Dopant controlled symmetry breaking and low energy phonon engineering in FeSe doped Bi(Formula presented) (Formula presented)Te(Formula presented) (Formula presented)probed by Raman and terahertz spectroscopy

Topological insulators host symmetry protected surface states whose functionality is strongly influenced by lattice dynamics and symmetry perturbations. Here, we demonstrate dopant-driven symmetry and phonon engineering in FeSe-doped Bi (Formula presented) (Formula presented) Te (Formula presented) (Formula presented) thin films by correlating structural evolution with Raman and terahertz (THz) measurements. At low FeSe concentrations (3%–7%), compressive strain and lattice contraction preserve global centrosymmetry while inducing local inversion symmetry breaking, activating the normally infrared (Formula presented) (Formula presented) phonon in Raman spectra. With increasing doping beyond 7%, the lattice undergoes a compressive to tensile strain crossover accompanied by lattice expansion and microstructural disorder, leading to phonon softening, linewidth broadening, and suppression of the (Formula presented) (Formula presented) mode. At 15% FeSe-doping, a distinct low frequency THz active mode near 36 cm (Formula presented) (Formula presented) emerges, indicating the formation of dipole active lattice excitations associated with spatially heterogeneous strain and local polar distortions. Hall measurements reveal a concurrent reduction in carrier mobility and concentration, linking lattice disorder and dopant induced phonon dynamics to suppressed charge transport. These results establish FeSe doping as an effective route to control lattice symmetry, low-energy phonons, and electronic transport in Bi (Formula presented) (Formula presented) Te (Formula presented) (Formula presented), providing a versatile platform for phonon and symmetry engineered topological and spintronic devices.