Results and Analysis

Abstract

In this chapter the experimental results of this thesis will be shown and the related analysis explained. Four topological insulator crystals ($\mathrm{{Bi}}_2\mathrm{{Se}}_3$, $\mathrm{{Bi}}_{2-x}\mathrm{{Ca}}_x\mathrm{{Se}}_3$, $\mathrm{{Bi}}_2\mathrm{{Se}}_2\mathrm{{Te}}$ and $\mathrm{{Bi}}_2\mathrm{{Te}}_2\mathrm{{Se}}$) have been optically studied by FTIR Spectroscopy, with increasing chemical compensation. They have been measured from 5 to 300 K and from subterahertz to visible frequencies. The effect of compensation is clearly observed in the infrared spectra through the suppression of the extrinsic Drude term together with the appearance of strong absorption peaks, that we assign to electronic transitions among localized impurities states. From the far-infrared spectral weight of the most compensated sample ($\mathrm{{Bi}}_2\mathrm{{Te}}_2\mathrm{{Se}}$), one can estimate a density of charge carriers on the order of $10^{17}$ cm$10^{-3}$ in good agreement with transport data. Those results demonstrate that the low electrodynamics in single crystals of TI, even at the highest degree of compensation presently achieved, is still influenced by three-dimensional charge excitations. Its spectral weight is, indeed, still nearly higher by two orders of magnitude than that expected from the topological surface states, estimated from optical conductivity of films of $\mathrm{{Bi}}_2\mathrm{{Se}}_3$ on sapphire substrate. Such films have been measured in the sub-THz and THz frequency region, in order to study their optical conductivity as a function of their thickness. One can observe no appreciable change in the free carriers contribution, while the $\alpha$ phonon intensity strongly decreases with decreasing thickness, demonstrating that the only contribution to the transport is due to surface carriers, not depending on bulk characteristics.The surface metallic state of the thin TI films has been finally studied by patterning the films by a grating, as explained in Chap. 2. This provides the possibility to detect surface plasmonic collective modes, due to the excitation of two dimensional charge density waves along the topological interface of the samples. In the last part of this thesis those plasmons will be analyzed, demonstrating that they have two dimensional nature characteristic of 2DEGs (see Sect. 1.2.1).