We have measured the temperature dependence of the first-order Raman scattering by phonons in Si, Ge, and $\ensuremath{\alpha}\ensuremath{-}\mathrm{S}\mathrm{n}$. The full widths at half maximum of the Raman lines, extrapolated to zero temperature, are 1.24\ifmmode\pm\else\textpm\fi{}0.07, 0.75\ifmmode\pm\else\textpm\fi{}0.03, and 0.81\ifmmode\pm\else\textpm\fi{}0.15 ${\mathrm{cm}}^{\ensuremath{-}1}$ for Si, Ge, and $\ensuremath{\alpha}\ensuremath{-}\mathrm{S}\mathrm{n}$, respectively. The reliability of the data obtained allows a critical examination of the theoretical calculations published so far. We show that the model assuming the decay of the Raman phonon into two acoustical phonons belonging to the same branch, first proposed by Klemens, does not represent adequately the temperature dependence of the Raman linewidth. The most important decay channels are shown to be combinations of optical and acoustical phonons. However, the more complete calculation by Cowley, which involves all possible decay channels, gives very large zero-temperature linewidths. We show that this arises mainly from the poor description of the phonon dispersion curves by the shell model used by Cowley, and that a better agreement between theory and experiment is to be expected by repeating the calculation with Weber's adiabatic bond-charge model.

Temperature dependence of the first-order Raman scattering by phonons in Si, Ge, and α − S n : Anharmonic effects

We have measured nonresonant and resonant Raman-scattering spectra from ZnO nanocrystals with an average diameter of 20nm. Based on our experimental data and comparison with the recently developed theory, we show that the observed shifts of the polar optical-phonon peaks in the resonant Raman spectra are not related to the spatial phonon confinement. The very weak dispersion of the polar optical phonons in ZnO nanocrystals does not lead to any noticeable redshift of the phonon peaks for 20-nm nanocrystals. The observed phonon shifts have been attributed to the local heating effects. We have demonstrated that even the low-power ultraviolet laser excitation, required for the resonant Raman spectroscopy, can lead to the strong local heating of ZnO nanocrystals. The latter causes significant (up to 14cm−1) redshift of the optical-phonon peaks compared to their position in bulk crystals. Nonresonant Raman excitation does not produce noticeable local heating. The obtained results can be used for identification ...

Micro-Raman investigation of optical phonons in ZnO nanocrystals

Vibrational thermodynamics of materials

Strong chemical activity and extreme instability in ambient conditions characterize carbyne, an infinite sp1 hybridized carbon chain. As a result, much less has been explored about carbyne as compared to other carbon allotropes such as fullerenes, nanotubes and graphene. Although end-capping groups can be used to stabilize carbon chains, length limitations are still a barrier for production, and even more so for application. We report a method for the bulk production of long acetylenic linear carbon chains protected by thin double-walled carbon nanotubes. The synthesis of very long arrangements is confirmed by a combination of transmission electron microscopy, X-ray diffraction and (near-field) resonance Raman spectroscopy. Our results establish a route for the bulk production of exceptionally long and stable chains composed of more than 6,000 carbon atoms, representing an elegant forerunner towards the final goal of carbyne’s bulk production. One-dimensional linear carbon chains reaching a length close to 800 nm have been synthesized at high temperature and high vacuum using double-walled carbon nanotubes as nanoreactors.

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Confined linear carbon chains as a route to bulk carbyne.

The high density of heat generated in power electronics and optoelectronic devices is a critical bottleneck in their application. New materials with high thermal conductivity are needed to effectively dissipate heat and thereby enable enhanced performance of power controls, solid-state lighting, communication, and security systems. We report the experimental discovery of high thermal conductivity at room temperature in cubic boron arsenide (BAs) grown through a modified chemical vapor transport technique. The thermal conductivity of BAs, 1000 ± 90 watts per meter per kelvin meter-kelvin, is higher than that of silicon carbide by a factor of 3 and is surpassed only by diamond and the basal-plane value of graphite. This work shows that BAs represents a class of ultrahigh-thermal conductivity materials predicted by a recent theory, and that it may constitute a useful thermal management material for high-power density electronic devices.

https://science.sciencemag.org/content/sci/early/2018/07/03/science.aat8982.full.pdf

High thermal conductivity in cubic boron arsenide crystals.

Systematic measurements by light scattering of the linewidth and frequency shift of the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{q}}=0$ optical phonon in silicon over the temperature range of 5-1400 K are presented. Both the linewidth and frequency shift exhibit a quadratic dependence on temperature at high temperatures. This indicates the necessity of including terms in the phonon proper self-energy corresponding to four-phonon anharmonic processes.

Anharmonic effects in light scattering due to optical phonons in silicon

The electronic Raman scattering efficiency for interband transitions between valenceband states of $p$-type silicon is calculated taking into account the detailed band structure. In contrast to the isotropic model, the real spectrum reflects the band anisotropy: the spectrum extends from zero to rather large scattering frequency with a long tail; there appears a peak at about 500-600 ${\mathrm{cm}}^{\ensuremath{-}1}$. The result is compared with the experiment of Jouanne and is found to be in fair agreement.

R. F. Wallis

Papers

Anharmonic effects in light scattering due to optical phonons in silicon

Interband electronic Raman scattering in p -silicon