N. P. Stepanov

Magnetic susceptibility of (Bi2 − xSbx)Te3 (0 < x < 1) alloys in the temperature range 2 to 50 K

The superconducting quantum interferometer device with Josephson junctions (SQUID magnetometer) is used to study the temperature dependences of the magnetic susceptibility of (Bi2−xSbx)Te3 (0 < x < 1) alloy crystals in the temperature range 2 to 50 K, at the magnetic field vector H orientations H | C3 and H ⊥ C3 with respect to the crystal trigonal axis C3. It is found that the magnetic susceptibility of the ion core of the samples under study is χG = −0.35 × 10−6 cm3/g, the contribution of lattice defects to magnetic susceptibility can be disregarded, and the contribution of free carriers is of a diamagnetic nature in the entire studied temperature range. It is shown that the contribution of free carriers to the resulting magnetic susceptibility and its anisotropy can be described within the Pauli and Landau-Peierls approach. In calculating the magnetic susceptibility, taking into account the constant concentration of free carriers in the state of pronounced degeneracy, it is found that the temperature dependence of the anisotropic effective masses varies with crystal chemical composition. This is possibly associated with the complex structure of the valence band and its variation as the Sb2Te3 content in the alloy increases.

Anisotropy of plasma reflection of solid solutions (Bi 2 − x Sb x )Te 3 (0 x < 1) in the Temperature Range 78–293 K

We study IR polarized reflection spectra of solid solutions (Bi2−xSbx)Te3 (0 < x < 1) in the temperature range 78–293 K and in the excitation range of plasma oscillations of free charge carriers. We reveal that, as the content of Sb2Te3 in the solid solution increases from 0 to 50%, the anisotropy of both plasma frequencies and magnetic susceptibility vary insignificantly. This indicates that, at 293 K, these crystals do not show significant changes in the composition of groups of free carriers near the chemical potential level. We have found that to describe the observed anisotropy of plasma frequencies, it is necessary to take into account the effect of heavy holes of the valence band. This is also confirmed by the results of the study of the temperature behavior of plasma reflection spectra, which exhibit regularities that are similar to anomalous temperature changes of the Hall coefficient. We show that a decrease in plasma frequencies with increasing temperature cannot be described by an increase in the effective mass of charge carriers in accordance with the expression m* ∼ T0.17, which was obtained upon interpretation of temperature dependences of a number of kinetic coefficients. We use the empirical Moss rule ɛ∞2Eg to estimate the rate of decrease of the thermal band gap dEg/dT = −1.6 × 10−4 eV/deg in a crystal that contains 25% Sb2Te3.

Magnetic susceptibility of Bi2Te3-Sb2Te3 alloys

The magnetic susceptibility of Czochralski-grown single crystals of Bi2Te3-Sb2Te3 alloys containing 0, 10, 25, 40, 50, 60, 65, 70, 80, 90, 99.5, or 100 mol % Sb2Te3 has been investigated. The magnetic susceptibility of these crystals was determined at the temperature T = 291 K and the magnetic field H oriented parallel (χ‖) and perpendicularly (χ⊥) to the trigonal crystallographic axis C3. A complicated concentration dependence of the anisotropy of magnetic susceptibility χ‖/χ⊥ has been revealed. The crystals with the free carrier concentration p ≈ 5 × 1019 cm−3 do not exhibit anisotropy of magnetic susceptibility. The transition to the isotropic magnetic state occurs for the compositions characterized by a significantly increased (from 200 to 300 meV) optical bandgap.

Optical properties of doped bismuth telluride crystals in the region of plasma effects

The optical properties of bismuth telluride crystals doped with donor-and acceptor-type impurities are studied. The fact that energy corresponding to the resonance frequency of plasma oscillations of free charge carriers (plasmons) approaches the band-gap energy is detected in the infrared spectral region, where the main elementary excitations in the electronic system of these materials are observed. The mentioned approach of energies varies the intensity of electron-plasmon interaction, which affects the recombination processes in the materials widely used for the fabrication of thermoelectric energy converters.

