Surface phonons and possible structural phase transition in a topological semimetal PbTaSe2

Topological insulators are a novel class of quantum materials characterized by protected gapless surface or edge states but insulating bulk states which is due to presence of spin-orbit interactions and time-reversal symmetry. Such an intriguing surface and bulk topology manifests itself in coupling with lattice dynamics due to electron-phonon scattering. Here we report 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. Our 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 temperatures indicates towards a thermally induced structural phase transition which is also supported by the SC-XRD results. Hence based on our findings we propose that M1-M4 modes are surface phonon modes, the material undergoes a thermally induced structural phase transition from alpha to beta phase at T ~ 150 K or is in close proximity to the beta phase and another transition below T(CDW+beta) ~ 100K which is possibly due to the interplay of remanent completely commensurate charge density wave (CCDW) of 1H-TaSe2 and beta phase.