Helical fluctuations in the Raman response of the topological insulator Bi₂Se₃

The topological insulator Bi₂Se₃ shows a Raman-scattering (RS) response related to topologically protected surface states amplified by a resonant interband transition. Most significantly this signal has a characteristic Lorentzian lineshape and spin-helical symmetry due to collision-dominated scattering of Dirac states at the Fermi level E_F on bulk-valence states. Its resonance energy, temperature, and doping dependence points to a high selectivity of this process. Its scattering rate (Γ ≈ 40 cm⁻¹ ≈ 5 meV) is comparable to earlier observations, e.g., in spin-polaron systems. Although the observation of topological surface states in RS is limited to resonance conditions, this study represents a quite clean case which is not polluted by symmetry forbidden contributions from the bulk.