G. Signorelli

Disorder-induced light scattering in solids: The origin of the Boson peak in glasses

Abstract Theoretical models relative to Raman scattering in disordered solids are discussed and compared with the available experimental evidence. In particular, experimental data relative to the Boson peak integrated intensity and computer simulations on three-dimensional site percolators, allow us to infer that the relevant mechanism that produces the peak is disorder-induced light scattering from acoustic modes of a disordered structure.

Acoustic nature of the boson peak in vitreous silica

New temperature-dependent inelastic X-ray scattering (IXS) and Raman scattering (RS) data are compared with each other and with existing inelastic neutron scattering (INS) data in vitreous silica, in the 300± 1775K region. The IXS data show collective propagating excitations up to Q ˆ 3:5nm i 1 . The temperature behaviour of the excitations at Q ˆ 1:6nm i 1 matches that of the boson peak found in INS and RS. This supports the fact that the acoustic origin of the excess of vibrational states gives rise to the boson peak in this glass.

Raman scattering from fractals: Simulation on large structures by the method of moments

We have employed the method of spectral moments to study the density of vibrational states and the Raman coupling coefficient of large 2- and 3- dimensional percolators at threshold and at higher concentration. We first discuss the over-and under-flow problems of the procedure which arise when -like in the present case- it is necessary to calculate a few thousand moments. Then we report on the numerical results; these show that different scattering mechanisms, all {it a priori} equally probable in real systems, produce largely different coupling coefficients with different frequency dependence. Our results are compared with existing scaling theories of Raman scattering. The situation that emerges is complex; on the one hand, there is indication that the existing theory is not satisfactory; on the other hand, the simulations above threshold show that in this case the coupling coefficients have very little resemblance, if any, with the same quantities at threshold.

Origin of light scattering from disordered systems

Anelastic light scattering is computed numerically for model disordered systems (linear chains and 2-dimensional site and bond percolators), with and without electrical disorder. A detailed analysis of the vibrational modes and of their Raman activity evidences that two extreme mechanisms for scattering may be singled out. One of these resembles scattering from finite size systems, while the other mechanisms originates from spatial fluctuations of the polarizability and is such that modes in even small frequency intervals may have very different Raman activities. As a consequence, the average coupling coefficient

Brillouin scattering intensities in glasses: Theory and experiment

C(omega)

Vibrational dynamics of percolating clusters: Fracton wavefunctions and Raman coupling coefficients

is the variance of a zero-average quantity. Our analysis shows that for both linear chains and 2-dimensional percolators the second mechanism dominates over the first, and therefore Raman scattering from disordered systems is essentially due to spatial fluctuations.Anelastic light scattering is computed numerically for model disordered systems (linear chains and 2-dimensional site and bond percolators), with and without electrical disorder. A detailed analysis of the vibrational modes and of their Raman activity evidences that two extreme mechanism for scattering may be singled out. One of these resembles scattering from finite size systems, while the other mechanisms originate from spatial fluctuations of the polarizability and is such that modes in even small frequency intervals may have very different Raman activities. As a consequence, the average coupling coefficient C(ω) is the variance of a zero-average quantity. Our analysis shows that for both linear chains and 2-dimensional percolators the second mechanism dominates over the first, and therefore Raman scattering from disordered systems is essentially due to spatial fluctuations.

The fluctuation origin of disorder-induced light scattering

Abstract The interpretation of light scattering data in homogeneous disordered solids, as far as the intensities and positions of the Brillouin peaks are concerned, is reviewed on the basis of the existing phenomenological theories. A microscopic, fully electrodynamic theory is presented, which allows the calculation of Brillouin intensities. The theory, when expanded in terms of fluctuations from the average reference structure, yields zero-order expressions for the elasto-optical coefficients entirely in terms of the refractive index of the medium. A comparison is made with recent data obtained with a new high-resolution high-luminosity high-contrast monochromator and with already published data on a number of glasses. The discrepancies encountered and the role played by higher-order terms in the fluctuations are discussed. It is suggested that the intensity of the depolarized longitudinal and transverse components contains relevant dynamic and structural information on these systems.

Theory of Raman and Brillouin scattering in disordered solid

Abstract We report a numerical study concerning the vibrational dynamics and the Raman coupling coefficient of two- and three-dimensional site-percolating structures, as well as an analysis of how to characterize the fracton wavefunctions. We discuss the difficulty of finding a scaling law for the Raman coupling coefficient in terms of the macroscopic parameters which characterize the fractal structure and dynamics; the oscillatory nature of the vibrational wavefunctions is examined.

Collective Dynamics in Water by High Energy Resolution Inelastic X-Ray Scattering

The calculation of the scaling laws for the Raman coupling coefficient C(ω) in fractals has been a big challenge for the theoreticians. Many scaling laws have been proposed and adapted to the numerical results which in the meantime were produced. In this paper the difficulties which have been faced by various authors in finding scaling relationships for this quantity are discussed on the basis of numerical results, which show that C(ω) devices from an intrinsically fluctuating quantity, i.e. it is the mean square deviation of a zero-average quantity.

Equivalence of the sound velocity in water and ice at mesoscopic wavelengths

Abstract The general scattering equation is derived and discussed from a microscopical point of view for both ordered and disordered harmonic solids. In particular in the framework of a pure dipole-induced dipole theory the disorder-induced light scattering spectra are discussed and some applications to model systems are shown.