Temperature and magnetic field dependent Raman study of electron-phonon interactions in thin films of Bi2Se3 and Bi2Te3 nanoflakes

We have investigated two-dimensional nanostructures of the topological insulators ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ and ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ by means of temperature and magnetic field dependent Raman spectroscopy. The surface contribution of our samples was increased by using thin films of dropcasted nanoflakes with the aim of enhancing their topological properties. Raman spectroscopy provides a contact-free method to investigate the behavior of topological properties with temperature and magnetic fields at lower dimensions. The temperature dependent Raman study reveals anharmonic phonon behavior for ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ indicative of a two-phonon relaxation mechanism in this material. Contrary to this, ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ shows clear deviations from a two-phonon anharmonic decay model at temperatures below 120 K exhibiting a hardening and broadening, especially of the ${A}_{1\text{g}}^{2}$ mode. Similarly, the magnetic field dependent self-energy effects are only observed for the ${A}_{1\text{g}}^{2}$ mode of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, showing a broadening and hardening with increasing field. We interpret our results in terms of corrections to the phonon self-energy for ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ at temperatures below 120 K and magnetic fields above 4 T due to electron-hole pair excitations associated with the conducting surface states. The phonon renormalization with increasing magnetic field is explained by a gap opening in the Dirac cone that enables phonon coupling to the changing electric susceptibility.