The electronic properties of magnetic semiconductors TlFeS 2 and TlFeSе 2 have been investigated experimentally by spectral ellipsometry and theoretically ab initio using the density functional theory (DFT). The imaginary and real parts of the dielectric function and the dispersion of the refractive indices, extinction coefficients, and absorption coefficients are found from ellipsometric measurements in the energy range of 0.7−6.5 eV. The direct band gap is estimated. The band structure, the origin of energy states, the optical functions, and the partial densities of states (PDOS) projected on atoms are determined from ab initio calculations. The calculation results are compared with the spectral ellipsometry data obtained in this study.

X-ray studies of TlFeS2 and TlFeSe2 crystals grown by high-temperature synthesis reveal their single-phase nature and isostructurality with the space group C2/m. Raman scattering and infrared reflection of light in TlFeS2 and TlFeSe2 compounds is studied at a temperature of 300 K. Characteristic frequencies in the Raman scattering and infrared spectra of phonons in TlFeS2 and TlFeSe2 are detected. An analysis of published works on the magnetic properties of TlFeS2 and TlFeSe2 allows the conclusion that they are quasi-one-dimensional antiferromagnets each having two characteristic temperatures TN3D and $$T_{c}^{{{\text{super-p}}}}$$ , between which quasi-one-dimensional antiferromagnetic ordering in TlFeS2 and TlFeSe2 is favorable. The temperature $$T_{c}^{{{\text{super-p}}}}$$ is introduced for the first time and characterizes a highly developed short-range magnetic order, in which superparamagnetic ordering exists.

By spectroscopic ellipsometry, the imaginary and real parts of the dielectric functions of Bi 2 Se 3 and Bi 2 Se 3 〈Cu〉 single crystals are obtained in the photon-energy range from 0.7 to 6.5 eV, and the energies of allowed direct transitions responsible for some optical properties are determined. Using the Cody–Lorentz optical oscillator and the Lorentz optical oscillator, the dispersion relation is adjusted in conformity with the experimental data. The values of the refractive index ( n ) and extinction coefficient ( k ) of the Bi 2 Se 3 single crystals are calculated in the energy range from 0.7 to 6.5 eV. It is established that the maximum absorption corresponds to the energies of 1.900 and 1.949 eV for Bi 2 Se 3 and Bi 2 Se 3 〈Cu〉, respectively.

X-ray studies of TlFeS 2 and TlFeSe 2 crystals grown by high-temperature synthesis reveal their single-phase nature and isostructurality with the space group C2/m. Raman scattering and infrared reflection of light in TlFeS 2 and TlFeSe 2 compounds is studied at a temperature of 300 K. Characteristic frequencies in the Raman scattering and infrared spectra of phonons in TlFeS 2 and TlFeSe 2 are detected. An analysis of published works on the magnetic properties of TlFeS 2 and TlFeSe 2 allows the conclusion that they are quasi-one-dimensional antiferromagnets each having two characteristic temperatures T N3D and T_c^super-p , between which quasi-one-dimensional antiferromagnetic ordering in TlFeS 2 and TlFeSe 2 is favorable. The temperature T_c^super-p is introduced for the first time and characterizes a highly developed short-range magnetic order, in which superparamagnetic ordering exists.

Vibrational properties of TlFeS2 and TlFeSe2 crystal lattices have been investigated experimentally using Raman scattering (RS) and infrared reflection (IR) of light, as well as theoretically, using density functional theory (DFT). Full vibrational representation, compiled on the basis of the analysis of factor symmetry groups, contains 12 RS active modes and 9 IR active modes. Of these, six active RS modes and three IR active modes have been experimentally detected and identified for TlFeS2, as well as four RS active modes and three IR active modes for TlFeSe2.

The lattice vibrational properties of TlFeS2 and TlFeSe2 crystals have been studied experimentally using Raman scattering (RS) and infrared reflection (IR) light, as well as theoretically using density functional theory (DFT). The complete vibrational representation, based on the analysis of the factor site symmetry, contains 12 active Raman modes and 9 IR active modes. 6 RS active and 3 IR active modes of them for TlFeS2, and 4 RS active and 3 IR active modes for TlFeSe2 were experimentally detected and identified.

Рентгеновские исследования кристаллов TlFeS2, TlFeSe2, полученных методом высокотемпературного синтеза, выявили их монофазность и изоструктурность с пространственной группой симметрии С2/m. Исследованы комбинационное рассеяние света и инфракрасное отражение света в соединениях TlFeS2, TlFeSe2 при температуре 300 K. Обнаружены и определены характерные частоты в рамановском рассеянии и инфракрасных спектрах фононов в TlFeS2, TlFeSe2. Из анализа опубликованных работ по магнитным свойствам TlFeS2, TlFeSe2 сделано заключение, что они являются квазиодномерными антиферромагнетиками, каждый из которых обладает двумя характеристическими температурами, TN3D и T_csuper-p, между ними и располагается квазиодномерное антиферромагнитное упорядочение в TlFeS2, TlFeSe2. Температура T_csuper-p введена впервые и характеризует сильно развитый ближний магнитный порядок, в котором существует суперпарамагнитное упорядочение. Ключевые слова: оптические фононы, комбинационное рассеяние света, инфракрасное отражение света, монофазность и изоструктурность кристаллов, квазиодномерные антиферромагнетики.

