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Theses

Etats tunnel dans les quasicristaux et dans les cristaux de grandes mailles

Abstract : Amorphous materials possess specific low-energy excitations which are not found in simple crystals. Hence they show peculiar and, most remarkably, nearly universal low-temperature properties. In the phenomenological tunneling state model, these excitations arise from a tunneling effect of groups of atoms between nearly degenerate configurations. In order to improve our understanding of the microscopic nature of these tunneling states and their relation with structural disorder, we have studied various materials with an intermediate degree of order between crystalline and amorphous matter. Thus, using the propagation of acoustic waves, we have measured the acoustic properties, sound velocity and attenuation, in several quasicrystals and crystals with large unit cells between 15mK and 40K. A first experiment in a single-grain quasicrystal i-AlCuFe with a very high structural quality revealed the existence of tunneling states with a density of states similar to that of amorphous solids. Besides, measurements in the approximant phase R-AlLiCu and in two imperfect quasicrystals i-AlLiCu, one of them after annealing to improve its structural quality, showed that the density of states of the tunneling states increases with the quality of the quasicrystalline order. Therefore, tunneling states are not related to structural defects such as phasons but are intrinsic to quasicrystals. This is confirmed by the observation of a strong anisotropy of the tunneling state-phonons coupling in a two-dimension decagonal quasicrystal d-AlNiCo. We have also studied materials with a stronger long-range order, i.e. crystals with large unit cells. Single crystals of olivine and cordierite (28 and 116 atoms per cell respectively) showed tunneling states with a small density of states which seems to increase with the size of the unit cell.
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Contributor : Fabrice Bert <>
Submitted on : Wednesday, December 11, 2002 - 10:22:05 AM
Last modification on : Wednesday, September 16, 2020 - 4:31:57 PM
Long-term archiving on: : Tuesday, September 11, 2012 - 6:55:20 PM

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Fabrice Bert. Etats tunnel dans les quasicristaux et dans les cristaux de grandes mailles. Physique [physics]. Université Paris Sud - Paris XI, 2001. Français. ⟨tel-00002113⟩

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