Polarized micro-Raman spectroscopy has been performed on spatially separated single-wall carbon nanotubes (SWNTs) in the form of individual nanotubes or thin ropes of only a few SWNTs. Different from bulk samples, the Raman spectra are composed of well-resolved peaks which allow a direct comparison of experimental data with theoretical calculations. Orientation-dependent measurements reveal maximum intensity of all Raman modes when the nanotubes are aligned parallel to the polarization of the incident laser light. The angular dependences clearly deviate from the selection rules predicted by theoretical studies. These differences are attributed to depolarization effects caused by the strongly anisotropic geometry of the nanotubes and to electronic resonance effects for excitation at 633 nm.

LaMnO(3) was studied by synchrotron x-ray diffraction, optical spectroscopies, and transport measurements under pressures up to 40 GPa. The cooperative Jahn-Teller (JT) distortion is continuously reduced with increasing pressure. There is strong indication that the JT effect and the concomitant orbital order are completely suppressed above 18 GPa. The system, however, retains its insulating state to approximately 32 GPa, where it undergoes a bandwidth-driven insulator-metal transition. Delocalization of electron states, which suppresses the JT effect but is insufficient to make the system metallic, appears to be a key feature of LaMnO(3) at 20-30 GPa.

The effect of pressure on optical phonon frequencies of MgB2 has been calculated using the frozenphonon approach based on a pseudopotential method. Grüneisen parameters of the harmonic mode frequencies are reported for the high-frequency zone-center E2g and B1g and the zone-boundary E2u and B2u modes at A. Anharmonic effects of phonon frequencies and the implications of the calculated phonon frequency shifts for the pressure dependence of the superconducting transition temperature of MgB2 are discussed. Also reported are Raman and optical reflectance spectra of MgB2 measured at high pressures. The experimental observations in combination with calculated results indicate that broad spectral features we observed in the Raman spectra at frequencies between 500 and 900 cm −1 cannot be attributed to first-order scattering by zone-center modes, but originate in part from a chemical species other than MgB2 at the sample surface and in part from a maximum in the MgB2 phonon density of states. Low-temperature Raman spectra taken at ambient pressure showed increased scattering intensity in the region below 300 cm −1 .

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