In this work, by spectroscopic ellipsometry the imaginary and real parts of the dielectric function of Bi2Se3 and Bi2Se3 single crystals were obtained in the photon energy range from 0.7 eV to 6.5 eV, and were determined the energies of allowed direct transitions responsible for some optical properties. By the help of Cody-Lorentz and Lorentz optical oscillators, the dispersion relation is adjusted in conformity with the experimental data. The values of the refractive index n and the extinction coefficient k of Bi2Se3 single crystals are calculated in the energy range from 0.7 eV to 6.5 eV. It was also determined that the maximum absorption corresponds to the transitions at 1.900 eV and 1.949 eV for Bi2Se3 and Bi2Se3, respectively.

By spectroscopic ellipsometry, the imaginary and real parts of the dielectric functions of Bi2Se3 and Bi2Se3〈Cu〉 single crystals are obtained in the photon-energy range from 0.7 to 6.5 eV, and the energies of allowed direct transitions responsible for some optical properties are determined. Using the Cody–Lorentz optical oscillator and the Lorentz optical oscillator, the dispersion relation is adjusted in conformity with the experimental data. The values of the refractive index (n) and extinction coefficient (k) of the Bi2Se3 single crystals are calculated in the energy range from 0.7 to 6.5 eV. It is established that the maximum absorption corresponds to the energies of 1.900 and 1.949 eV for Bi2Se3 and Bi2Se3〈Cu〉, respectively.

AbstractThe temperature dependences of the Raman-active frequencies $$E_{g}^{2}$$ , $$A_{{1g}}^{2}$$ in layered Bi_2Se_3 single crystals are studied. The contribution of the thermal expansion to a temperature variation in the frequencies is determined. The decay of the anisotropy of the elastic properties in layered Bi_2Se_3 single crystals due to strong spin–orbit coupling is shown. The Gruneisen-mode parameters for phonons $$E_{g}^{2}$$ and $$A_{{1g}}^{2}$$ are calculated. Systematic features in the dependences of vibrational frequencies on the atomic masses in layered Bi_2Te_3, Bi_2Se_3, and Sb_2Te_3 single crystals are determined.

The temperature dependences of the Raman-active frequencies $$E_{g}^{2}$$ , $$A_{{1g}}^{2}$$ in layered Bi2Se3 single crystals are studied. The contribution of the thermal expansion to a temperature variation in the frequencies is determined. The decay of the anisotropy of the elastic properties in layered Bi2Se3 single crystals due to strong spin–orbit coupling is shown. The Gruneisen-mode parameters for phonons $$E_{g}^{2}$$ and $$A_{{1g}}^{2}$$ are calculated. Systematic features in the dependences of vibrational frequencies on the atomic masses in layered Bi2Te3, Bi2Se3, and Sb2Te3 single crystals are determined.

The raman spectra of ZnGa 2 Se 4 single crystals are studied at 300 K in various experimental configurations. The observed phonon frequencies are identified by their types of symmetry. The optical phonon frequencies are related to particular atomic displacements in the unit cell.

The raman spectra of ZnGa2Se4 single crystals are studied at 300 K in various experimental configurations. The observed phonon frequencies are identified by their types of symmetry. The optical phonon frequencies are related to particular atomic displacements in the unit cell.

The Raman spectra of CdGa2S4x Se4(1 − x) alloys (x = 0.1, 0.2, … 0.9) are studied. Both the singlemode and double-mode behavior of optical phonons are observed in CdGa2S4x Se4(1 − x) alloys. The observed optical mode at 138 cm−1 is independent of the composition. It seems likely that this mode is the “breathing mode” and is caused by atomic motion in the anion sublattice relative to vacancies. It is shown that the high-frequency modes of symmetry B 1(LO, TO) and B 2(LO, TO) are caused by the in-phase motion of atoms in the anion sublattice along the tetragonal axis c relative to trivalent Ga atoms. The doubly degenerate symmetry modes E 1(LO, TO) and E 2(LO, TO) are caused by the in-phase motion of atoms in the anion sublattice relative to trivalent Ga atoms of the cation sublattice in the xy plane (dipole 2Ga–4C), where C is S, Se. The optical symmetry modes B 3(LO, TO) and B 4(LO, TO) are associated with the motion of atoms in the anion sublattice relative to Cd atoms along the tetragonal axis c. The doubly degenerate modes E 3(LO, TO) and E 4(LO, TO) are associated with the motion of atoms in the anion sublattice relative to Cd atoms (dipole Cd–4C). The low-frequency modes B 5(LO, TO) and E 5(LO, TO) are the analogs of acoustic phonons at the edge of the Brillouin zone of sphalerite.