Absorption edge, urbach tail, and electron-phonon interactions in topological insulator Bi2Se3 and band insulator (Bi0.89In0.11)2Se3

We employ infrared transmission spectroscopy to explore the temperature-dependent absorption edge and electron-phonon (e-ph) interaction in topological insulator Bi₂Se₃ and band insulator (Bi_0.89In_0.11)₂Se₃ films. Upon heating from 5 K to 300 K, the absorption edge shifts from 262 to 249 meV for Bi₂Se₃ and from 367 to 343 meV for (Bi_0.89In_0.11)₂Se₃. By analyzing the temperature dependence of the Urbach tail, the significant role of Raman-active phonon mode _E^{g 2} in e-ph interaction is identified, which agrees well with the ab initio calculation.