The pressure response of double-wall carbon nanotubes has been investigated by means of Raman spectroscopy up to 10 GPa. The intensity of the radial breathing modes of the outer tubes decreases rapidly but remain observable up to 9 GPa, exhibiting a behavior similar (but less pronounced) to that of single-wall carbon nanotubes, which undergo a shape distortion at higher pressures. In addition, the tangential band of the external tubes broadens and decreases in amplitude. The corresponding Raman features of the internal tubes appear to be considerably less sensitive to pressure. All findings lead to the conclusion that the outer tubes act as a protection shield for the inner tubes whereas the latter increase the structural stability of the outer tubes upon pressure application.
We present a Raman study of the pressure induced structural phase transitions in the tungstates Caw04 and SrWO4. Raman spectra up to 24 GPa at room temperature reveal reversible pressure induced phase transitions at 10 and 11.5 GPa for Caw04 and SrW04, respectively. The lowest B, mode, associated with the wo4-wo4 vibration along the c-axis, exhibits a softening in the scheelite structure, while in the high-pressure phase its slope becomes positive. Above the phase transition the modes originating from the antisymmetric v3(E,) and vg(Bg) stretching modes of the W 0 4 group exhibit a frequency jump to higher and lower frequencies. In the high pressure phase the modes originating from v3 show an unusual, strongly nonlinear softening in a small pressure region before becoming almost pressure independent.
We have studied the effect of isotopic substitution on the superconducting Tc in the 90-K superconductors Ba2YCu307 and Ba2EuCu307 by replacing ^^O with the heavier isotope '^O. Samples with approximately 75% of the '^O replaced by '^O were prepared by gas-phase ion exchange. In these samples the phonon frequencies, measured by Raman spectroscopy, are reduced by the expected ~4%. The transition temperatures, however, are found to change by less than 0.2%. This change in Tc is much less than that expected for strongly coupled phonon-mediated superconductivity.
PACS: 63.20.Dj; 78.30.Am; 87.64.Je This paper reports detailed lattice dynamics studies involving experimental Raman scattering measurements and theoretical rigid ion model calculations of the rare earth aluminum garnets (RE 3 Al 5 O 12 ). The studies are fairly involved as these garnets have complex crystal structure with 80 atoms/primitive cell. Our calculations have provided a theoretical understanding of the mode eigenvectors, phonon dispersion relations, density of states, and effective charges of these materials. The calculated Raman mode eigenvectors reveal that they correspond to mixtures of molecular modes of the basic polyhedra, implying thus strongly coupled polyhedra. The assignment of the Raman and infrared active modes has been elucidated and the frequencies of some modes, which do not appear in the vibrational spectrum, have been calculated. Our calculations show a differentiation of the effective charges at the various symmetry sites, which leads to a mostly covalent and to an almost ionic character for the tetrahedral Al-O and the dodecahedral RE-O bonds, respectively. Finally, the dispersion curves along the [100] direction of the Brillouin zone as well as the one and two-phonon density of states have been calculated and discussed.
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