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Register a new userThe Raman coupling function in disordered solids: a light and neutron scattering study on glasses of different fragility
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We report new inelastic Raman and neutron scattering spectra for glasses with different degrees of fragility; the data are compared for each sample to obtain the Raman coupling function $C(omega)$. The study indicates a general linear behaviour of the $C(omega)$ near the Boson peak maximum, and evidence a correlation between vibrational and relaxational properties, already observed in recent publications.
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The Raman coupling coefficients of site- and bond-percolators at concentration higher than percolation threshold are computed for two scattering mechanisms: Bond Polarizability (BPOL) and Dipole-Induced-Dipole (DID). The results show that DID does not follow a scaling law at low frequency, while in the case of BPOL the situation is less clear. The numerically computed frequency dependence in the case of BPOL, which can be considered a good scattering mechanism for a wide class of real glasses, is in semiquantitative agreement with experimental results.
New Raman and incoherent neutron scattering data at various temperatures and molecular dynamic simulations in amorphous silica, are compared to obtain the Raman coupling coefficient $C(omega)$ and, in particular, its low frequency limit. This study indicates that in the $omega to 0$ limit $C(omega)$ extrapolates to a non vanishing value, giving important indications on the characteristics of the vibrational modes in disordered materials; in particular our results indicate that even in the limit of very long wavelength the local disorder implies non-regular local atomic displacements.
The Raman response of the metallic glass Ni$_{67}$Zr$_{33}$ is measured as a function of polarization and temperature and analyzed theoretically. Unexpectedly, the intensity in the range up to 300wn increases upon cooling, which is counterintuitive when the response originates from vibrations alone as in insulators. The increase finds a natural explanation if the conduction electrons are assumed to scatter on localized vibrations with a scattering probability proportional to the Debye-Waller factor. None of our assumptions is material specific, and the results are expected to be relevant for disordered systems in general.
Much progress has been made over a long period, spanning more than a century, in understanding the atomic arrangement on various length scales of noncrystalline chalcogens and their transitions upon certain external stimuli. However, it is broadly admitted that there are still several unsettled issues that call for proper rationalization. The current review presents an assessment of Raman scattering studies of noncrystalline phases of elemental chalcogens and their mixtures. First, a few remarks on the analysis of Raman data, related to polarization details and spectra reduction are presented. The effect of temperature, pressure and irradiation on the structure of chalcogens is reviewed in detail. As only selenium can form a stable glass at ambient conditions, the interest on sulfur and tellurium has been placed in the melt and the amorphous phase, respectively, whereas reference is also made to the sporadic structural studies of glassy sulfur at low temperatures. It is shown how Raman scattering can be exploited to explore unique phenomena emerging in the liquid state of sulfur, offering valuable information on the details of lambda transition including various thermodynamic related properties. The subtle nature of this transition in selenium is also discussed. Tellurium is not only impossible to be prepared in the bulk glassy state, but also forms a very liable to crystallization amorphous film. Therefore, the emphasis is placed on light induced nanostructuring and effects related to photo amorphization and photo oxidation.
By using very general arguments, we show that the entropy loss conjecture at the glass transition violates the second law of thermodynamics and must be rejected.
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