Search

The crystal structure of CuZnSiSe semiconductor, grown by the chemical vapor transport method, was determined by the powder and single-crystal X-ray diffraction and shown to be orthorhombic wurtzstannite (space group P mn2). Lattice dynamics of CuZnSiSe is studied by the low-temperature off-resonant polarized and resonant Raman scattering. Out of the total 45 optical vibrational modes allowed by the symmetry, 37 are observed in the spectra and are assigned to the specific lattice eigenmodes based on their polarization properties and a comparison of frequencies with the results of density functional lattice dynamics calculations. Good agreement between theoretical and experimental mode frequencies is established. The vibrational patterns for the most intense spectral lines in experimental spectra as well as pronounced resonant enhancement of Si-related modes are discussed.

Density-functional calculations of the electronic band structure, optical properties, and lattice dynamics of kesterite CuZnGeSe semiconductor (space group ) are reported. Theoretical optical properties of the material are shown to agree quantitatively with available spectroscopic ellipsometry experimental data that allow detailed interpretation of the main spectral features around the fundamental absorption edge. A good agreement between theoretical and experimental vibrational mode frequencies is also established and the assignment of observed phonon modes to specific lattice eigenmodes is performed.

Experimental Raman scattering study of mixed AgxGaxGe1−xSe2 (0.167 ≤ x ≤ 0.333) single crystals is reported. The spectra transformation upon composition variation exhibits the two-mode type of behavior, with lines of the end-member compounds of the solid solution (AgGaSe2 and GeSe2) dominating in the spectra, whose relative intensities vary with composition.

Chromium dioxide, CrO2, attracts significant interest as it has a unique electronic band structure, resulting in half-metallic ferromagnetism (TC ≈ 390 K) with completely spin-polarized electrons at EF. In this work we report the polarized Raman spectra of CrO2 between 5 and 420 K, as obtained from (110) and (001) oriented thin films on TiO2 substrates of the same orientation. The Raman lines corresponding to the four Raman allowed phonon modes of A1g, B1g, B2g and Eg symmetry were identified. The two low frequency Raman lines are very narrow, in particular at low temperatures, indicating weak scattering of the corresponding B1g and Eg phonons. The Raman intensities are not strongly affected by the magnetic ordering, whereas the line position versus T dependencies change their slope near TC for the A1g and Eg modes. At low temperatures the variations with T of the phonon linewidths follow the dependence expected for a phonon–phonon scattering. The additional broadening, which appears with approaching TC is tentatively assigned to the scattering of phonons from collective spin fluctuations.

Polarized Raman scattering spectra of PrBa2Cu4O8 were studied in the temperature range 10 to 300 K. An anomalous temperature dependence of the two-magnon peak was found, which softens upon cooling from about 2590 cmy1 at 300 K to 2480 cmy1 at 10 K. The observed behavior is opposite to what is known for RBa2Cu3O6, R2CuO4 (R ˆ rare earth) and KMnF3-type antiferromagnets, where the two-magnon peak shifts to higher frequencies upon decreasing the in-plane Cu±O bond length due to either lowering temperature or applying pressure. The experimental data are discussed in view of the semiconductor±metal transition which, according to recent optical and transport studies, occurs in PrBa2Cu4O8 at T* 150 K.

Resonant Raman scattering in bulk Zn1–x CdxSe crystals obtained from the vapour phase by the “free-growing” method is studied experimentally. The existence of seventh-order LO-phonon lines under condition of excitation above the band gap and fourth-order LO-phonon lines under condition of excitation below the band gap is observed for mixed semiconductor crystals for the first time. The obtained ratios of intensities of different lines in multiple-phonon spectra are compared with the results of the theory involving the continuous exciton spectrum as an intermediate state. [Russian Text Ignored]

Temperature studies of lattice dynamics and ionic conductivity, within the temperature range of 100 to 700 K, are performed for PbF2 and PbF2:6% LiF superionic crystals. The doped crystals exhibit a decreased activation energy of the conductivity. A decrease, by about 70 degrees, in the temperature at which the ionic conductivity changes and Raman spectra anomalies occur is observed. It is found that at temperatures significantly lower than that of the superionic state transition, the Raman line form is determined by processes of phonon scattering at thermally excited defects. Fur PbF2- und PbF2:6% LiF-Superionenkristalle werden Temperaturuntersuchungen der Gitterdynamik und Ionenleitfahigkeit im Temperaturbereich von 100 bis 700 K durchgefuhrt. Die dotierten Kristalle zeigen eine verringerte Aktivierungsenergie der Leitfahigkeit. Es wird ein Abfall von etwa 70 Grad fur die Temperatur bei der die Ionenleitfahigkeit sich andert und Anomalien im Ramanspektrum auftreten, beobachtet. Es wird gefunden, das bei Temperaturen, die betrachtlich niedriger sind als die des Superionenzustandsubergangs, die Raman-Linienform durch Prozesse der Phononenstreuung an thermisch angeregten Defekten bestimmt wird.

The results of Raman scattering studies of polariton and plasmon-polariton (plasmariton) excitations in 6H-SiC are presented. Polariton anisotropy and resonant coupling with one- and two-phonon host lattice vibrations are discussed. The effect of free charge carriers upon the dispersion of polaritons in nitrogen-doped 6H-SiC crystals is studied. Good agreement of theoretical calculations with the experimental results is obtained. [Russian Text Ignored].