Phonon drag thermopower in doped bismuth

The low-temperature (2<T<80 K) thermopower in bismuth doped by tellurium, a donor impurity (0<c≤0.07 at. % Te), is dominated by the phonon component, which shifts to higher temperatures with increasing dopant concentration. The temperature and concentration dependences of the phonon thermopower of doped bismuth are satisfactorily described by the theory of phonon drag of electrons. The theory is developed for a strongly anisotropic electron spectrum and includes both direct and two-step phonon drag.     

Room-temperature synthesis of Mn3O4 nanorods

Nanocrystalline Mn₃O₄ hausmannite has been easily prepared from manganese(II) acetate tetrahydrated, dissolved in a mixture of N,N’-dimethylformamide (DMF) and water (10%) at room temperature, without post-treatment of heating. Stability of the Mn₃O₄ colloidal dispersion was monitored by UV-visible electronic absorption spectroscopy. X-ray powder diffraction (XRD) pattern demonstrates its good phase purity. Analysis by transmission electron microscopy (TEM) image shows homogeneous nanorods with a narrow size distribution, being the average diameter and length of 6.58 nm and 17.44 nm, respectively. Moreover, the facility of the hausmannite nanorods to adhere to glass wall in this solvent mixture has allowed the formation of thin films which were analyzed by atomic force microscopy (AFM). PACS 81.07.Bc; 81.16.Be; 61.46.+w     

Electron relaxation mechanisms in n-Bi-Sb semiconducting alloys

The magnetic field dependence of diffusion thermal electromotive force α₂₂(H) (?T ∥ C ₁) in degenerate n-Bi-Sb semiconducting alloys, in which only L electrons participate in transfer phenomena, had a maximum at H ∥ C ₃. The electron relaxation time was determined from the magnetic field value corresponding to this maximum. The dependences of the electron relaxation time on temperature and the concentration of alloy components and the dopant (on the concentration of electrons) were used to separate electron relaxation time components corresponding to scattering by phonons, ionized impurities, and component concentration fluctuations. The latter (“alloy”) mechanism of electron scattering by concentration fluctuations was for the first time considered for Bi-Sb alloys; its contribution was found to be comparable with those of the other scattering mechanisms. The obtained relaxation times were used to calculate theoretical magnetic field dependences of thermal electromotive force and the Nernst-Ettingshausen coefficient. The calculation results were in satisfactory agreement with experiment.     

Quantum oscillations of the resistivity and hall coefficient and the quantum limit in Bi<Subscript>0.93</Subscript>Sb<Subscript>0.07</Subscript> alloys in a magnetic field along the trigonal axis

The dependences of the electrical resistivity ρ and the Hall coefficient R on the magnetic field have been measured for single-crystal samples of the n-Bi_0.93Sb_0.07 semiconductor alloys with electron concentrations in the range 1 × 10¹⁶ cm⁻³ < n < 2 × 10¹⁸ cm⁻³. It has been found that the measured dependences exhibit Shubnikov-de Haas quantum oscillations. The magnetic fields corresponding to the maxima of the quantum oscillations of the electrical resistivity are in good agreement with the calculated values of the magnetic fields in which the Landau quantum level with the number N intersects the Fermi level. The quantum oscillations of the Hall coefficient with small numbers are characterized by a significant spin splitting. In a magnetic field directed along the trigonal axis, the quantum oscillations of the resistivity ρ and the Hall coefficient R are associated with electrons of the three-valley semiconductor and are in phase with the magnetic field. In the case of a magnetic field directed parallel to the binary axis, the quantum oscillations associated both with electrons of the secondary ellipsoids in weaker magnetic fields and with electrons of the main ellipsoid in strong magnetic fields (after the overflow of electrons from the secondary ellipsoids to the main ellipsoid) are also in phase. In magnetic fields of the quantum limit ħω _c /2 ≥ E _F, the electrical conductivity increases with an increase in the magnetic field: σ₂₂(H) ∼ H ^k . A theoretical evaluation of the exponent in this expression for a nonparabolic semiconductor leads to values of k close to the experimental values in the range 4 ≤ k ≤ 4.6, which were obtained for samples of the semiconductor alloys with different electron concentrations. A further increase in the magnetic field results in a decrease of the exponent k and in the transition to the inequality σ₂₂(H) ≤ σ₂₁(H).     

Optical spectroscopy of europium molybdate in the crystalline and amorphous States

Comparative spectroscopic studies of crystalline and amorphous samples of Eu₂(MoO₄)₃ were carried out. Amorphous samples were obtained through exposure of the β' crystal phase to a high pressure of ∼9 GPa. It was established that the transition to the amorphous state is accompanied by substantial changes both in the luminescence spectrum and the luminescence excitation spectrum. The long-wavelength absorption edge is estimated to shift by ∼0.8 eV, which is much more significant than in the case of amorphization of classical semiconductors.     

Equilibrium of acyl transfer between pyridine N-oxides and their acylonium salts

Transfer of acyl groups from N-acyloxypyridinium salts to pyridine N-oxides in acetonitrile was studied. The equilibrium constants of acyl exchange were determined. These quantities vary in the range covering eight orders of magnitude, depending on the structure of the reagents, and are independent of the structure of the acyl group.     

Phenylhydrazines. Structure and basicity

The main properties of phenylhydrazines are governed by the p-character of the unshared electron pair of the nitrogen of imino group changing in keeping with sp ³ → sp ²-regibridization of its atomic orbitals under the steric and electronic (-I effect of amino group) effect of substituents.     

Analysis of the spin splitting of maxima of quantum oscillations of the resistivity of n-Bi-Sb semiconductor alloys in a magnetic field parallel to the bisector axis

In samples of semiconductor alloys n-Bi_0.93Sb_0.07 with different electron concentrations (n ₁ = 8 × 10¹⁵ cm^?3, n ₂ = 1.2 × 10¹⁷ cm^?3, and n ₃ = 1.9 × 10¹⁸ cm^?3), dependences of the electrical resistivity on magnetic fields up to 45 T parallel to the current and the bisector axis (H ‖ C ₁ ‖ j) have been measured at temperatures of 1.5, 4.5, and 10 K. The obtained dependences ρ₂₂(H) demonstrate quantum oscillations of the resistivity (Shubnikov-de Haas effect), and, in high magnetic fields, there is a resistivity maximum far away from other maxima. On assumption that this maximum is related to the spin-split Landau level N = 0^? for electrons of the main ellipsoid, the spin-splitting parameters are calculated for electrons of the main ellipsoid: γ₁ = 0.87, γ₂ = 0.8, and γ₃ = 0.73. Using these values, the oscillation maxima can be reliably related to the numbers of split Landau levels for electrons of the main and secondary ellipsoids. The dependences of the resistivity ρ₁₁ and the Hall coefficient R _31.2 on magnetic field have been measured in a transverse magnetic field at H ‖ C ₁ and j ‖ C ₂ on the sample with the electron concentration n ₄ = 1.4 × 10¹⁷ cm^?3. Using similar analysis, the spin-splitting parameter is found to be γ₄ = 0.85, which is close to the value of γ₂ = 0.8 obtained for the sample with close electron concentration (n ₂ = 1.2 × 10¹⁷ cm^?3) during the measurements in a longitudinal magnetic field. The quantum oscillation maxima of Hall coefficient R _31.2 are shifted to the range of high magnetic fields as compared to the quantum oscillation maxima of resistivity ρ₁₁.