Features of reflection spectra of single crystals of Bi2Te3-Sb2Te3 solid solutions in the region of plasma effects

Reflectance spectra of single crystals of Bi2Te3-Sb2Te3 solid solutions containing 0, 10, 25, 40, 50, 60, 65, 70, 80, 90, 99.5, and 100 mol % of Sb2Te3 have been studied in the range of 400–4000 cm−1 at the temperature T = 291 K and with orientation of the vector of the electric-field strength E perpendicular to the trigonal axis of the crystal C3 (E ⊥ C3). The shape of the spectra is characteristic of plasma reflection; the spectra include the features in the range 1250–3000 cm−1 corresponding to the optical band gap Eg opt. The features become more pronounced as the content of Bi2Te3 is increased to 80 mol % in the composition of the Bi2Te3-Sb2Te3 solid solution. A further increase in the content of Sb2Te3 is accompanied by discontinuities in the functional dependences of the parameters characterizing the plasma oscillations of free charge carriers on the solid-solution composition and also by a sharp increase in Eg opt.

Magnetic susceptibility of (Bi 2 − x Sb x )Te 3 (0 x < 1) crystals from 2 to 400 K

The magnetic susceptibility (χ) of crystals of (Bi2 − xSbx)Te3 (0 < x < 1) solid solutions has been measured at temperatures from 2 to 400 K. The χ of the crystals containing 10 and 25 mol % Sb2Te3 increases with temperature in the range 50 to 220 K, where the Hall coefficient of Bi2Te3 increases anomalously. The increase in diamagnetic susceptibility and Hall coefficient with temperature is shown to be caused by a reduction in light-hole concentration, accompanied by a decrease in light-hole effective mass. With increasing Sb2Te3 content, the shape of the χ(T) curve changes as a consequence of changes in band structure, which increase the influence of heavy, paramagnetic holes.

Plasmon-phonon-polaritons in p-doped Bi-Sb alloys

The phonon spectra of Bi crystals and Bi-Sb alloys indicate the presence of phonon-polaritons caused by the polarization of valence electrons, which reduces the rigidity of the transverse mode of optical lattice vibrations and increases the rigidity of the longitudinal mode of vibrations. Doping Bi-Sb alloys with Sn (p-type impurity) decreases the difference in the energy of plasma oscillations of free charge carriers and the energy of longitudinal optical phonons. In this case, the behavior of the dielectric function is described in terms of the model corresponding to the excitation of plasmon-phonon-polaritons.

Electron-plasmon interaction in acceptor-doped bismuth crystals

By doping the semimetal bismuth, it is possible to equate a plasma-oscillation energy with a band gap at the L point of the Brillouin zone. In this case, modifications in the reflection spectra are observed. The analysis of experimental data shows that the plasmon excitation is an efficient mechanism of electron-hole recombination provided that ℏωp=EG.

Reflection Spectra of Doped Bismuth-Antimony Crystals in the Far-Infrared Region of the Spectrum

The plasma reflection spectra of the doped bismuth and bismuth-antimony alloyed crystals were studied experimentally at liquid-nitrogen temperature within a range of 30–600 cm−1. Specificities in the optical functions of the samples are revealed within the infrared range that lies on the low-frequency side of the plasma edge and coincides with the optical-phonon frequencies in bismuth. When the plasma frequency is approached, the character of the interaction of radiation with anisotropic electron-hole plasma changes considerably.

Magnetic susceptibility of Bi2 − xSbxTe3 (0 < x < 1) solid solution crystals in the range from 2 to 400 K

The magnetic susceptibility of single crystals of Bi2Te3-Sb2Te3 solid solutions containing 10, 25, and 50 mol % Sb2Te3 has been measured using a superconducting Josephson interferometer in the temperature range from 2 to 400 K at 3-K intervals in magnetic fields of up to 2.38 × 106 A/m at two orientations of the magnetic field H with respect to the trigonal axis C3 of the crystals (H ‖ C3 and H ⊥ C3) with a relative uncertainty within 2%. A new approach has been proposed for separately assessing the contributions of the ionic core and free charge carriers to the magnetic susceptibility, which uses experimental data on magnetic susceptibility and effective mass anisotropy. Analysis of the data obtained in this study demonstrates that the temperature variation of carrier susceptibility in the region of degenerate and nondegenerate states of the electronic system can be described in terms of the Pauli and Landau-Peierls approaches if one takes into account the complex structure of the valence band and “nonparabolicity” of the light hole energy spectrum.