Raman signature of multiple phase transitions and quasiparticle excitations in the putative Kitaev spin liquid candidate Na2Co2TeO6

Two-dimensional cobalt-based honeycomb oxide Na₂Co₂TeO₆ is an important candidate for the realization of Kitaev physics and may provide a future platform for quantum computation and quantum technology. Here, for the quasi-two-dimensional Na₂Co₂TeO₆ single crystals, we present detailed temperature and polarization-dependent inelastic light scattering (Raman) measurements. Our investigation uncovers the Raman spectroscopic signature of underlying multiple phase transitions i.e., a long-range antiferromagnetic transition (TN) at {~}30 K, a ferroelectric transition (TFE) at {~}70 K, and a crossover from a pure paramagnetic phase to a quantum paramagnetic phase around {~}150 K reflected in the renormalized self-energy parameters of the Raman active phonon modes. A distinct signature of spin reorientation deep into the antiferromagnetic phase around TSR~17 K is observed, marked by the clear change in the frequency and linewidth slopes. We also observed an asymmetric phonon mode in the low-frequency region, and it appears below the transition temperature {~}50 K, attributed to the magnetic excitations other than the magnon. The Raman signature of multiple crystal-field excitations at low temperature along with lifting of the Kramer's degeneracy is also observed.