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Register a new userThe Raman coupling function in amorphous silica and the nature of the long wavelength excitations in disordered systems
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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.
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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.
New temperature dependent inelastic x-ray (IXS) and Raman (RS) scattering data are compared to each other and with existing inelastic neutron scattering data in vitreous silica (v-SiO_2), in the 300 - 1775 K region. The IXS data show collective propagating excitations up to Q=3.5 nm^-1. The temperature behaviour of the excitations at Q=1.6 nm^-1 matches that of the boson peak found in INS and RS. This supports the acoustic origin of the excess of vibrational states giving rise to the boson peak in this glass.
We report a numerical study of Anderson localization in a 2D system of non-interacting electrons with spin-orbit coupling. We analyze the scaling of the renormalized localization length for the 2D SU(2) model and estimate its $beta$-function over the full range from the localized to the metallic limits.
We report a numerical investigation of the fluctuations of the Lyapunov exponent of a two dimensional non-interacting disordered system. While the ratio of the mean to the variance of the Lyapunov exponent is not constant, as it is in one dimension, its variation is consistent with the single parameter scaling hypothesis.
We study a disordered vibrational model system, where the spring constants k are chosen from a distribution P(k) ~ 1/k above a cut-off value k_min > 0. We can motivate this distribution by the presence of free volume in glassy materials. We show that the model system reproduces several important features of the boson peak in real glasses: (i) a low-frequency excess contribution to the Debye density of states, (ii) the hump of the specific heat c_V(T) including the power-law relation between height and position of the hump, and (iii) the transition to localized modes well above the boson peak frequency.
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