Observation of charge dressing in Bi 4 Te 3 topological insulator thin films: A terahertz and infrared spectroscopic study

Bi 4 Te 3 has been first realized as an element of the adaptive series ( Bi 2 ) m ( Bi 2 Te 3 ) n —deriving from stacking metallic Bi and topological insulator Bi 2 Te 3 blocks—and studied for its promising thermoelectric properties. Recently, Bi 4 Te 3 has also been found as a topological insulator hosting a complex band structure with three Dirac cones in k space. Here we report measurements of the optical conductivity of five Bi 4 Te 3 thin films with thickness varying from 11 to 32 nm. This extensive study, based on terahertz time-domain spectroscopy and Fourier-transform infrared spectroscopy, provides a broadband optical conductivity. By employing a Kramers-Kronig constrained Drude-Lorentz model, the spectral contributions to the optical conductivity of each sample have been singled out. In contrast to an ideal topological insulator—with perfectly insulating bulk—in Bi 4 Te 3 far-infrared and mid-infrared bands indicate the presence of localized massive carriers, which dope the bulk system, as also observed in chemically compensated single crystals of other bismuth-based topological insulators. In addition, the localized bulk charges strongly renormalize the electronic mass. The charge dressing enhances as the thickness increases, indicating a trivial metallic behavior of Bi 4 Te 3 thin films. This study elucidates the inner relationship between the optical properties of a topological insulator like Bi 4 Te 3 and its complex band structure, since the detection of the topological massless carriers by optical measurements is prevented by the occurrence of novel and unusual conductive properties.