Surface Phonons and Possible Structural Phase Transition in a Topological Semimetal PbTaSe2

Topological insulators are characterized by protected gapless surface or edge states but insulating bulk states which is due to the presence of spin-orbit interactions and time-reversal symmetry. Here, an in-depth investigation of a topological nodal line semimetal PbTaSe₂ via temperature, polarization dependent Raman spectroscopy, and temperature dependent single crystal X-ray diffraction (SC-XRD) measurements is reported. The analysis shows signature of electron-phonon coupling as reflected in the Fano asymmetry in line shape of M1-M4 modes and anomalous temperature variation of line-width of P3-P4 modes. Further polarization dependent phonon symmetry changes at different temperature (6K and 300K), discontinuities in bulk phonon dynamics for P2-P5 modes, and disappearance of phonon modes, i.e., M1-M5, on decreasing temperature indicates toward a thermally induced structural phase transition which is also supported by the SC-XRD results. Hence based on the findings, it is proposed that M1-M4 modes are surface phonon modes, the material undergoes a thermally induced structural phase transition from α to β phase at T_α→β ≈ 150 K or is in close proximity to the β phase and another transition below T_CDW+β ≈ 100K which is possibly due to the interplay of remanent completely commensurate charge density wave (CCDW) of 1H-TaSe₂ and β phase.