Scientific Report 2006-2008

Introduction

The research carried out at the laboratory concers:

• Disordered systems
  • Vibrations in glasses
  • Theory of the glass transition
  • Soft matter
• Nanostructures
  • Ultratrasparency in nanostructured glass-ceramics
  • Theory of the vibrational dynamics in nanoparticles and of Raman and Brillouin activity
  • Nanostructures of silica with metallic clusters e fluorofores for biosensors
  • Nanostructured systems for applications to photonic
• Biosystems
  • Dynamics of self-assembly in systems of biological interest
  • Vibrational spectroscopy in complex molecular systems
  • Spectroscopy methods for cultural goods

The strong integration between theory and experiments has characterized most of the activities of our laboratory. On the experimental side, at our laboratory several techniques, ranging from time-resolved visible and IR spectroscopy to fluorescence line-narrowing, non-linear spectroscopies and Raman and Brillouin scattering, are employed in order to characterize the systems and the materials under investigation. However, most of our studies with the best impact in the field of condensed matter, which utilize X-rays, neutrons and UV inelastic scattering techniques, are carried out at large national and international facilities (ESRF, ILL, ELETTRA, LLB, JCNS).
On the theoretical side, the main goal is to develop original models to gain physical insight on still unsettled topics of disordered systems (vibrational anomalies, aging at off-equilibrium, dynamic slowing down at the glass transition, exotic phases in colloids). The developed models are studied both analytically, by utilizing the tools provided by the statistical mechanics of disordered systems, and via Monte Carlo simulations., by designing new optimized algorithms.

Highlights

1. Thermodynamic signature of growing amorphous order in glass-forming liquids
   G. Biroli, J. Bouchaud, A. Cavagna, T. Grigera, P. Verrocchio
   Nature physics 4, 771, (2008)
   
   Supercooled liquids show a dramatic slowdown of their dynamics upon cooling. Several theories relate this to increasing spatial correlations. In this work we have found for the first time evidence of such phenomenon. Through simulations with fixed boundary conditions, we see that the influence of the boundary propagates into the bulk over larger and larger length scales.
2. 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 and G. Ruocco
   Phys. Rev. Lett. 97, 035501 (2006)
   
   In this work we report measurements of the sound attenuation coefficient in vitreous silica, obtained by inelastic UV light scattering. These data shows in silica glass a crossover between a low frequency (temperature-dependent) and a high frequency (temperature-independent) acoustic attenuation mechanism. Such crossover may be associated with local elastic constant fluctuations.
3. Phase diagram of a polydisperse soft-spheres model for liquids and colloids
   L. Fernandez, V. Martin-Mayor and P. Verrocchio
   Phys. Rev. Lett. 98, 085702 (2007)
   
   The phase diagram of soft spheres with size dispersion is studied by means of an optimized Monte Carlo algorithm which speeds up considerably the equilibration. The system ubiquitously undergoes a first-order freezing transition. While for a small size dispersion the frozen phase has a crystalline structure, large density inhomogeneities appear in the highly disperse systems
4. Ultratransparent glass ceramics: The structure factor and the quenching of the Rayleigh scattering
   M. Mattarelli, M. Montagna, and P. Verrocchio
   Appl. Phys. Lett. 91, 061911 (2007)
   
   Glass ceramics with nanocrystals have a particularly high transparency. In this work we attribute this to spatial correlation of the nanoparticles. The physical mechanism producing high transparency in glass ceramics is demonstrated to be the low density fluctuation in the number of scatterers
5. Vibrational properties of inclusion complexes: The case of indomethacin-cyclodextrin
   B. Rossi, P. Verrocchio, G. Viliani, G. Scarduelli, G. Guella and I. Mancini
   J. Chem. Phys. 125, 044511 (2006)
   
   Vibrational properties of inclusion complexes with cyclodextrins are studied by means of Raman spectroscopy and simulations. In particular, Raman spectra of indomethacin undergo notable changes when such complexes are formed. By using both ab initio quantum chemical calculations and molecular dynamics, we relate such changes to the geometry of the inclusion process.

Papers

Organized conferences

• X International Workshop on Disordered Systems
  18-21 march 2006, Molveno (TN), Italy
• XI International Workshop on Complex Systems
  17-20 march 2008, Andalo (TN), Italy

Large projects

Our laboratory is affiliate to SOFT, an INFM-CNR research and development center. SOFT has supported our activities mainly with the funding of two staff position (researcher). Furthermore it has co-funded the standard activities of our laboratory with approximately 5.000 € per year.

Projects

• PRIN
  • PRIN07 (2008-2009)
    Dinamica vibrazionale e rilassamenti in vetri densificati e in sistemi disordinati confinati
    Responsabile locale: A. Fontana
  • PRIN05 (2006-2007)
    Dinamica vibrazionale e fenomeni di rilassamento in sistemi disordinati
    Responsabile nazionale: G. Viliani
  • PRIN04 (2005-2006)
    Nanocomposite glasses for photonics
    Responsabile nazionale: M. Montagna
• PAT
  • MATIS (2008-2010)
    Studio del degrado di materiali per l'arte contemporanea con tecniche di imaging spettroscopico
    Responsabile: M. Mattarelli
  • PAT-CRS (2008-2009)
    Analisi microRaman di materiali
    Responsabile: M. Montagna

PhD thesis

• Laura Orsingher (XXI ciclo) Vibrational dynamics in tetrahedral strong glasses: the cases of densified GeO₂ and of GeSe₂.
• Matteo Martini (XXI ciclo) Synthesis and optical properties of fluorescein encapsulated in gold (core)/ silica (shell) architectures.
• Giacomo Gradenigo (XXII ciclo) Mosaic scenario of the glass transition.
• Marco Zanatta (XXIV ciclo) Vibrational dynamics in strong glasses: the cases of densified SiO₂ and of SiSe₂.