Nuclear Spin Relaxation in Aerogels and Porous Glasses
Part of the
NATO ASI Series
book series (NSSB, volume 323)
It has been known for two decades that most of the physical properties are drastically different in amorphous materials as compared to their crystalline counterparts, specially when examined at low temperatures1,2. These specific behaviors have been accounted for by the phenomenological two-level-system (TLS) model3,4 or its more recent generalization through soft modes5. One of the properties specific to the glassy state concerns the nuclear magnetic relaxation, which presents common features over numerous inorganic glasses6,7,8. Though different mechanisms have been debated, all of them require the TLS model to account for the weak température-dépendance of the relaxation. The lack of structural model for these TLS prevents precise estimation of their coupling with the spins and then an absolute evaluation of the relaxation. In the case of electron spin resonance in bio-polymers, relaxation involving fractal-structure vibrations (fractons) have been considered9. Primarily introduced by S.Alexander and R. Orbach10 on a theoretical basis, fractons have been experimentally tracked during these last years, with peculiar attention paid on aerogel dynamics11-14. Aerogels are materials whose fractal structure has been evidenced by small angle scattering15,16 in the reciprocal space and by nuclear spin relaxation in the direct space17. Their density of state, deduced from inelastic light and neutron scattering, presents a weak energy dependance in part of the giga-Hertz range (typically 3 to 100 GHz). Nevertheless, this density cannot be described by a single spectral dimension as initially proposed, and the relevance of scalar or tensorial models is still a subject of debate. Moreover, the characteristic structural length determined from small angle scattering and the one deduced from the dynamics measurements present disrepancies which are not yet understood.
KeywordsPorosity Silicate Fluorozirconate
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