G Baldi

Publications (2)7.37 Total impact

• Source

  Available from: ArXiv

  Article: Vibrational excitations in systems with correlated disorder

  W. Schirmacher, B. Schmid, C. Tomaras, G. Viliani, G Baldi, G Ruocco, T Scopigno
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  ABSTRACT: We investigate a $d$-dimensional model ($d$ = 2,3) for sound waves in a disordered environment, in which the local fluctuations of the elastic modulus are spatially correlated with a certain correlation length. The model is solved analytically by means of a field-theoretical effective-medium theory (self-consistent Born approximation) and numerically on a square lattice. As in the uncorrelated case the theory predicts an enhancement of the density of states over Debye's $\omega^{d-1}$ law (``boson peak'') as a result of disorder. This anomay becomes reinforced for increasing correlation length $\xi$. The theory predicts that $\xi$ times the width of the Brillouin line should be a universal function of $\xi$ times the wavenumber. Such a scaling is found in the 2d simulation data, so that they can be represented in a universal plot. In the low-wavenumber regime, where the lattice structure is irrelevant there is excellent agreement between the simulation at small disorder. At larger disorder the continuum theory deviates from the lattice simulation data. It is argued that this is due to an instability of the model with stronger disorder.
  12/2007;
• Source

  Available from: Daniele Fioretto

  Article: Evidence for a crossover in the frequency dependence of the acoustic attenuation in vitreous silica.

  C Masciovecchio, G Baldi, S Caponi, L Comez, S Di Fonzo, D Fioretto, A Fontana, A Gessini, S C Santucci, F Sette, G Viliani, P Vilmercati, G Ruocco
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  ABSTRACT: We report measurements of the sound attenuation coefficient in vitreous silica, for sound waves of wavelength between 50 and 80 nm, performed with the new inelastic UV light scattering technique. These data indicate that in silica glass a crossover between a temperature-dependent (at low frequency) and a temperature-independent (at high frequency) acoustic attenuation mechanism occurs at Q approximately equal to 0.15 nm(-1). The absence of any signature in the static structure factor at this Q value suggests that the observed crossover should be associated with local elastic constant fluctuations.
  Physical Review Letters 08/2006; 97(3):035501. · 7.37 Impact Factor