Absorption edge, urbach tail, and electron-phonon interactions in topological insulator Bi2Se3 and band insulator (Bi0.89In0.11)2Se3
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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 (Bi0.89In0.11)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 (Bi0.89In0.11)2Se3. By analyzing the temperature dependence of the Urbach tail, the significant role of Raman-active phonon mode E2g
in e-ph interaction is identified, which agrees well with the ab initio calculation.
Topological insulators (TIs) are a class of quantum systems with an insulating bulk and a conducting surface. Their metallic surface states are formed by Dirac fermions with spin-momentum locking facilitated by strong spin?orbit coupling.1,2 As an archetypical three-dimensional TI, Bi2Se3 has a large bulk gap of around 0.3?eV with the simplest surface state, opening up the possibility of many experiments. Indium doping in Bi2Se3 can effectively tune the spin?orbit coupling and drive the bulk topological phase transition. (Bi1-xInx)2Se3 shares the common rhombohedral D3d5 structure, and a topological-to-trivial transition is observed in a range x from 0.03 to 0.07.3,4 Despite intensive investigations of TIs, experimental studies for the temperature dependence of the optical gap in TIs are quite controversial. While Post et al. reported that TI Bi2-xSbx Te3-ySey has an optical gap that shrinks with increasing temperature,5 they found that the optical gap of (Bi, Sb)2Te3 does not show an apparent temperature dependence below room temperature.6 There is a lack of comparison between the energy gap and theoretical models, which could be used to show that temperature induced shifts are reasonable. Temperature dependences of the optical gap in TIs are seldom explained by any empirical model, such as the Bose-Einstein model, in which the parameter is related to the known Debye temperature of the material.