Field-Induced Spin Continuum in Twin-Free Na₃Co₂SbO₆ Revealed by Magneto-THz Spectroscopy

The honeycomb magnet Na_3Co_2SbO_6 recently emerged as a promising candidate for realizing Kitaev quantum spin liquid with relatively low levels of structural disorder. While the precise spin Hamiltonian remains controversial, the potential existence of a quantum spin liquid or other novel quantum magnetic phases continues to stimulate investigation. Here, we study the temperature and magnetic field-dependent spin excitations of Na_3Co_2SbO_6 on a twin-free single crystal using magneto-terahertz (THz) spectroscopy, focusing on magnetic anisotropy and field-induced unusual phases. We observe a low-energy continuum excitation above T_\rm N and a 0.5 THz (2 meV) spin wave excitation in magnetic order under zero field. Upon applying an in-plane magnetic field, the spin waves transform to a magnetic continuum over an intermediate field range, above which the system enters a spin-polarized state. Crucially, the spin excitation spectra reveal striking anisotropy between the \boldsymbol a and \boldsymbol b crystallographic axes, demanding description by a strongly anisotropic spin model. These findings establish Na_3Co_2SbO_6 as a model system for investigating field-tunable quantum magnetism and potential spin liquid behavior in highly anisotropic systems.