Electrodynamic properties of nanoporous silicon in the range from terahertz to infrared frequencies

The dielectric response (conductivity and permittivity) spectra of a series of nanoporous silicon samples prepared by anodization of low-resistivity single-crystal silicon are measured, for the first time, using terahertz and IR spectroscopy in the frequency range 7–4000 cm^?1 at room temperature. The spectra obtained are analyzed in terms of the effective medium theory with a size-dependent dielectric response function of nanoinclusions and averaged dielectric characteristics of the surrounding medium. The geometric and dielectric characteristics of silicon nanoinclusions are determined. The dielectric properties of inclusions are found to be affected by nanosize effects, namely, carrier scattering at crystallite boundaries and a broadening of the band gap due to quantum confinement. The spectra of the samples prepared by adding iodine to the electrolyte exhibit a resonance at frequencies of 150–300 cm^?1. The nature of the resonance can be associated with the presence of chemisorbed iodine on the surface of porous silicon. Possible mechanisms responsible for the changes in broadband conductivity and permittivity spectra of single-crystal silicon upon transformation into a nanoporous structure are discussed.     

Free radical synthesis of 2-alkoxytetrahydrofurans

Conclusions A new method is presented for obtaining 2-alkoxy tetrahydrofurans by reacting alcohols with, - unsaturated aldehydes in the presence of a peroxide.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2829–2831, December, 1968.     

On the problem of the LO-TO splitting of the soft mode in CaTiO3

The dispersion analysis of the panorama reflection spectrum of ceramic calcium titanate has been performed following the additive and factorized dispersion models with the goal to reveal the degree of adequacy of each of these models for determining the dispersion parameters of phonon modes. The correspondence of longitudinal and transverse IR-active vibrations has been established for the cubic and orthorhombic phases. Giant LO-TO splitting of the low-frequency soft mode (700 cm^?1), which leads to the inversion of the frequencies of all other IR vibrations in this region, has been found. This result is in good agreement with first-principles calculations.     

Terahertz spectroscopy of AuFe spin glasses

The electrodynamic response of spin glasses (in the form of thin AuFe films) in the terahertz frequency range has been studied using backward-wave oscillator (BWO) spectroscopy (10–40 cm^?1) and optical ellipsometry (5000–33000 cm^?1) techniques at temperatures from 5 to 295 K. The room-temperature dynamic conductivity spectra of AuFe films are typical of metals and can be described within the framework of the Drude theory of conduction by free charge carriers. Changes in the microscopic parameters of charge carriers in AuFe films with increasing iron content, which are related to additional scattering of carriers on the impurity magnetic moments, have been studied on the quantitative level, including the carrier relaxation frequency and characteristic time, plasma frequency, and conductivity. It is established that the spin-glass phase at a temperature of ～5 K exhibits dispersion of the conductivity in the frequency range 10–40 cm^?1, which can be related to the appearance of a mobility gap in the subsystem of free electrons involved in the RKKY interaction between magnetic centers (Fe atoms).     

Raman spectra of cadmium titanate

Polarized Raman spectra of CdTiO₃ single crystals are recorded for the first time over the frequency range 5 < ν < 1000 cm^?1 at temperatures of 10 to 1200 K. The emphasis was on the low-frequency range, where an anomalous temperature dependence of a few phonon modes was observed. At high temperatures, four phonon modes exhibiting a behavior typical of soft modes were found to exist. These phonon modes are assumed to restore the cubic symmetry of the lattice. Their extrapolated temperature dependences suggest that there exists a sequence of three hypothetical high-temperature phase transitions analogous to those observed in the genuine perovskite CaTiO₃. At temperatures below 78 K, the Raman spectrum exhibits new lines associated with polar distortions of the unit cell. At low frequencies, three lines are observed whose parameters exhibit an anomalous behavior typical of soft modes in a ferroelectric phase. Several different polar states are assumed to exist at low temperatures.