Raman signature of multiple phase transitions and quasi-particle excitations in putative Kitaev spin liquid candidate Na2Co2TeO6

Abstract: Two-dimensional cobalt-based honeycomb oxide Na2Co2TeO6 is an important candidate for the realization of Kitaev physics and may provide future platform for the quantum computation and quantum technology. Here, we report an in-depth temperature as well as polarization dependent inelastic light scattering (Raman) measurements on the single crystals of quasi-two-dimensional Na2Co2TeO6. Our study reveal signature of multiple phase transitions i.e. long-range zigzag antiferromagnetic transition (TN) at ~ 30 K, ferroelectric transition (TFE) at ~ 70 K, and a crossover from 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 AFM 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 Kramers degeneracy is also observed. Signature of the underlying broad magnetic continuum in the quantum paramagnetic phase and its temperature dependence suggest presence of frustrated magnetic interaction in the quantum paramagnetic phase below ~ 150 K.