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Acoustic and thermal transport properties of hard carbon formed from C60 fullerene

We report on an extended investigation of the thermal transport and acoustical properties on hard carbon samples obtained by pressurization of C60 fullerene. Structural investigations performed by different techniques on the same samples indicate a very inhomogeneous structure at different scales, based on fractal-like amorphous clusters on the mm to sub-millimetre scale, which act as strong acoustic scatterers, and scarce micro-crystallites on the nm scale. Ultrasonic experiments show a rapid increase in the attenuation with frequency, corresponding to a decrease in the localization length for vibrations. The data give evidence for a crossover from extended phonon excitations to localized fracton excitations. The thermal conductivity is characterized by a monotonous increase versus temperature, power law T1.4, for T ranging from 0.1 to 10 K, without any well defined plateau, and a strictly linear-in-T variation between 20 and 300K. The latter has to be related to the linear-in-T decrease of the sound velocity between 4 and 100 K, both linear regimes being characteristic of disordered or generally aperiodic structures, which can be analysed by the Ť phonon-fracton hopping ť model developed for fractal and amorphous structures.

fullerene and related materials; X-ray difraction; high resolution transsmision electron microscope; transport properties; acoustic propeties

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