Rotational ordering transition in single-crystal c-60 studied by raman-spectroscopy - reply

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Raman scattering in electronically excited C60

The low-temperature Raman spectrum of 2a0fcc single crystal C60 recorded using low-irradiance 514 nm excitation shows only a single peak in the region of the 1468 cm-1 out-of-phase ring mode. For higher irradiances a new broad peak appears in the spectrum at a somewhat lower frequency. Simultaneously, an increase of the luminescence is observed. As the irradiance increases the new peak gains intensity and shifts to lower frequencies, whereas the original peak slowly disappears. It is argued that this peak originates from electronically excited C60. A simple electronic four-level model is proposed to explain the observed effects.

Rotational Ordering Transition in Single-Crystal C60 Studied by Raman Spectroscopy

Raman spectroscopic data are presented on the rotational ordering fcc→sc transition in single-crystal C60. Splittings and activation of new modes reveal the influence of the cubic structure, and are in agreement with the crystal symmetry in both phases. The temperature dependence of the spectra is consistent with a sharp first-order phase transition near 252 K. A simple model is proposed for the explanation of the temperature dependence of the spectra, involving a large vibrational-rotational coupling in the fcc phase, and essentially no rotational disorder in the sc phase.

Raman scattering in single crystal C60

Raman spectra (using 514 and 740 nm excitation) of high-purity single crystal C60 are presented for the high- and low-temperature phases, showing activity of all “gerade” modes. Within the experimental accuracy, the spectra are found to be consistent with the selection rules for Raman scattering in both the high-temperature fcc and low-temperature 2a0-fcc phases of solid C60. A complete assignment of the observed peaks is proposed.

Lattice vibrations in crystalline C70

Raman spectroscopy in single-crystalline C70 at low temperatures reveals a large number of Raman-active intramolecular modes, consistent with the selection rules and in agreement with theoretical predictions. Librational and vibrational lattice modes are observed in the low-frequency region of the spectrum, consistent with the selection rules for monoclinic P1121/m-C70. We discuss the lattice modes in terms of a simplified model of the crystal potential. Near 280 K we observe a phase transition, which is assigned to the order-disorder transition for the rotational motion around the long molecular axis.

High frequency electron spin resonance in Erm@C2n

High-field electron spin resonance experiments have been performed on an Er fullerene extract, containing about 1% endohedral Er fullerenes (Erm@C82). The ESR spectra of (Erm@C82) show two resonances, which we propose to originate from the (Er@C82) molecules, one resonance from an unpaired electron on the fullerene cage and one from the Er3+ 4f electrons.

Optical properties of incommensurately modulated calaverite, AuTe2

The optical reflection spectrum of incommensurately modulated calaverite is presented. Using a Kramers-Kronig analysis the dielectric function is derived from the reflection spectrum. The results for the imaginary part of the dielectric function are qualitatively discussed in terms of absorption by phonon, free electron and interband scattering. The phonon contribution clearly shows the influence of the incommensurability by the activity of phonons with k ≠ 0. The free electron scattering can be understood in terms of a simple Drude model. The interband scattering is found at a lower frequency than predicted from electronic band structure calculations.

Raman study of the ferroelectric semiconductor Sn2P2Se6

The Raman spectrum of Sn2P2Se6 is found to be in good agreement with the selection rules in the paraelectric and ferroelectric phases. The incommensurability has only a minor influence on the phonon scattering in Sn2P2Se6. A broad central peak is found in the ferroelectric phase, whose intensity reaches a maximum near the lock-in phase transition. Line-shape distortions of some of the phonon modes indicate a Fano type of coupling to the excitations causing the central peak.

Charge and Sodium Ordering in β-Na0.33V2O3

Polarized Raman and optical spectra for the quasi one-dimensional metallic vanadate β-Na0.33V2O3 are reported for various temperatures. The spectra are discussed in the light of the sodium and charge ordering transitions occurring in this material, and demonstrate the presence of strong electron–phonon coupling.

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