Disorder-enriched magnetic excitations in the Kitaev quantum spin liquid candidate Na${}_2$Co${}_2$TeO${}_6$

Using optical magneto-spectroscopy, we investigate the magnetic excitations of Na${}_2$Co${}_2$TeO${}_6$ in a broad magnetic field range ($0\mathrm{T}\le B\le 17.5\mathrm{T}$) at low temperature. Our measurements reveal rich spectra of in-plane magnetic excitations with a surprisingly large number of modes, even in the high-field spin-polarized state. Theoretical calculations find that the Na-occupation disorder in \NCTO plays a crucial role in generating these modes. Our work demonstrates the necessity to consider disorder in the spin environment in the search for Kitaev quantum spin liquid states in practicable materials.