Magnetic excitations and interactions in the Kitaev hyperhoneycomb iridate β-Li₂IrO₃

We present a thorough experimental study of the three-dimensional hyperhoneycomb Kitaev magnet β−Li₂IrO₃, using a combination of inelastic neutron scattering (INS), time-domain terahertz spectroscopy (TDTS), and heat capacity measurements. The main results include a massive low-temperature reorganization of the INS spectral weight that evolves into a broad peak centered around 12 meV, and a distinctive peak in the terahertz data at 2.8(1) meV. A detailed comparison to powder-averaged spin-wave theory calculations reveals that the positions of these two features are controlled by the anisotropic Γ coupling and the Heisenberg exchange J, respectively. The refined microscopic spin model places β−Li₂IrO₃ in close proximity to the Kitaev spin liquid phase.