Disorder-Enriched Magnetic Excitations in a Heisenberg-Kitaev Quantum Magnet Na2Co2TeO6

Using optical magnetospectroscopy, we investigate the magnetic excitations of Na2Co2TeO6 in a broad magnetic field range (0 T <= B <= 17.5 T) at low temperature. Our measurements reveal rich spectra of inplane 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 Na2Co2TeO6 plays a crucial role in generating these modes. Our Letter demonstrates the necessity to consider disorder in the spin environment in the search for Kitaev quantum spin liquid states in practicable materials.