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

We present a thorough experimental study of the three-dimensional hyperhoneycomb Kitaev magnet beta-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 Gamma coupling and the Heisenberg exchange J, respectively. The refined microscopic spin model places beta-Li2IrO3 in close proximity to the Kitaev spin liquid phase.