energy-dependent suppression. Our itinerant approach, built on a material-specific DFT-derived band structure, captures this three-dimensional-to-two-dimensional crossover without phenomenological tuning, thereby providing a stringent benchmark for realistic electronic-structure models of iron-based superconductors.