U. D. Venkateswaran

High Pressure Studies of the Raman‐Active Phonons in Carbon Nanotubes

We report high pressure Raman studies on as-prepared, purified, and solubilized single-walled and aligned multi-walled carbon nanotubes. The pressure dependence of radial (R) and tangential (T) vibrational bands in these samples is measured and compared with the results from other studies. In single-walled nanotubes, an abrupt drop in the intensity of these bands is seen near 2 GPa, indicative of a phase transition. Experiments on single (unbundled) tubes reveal a ≈10 cm—1 upshift of the R band relative to its frequency in bundled tubes. This is opposite to the predictions of calculations that include the intertube van der Waals interaction only and is explained by the changes in the electronic band dispersion driven by tube–tube interactions. Surprisingly, the pressure dependence of the R and T bands in unbundled tubes is very similar to that seen in bundled tubes, which indicates that the changes in the electronic band structure might significantly influence the pressure dependence.

Photo-produced defect luminescence in solid C60

Abstract We report changes in the photoluminescence (PL) spectra of solid C60 crystals and films when exposed to low levels of visible laser radiation. New PL features at 1.73 and 1.76 eV are seen to emerge at low temperatures upon laser irradiation at low fluences in C60 crystals grown from purified powder. In rapidly grwon crystals, the 1.76 eV peak dominates the PL spectrum. These new peaks are interpreted as the emission from metastable defects (X-traps) which are either photo-induced or produced during growth. In analogy with solid anthracene emission, the variations observed in the positions and relative intensities of the PL peaks in solid C60 are thought to arise from differences due to the presence of chemical impurities or physical imperfections, growth methods, irradiation of light, temperature cycles, and strain in the crystal.

Temperature dependence of the Raman scattering in HgBa2CuO4+δ

Abstract The temperature dependence of the high frequency Raman mode in HgBa 2 CuO 4+ δ belonging to the vibrations of the apical oxygen has been studied in the temperature range of 300–10xa0K in samples with different values of δ . The frequency, linewidth, and lineshape of this mode are found to exhibit a non-monotonic variation with temperature. The trends in the temperature dependent variation are found to be qualitatively the same in over-, under-, and optimally oxygen doped samples, although there are quantitative sample dependent differences. We show that the maximum change in the frequency and the asymmetry of this peak are linear in the excess oxygen content, δ .