A comparative study of the electronic and optical properties of Mn-Bi-Te layered compounds was carried out using spectroscopic ellipsometry (SE) over a photon energy range of 0.7–6.5 eV at room temperature and density functional theory (DFT)-based first-principle calculations within the general gradient approximation with Hubbard like correction (GGA+U) and allowance for a spin-orbital coupling. The total energies of the above compounds in ferromagnetic (FM) and antiferromagnetic (AFM) spin configurations are obtained by taking the long-range van der Waals interaction into account. The stability of the AFM state of MnBi2Te4 and MnBi4Te7 over the corresponding FM counterpart is disclosed. The SE-based and calculated dielectric functions are compared. It is shown that interband optical transitions in the accessed photon energy range mainly occur between Mn 3d + Te 5p states of the valence band and Bi 6p + Te 5p with a small admixture of Mn 3d states of the conduction band.

With infrared spectroscopy, we studied the bulk electronic properties of the topological antiferromagnet $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ with $T_N\simeq 25\mathrm{K}$. With the support of band-structure calculations, we assign the intra- and interband excitations and determine the band gap of $E_g\approx 0.17$ eV. We also obtain evidence for two types of conduction bands with light and very heavy carriers. The multiband free-carrier response gives rise to an unusually strong increase of the combined plasma frequency, ${\omega }_{\mathrm{pl}}$, below 300 K. The band reconstruction below $T_N$ yields an additional increase of ${\omega }_{\mathrm{pl}}$ and a splitting of the transition between the two conduction bands by about 54 meV. Our study thus reveals a complex and strongly temperature-dependent multiband low-energy response that has important implications for the study of the surface states and device applications.

Further to the structure of the intrinsic magnetic topological insulators $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4·n\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)$ with $n<4$, where index $n$ is the number of quintuple Te-Bi-Te-Bi-Te building blocks inserted between the neighboring septuple Te-Bi-Te-Mn-Te-Bi-Te building blocks, the structure of the members with $n=4$, 5, and 6 was studied using x-ray powder diffraction. The unit-cell parameters and atomic positions were obtained. The obtained and available structural data were summarized to show that the crystal structure of all members of $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4·n\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)$ follows the cubic close-packing principle, independently of the space group of the given member. Confocal Raman spectroscopy was then applied. Comparative analysis of the number, frequency, symmetry, and broadening of the vibration modes responsible for the lines in the Raman spectra of the systems with $n=1$,…,6, as well as $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4\left(n=0\right)$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ ($n=\infty$) has shown that lattice dynamics of $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4·n\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)$ with $n>0$ overwhelmingly dominates by the cooperative atomic displacements in the quintuple building blocks.

The electronic, optical, and lattice oscillatory properties of CdGa₂Te₄ have been studied experimentally using spectral ellipsometry, Raman scattering (R) and infrared (IR) spectroscopy, as well as theoretically using the density functional theory (DFT). Eight Raman-active modes and twelve IR-active modes are detected and identified from consideration of the point symmetry group. Based on the analysis of the electronic spectrum and the density of energy states, the nature of the chemical bond in this semiconductor compound is determined. The theoretically calculated results are compared with the experimental data of the present work and with the results of experimental data available in the literature obtained by infrared spectroscopy and Raman scattering.