Three-dimensional spin susceptibility in Ba$_{0.75}$K$_{0.25}$Fe$_{2}$As$_{2}$: Out-of-plane modulation revealed by neutron spectroscopy and theoretical modeling

Naoki Murai; Katsuhiro Suzuki; Masamichi Nakajima; Maiko Kofu; Seiko Ohira-Kawamura; Yasuhiro Inamura; Ryoichi Kajimoto

doi:10.1103/w1lf-837b

paper

Three-dimensional spin susceptibility in Ba $_{0.75}$ K $_{0.25}$ Fe $_{2}$ As $_{2}$ : Out-of-plane modulation revealed by neutron spectroscopy and theoretical modeling

arXiv:2412.16870 · doi:10.1103/w1lf-837b

Abstract

We present a combined experimental and theoretical investigation of the spin dynamics in the iron-based superconductor Ba $_{0.75}$ K $_{0.25}$ Fe $_{2}$ As $_{2}$ . Time-of-flight inelastic neutron scattering measurements reveal the three-dimensional (3D) nature of the spin fluctuations, manifested as out-of-plane modulations of the low-energy magnetic intensity. As the energy increases, this 3D-like modulation gradually fades away, leading to a more two-dimensional (2D) profile -- a clear signature of a 3D-to-2D crossover in the spin dynamics. By incorporating a realistic 3D electronic band structure derived from density functional theory (DFT), we reproduce the experimentally observed features of the spin susceptibility, including the pronounced out-of-plane modulation at low energies and its gradual evolution into a more 2D character at higher energies. The calculated susceptibility exhibits a peak at the experimental ordering wavevector $q_{AFM} = (0.5, 0.5, 1)$ , demonstrating that the DFT-derived 3D model accurately captures the tendency toward out-of-plane antiferromagnetic (AFM) order. Notably, electronic states away from the Fermi level play a crucial role in shaping the susceptibility peak at $q_{AFM}$ , highlighting the limitations of the Fermi surface nesting picture in explaining the out-of-plane AFM instability. The demonstrated agreement between experiment and theory serves as a benchmark for validating the DFT-derived model as a realistic description of the material-specific electronic structure.

17 pages, 5 figures + supplemental material

Three-dimensional spin susceptibility in Ba0.75​K0.25​Fe2​As2​: Out-of-plane modulation revealed by neutron spectroscopy and theoretical modeling

Abstract

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Three-dimensional spin susceptibility in Ba $_{0.75}$ K $_{0.25}$ Fe $_{2}$ As $_{2}$ : Out-of-plane modulation revealed by neutron spectroscopy and theoretical modeling