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

Abstract 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 PbTaSe 2 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‐TaSe 2 and β phase.