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Effets des anisotropies de spins dans les oxydes de basse dimension

Abstract : The spin 1/2 magnetic oxides such as CuGeO3, SrCu2(BO3)2 and NaV2O5 raise many interesting questions as to the low-temperature phases, which are disordered by the quantum fluctuations. In this thesis, we study magnetic properties of these compounds in relation with experiments. In particular, unexpected results have been seen by two novel experimental techniques: the rotation of the polarisation in inelastic neutron scattering measurements and forbidden optical transitions. In order to explain these results, we needed to consider anisotropic spin interactions, such as the Dzyaloshinski-Moriya interaction. We studied the effects of such an interaction on quantum-disordered systems and explained the fine structure of the triplet excitation in SrCu2(BO3)2. The rotation effect of the polarisation is due to a magnetic-nuclear interference and is related to the anisotropy of the system. We found the selection rules with a perturbative calculation in the spin-phonon coupling and compared with experimental results. Similarly, we calculated the selection rules for optical transitions, considering not only the magnetic dipolar operator, but also the electric dipolar operator, thus generalising the Fleury-Loudon theory. The question of estimating the Dzyaloshinski-Moriya interaction, both static and dynamic, is revisited for the special geometry of these compounds within the superexchange theory. It is shown that such an interaction, in certain cases, can be as large as the exchange.
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Contributor : Olivier Cepas <>
Submitted on : Wednesday, July 17, 2002 - 8:05:51 AM
Last modification on : Wednesday, November 29, 2017 - 10:26:44 AM
Long-term archiving on: : Tuesday, September 11, 2012 - 5:35:31 PM


  • HAL Id : tel-00001494, version 1



Olivier Cepas. Effets des anisotropies de spins dans les oxydes de basse dimension. Matière Condensée [cond-mat]. Université Joseph-Fourier - Grenoble I, 2000. Français. ⟨tel-00001494⟩



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