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Etude par RMN des corrélations magnétiques dans les supraconducteurs à haute température critique: effets des impuretés

Abstract : All the high-Tc superconductors contain CuO2 planes which govern most of their physical properties. When increasing the number of doped holes per plane, these cuprates change from an antiferromagnetic/insulating behavior to a superconducting/correlated metal behavior (underdoped case). Above a certain limit of doping, the superconductivity progressively disapear, apparently together with the correlations (overdoped case). It is therefore of the first importance to get a precise description of the role and the nature of these correlations occuring within the planes above Tc in the different parts of the phase diagram. In this aim, we performed an NMR study of compounds containing one or two planes per cell unit, pure or with substitutional impurities. A spin pseudogap due to the presence of magnetic correlations has been shown to occur in the underdoped YBa2Cu3O6.6 compound (2 planes/cell) above Tc. Our 17O NMR measurements show that this pseudogap also occurs in HgBa2CuO4+y (1 plane/cell). This pseudogap therefore appears as an intrinsec property of the CuO2 planes. We also present a study of substitution impurity effects (Zn2+, Ni2+ or Li+ in Cu2+ plane site) on the magnetism of the planes above Tc. The 17O, 7Li NMR and SQUID measurements show that each impurity induces a local magnetic moment within the planes without changing their doping content. The measured values of these moments are anomalous and can be explained by the frustration of some in-plane preexisting antiferromagnetic correlations. In a second step, our approach consists in considering these moments as localized perturbations which allow to probe the susceptibility chi’(q non zero) of the planes by 17O NMR. The NMR indeed probes the polarisation of the electronic spins of the planes, which is proportionnal to the spatial susceptibility chi’(r), r being the distance to the impurity. Our measurements show an anomalous temperature dependence in the underdoped compounds, which can be explained by the presence of antiferromagnetic correlations. This anomalous dependence disapear with increasing doping, indicating that the system changes from a strongly correlated behavior to a Fermi Liquid one. This approach is a new way to probe chi’(q), which only transverse relaxation measurements could measure in an incomplete way up to now in cuprates.
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https://tel.archives-ouvertes.fr/tel-00001972
Contributor : Julien Bobroff <>
Submitted on : Monday, November 18, 2002 - 9:24:57 AM
Last modification on : Wednesday, October 14, 2020 - 4:00:13 AM
Long-term archiving on: : Tuesday, June 2, 2015 - 3:25:27 PM

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  • HAL Id : tel-00001972, version 1

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Julien Bobroff. Etude par RMN des corrélations magnétiques dans les supraconducteurs à haute température critique: effets des impuretés. Matière Condensée [cond-mat]. Université Paris Sud - Paris XI, 1997. Français. ⟨tel-00001972⟩

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