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L'effet Nernst dans les systèmes corrélés : étude des fluctuations supraconductrices dans NbxSi1-x et des ordres électroniques dans PrFe4P12

Abstract : Although the Nernst effect was poorly used since its discovery in 1886, it acquired recently an important place in the field of strongly correlated systems.In this PhD. work, we use the Nernst effect to study two examples of correlated systems, the superconductor NbxSi1−x and the heavy fermion PrFe4P12.A study of superconducting amorphous thin films of Nb0.15Si0.85 shows that, above the superconducting transition, the Nernst signal observed is in perfect agreement with the theoretical prediction of Ussiskhin, Sondhi and Huse (USH) within the limit of weak magnetic field and close to Tc. The USH theory, based on the existence of short lived Cooper pairs above Tc, directly connects the Nernst coefficient to the zero field correlation length. This correlation length sets the size of the fluctuating Cooper pairs. More generally, the Nernst signal is shown to be determined by a unique length at arbitrary temperature and magnetic field. Consequently, the USH theory appears to be the low field limit of amore general theory which would connect the Nernst coefficient to the correlation length. These results show that the Nernst signal observed above Tc until very high temperature (30 × Tc) and very high magnetic field (3 × Bc2) in amorphous films of NbxSi1−x is generated by fluctuating Cooper pairs. The second study, carried out on the heavy fermion compound PrFe4P12, enabled us to characterize the phases appearing in this material at low temperature. The exceptional amplitude of the Nernst effect observed in the ordered phase at low magnetic field of PrFe4P12 is the consequence of three independent factors : a low carriers density, an increase in the effective mass and a large mean free path. This behavior is characteristic of a semi-metallic heavy fermion. The important increase in the thermoelectric power in the ordered phase is revealing of an important destruction of the Fermi surface. The phase which appears in high field for the direction [111] seems also related to a reorganization of the Fermi surface, in a less important way though, with a non Fermi liquid behavior. The sign change of the Nernst coeficient with the onset of the ordering at high magnetic field appears to be the consequence of a metamagnetic transition.
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https://tel.archives-ouvertes.fr/tel-00443883
Contributor : Alexandre Pourret Connect in order to contact the contributor
Submitted on : Monday, January 4, 2010 - 8:59:59 PM
Last modification on : Thursday, November 18, 2021 - 4:04:38 AM
Long-term archiving on: : Thursday, June 17, 2010 - 7:48:54 PM

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  • HAL Id : tel-00443883, version 1

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Alexandre Pourret. L'effet Nernst dans les systèmes corrélés : étude des fluctuations supraconductrices dans NbxSi1-x et des ordres électroniques dans PrFe4P12. Matière Condensée [cond-mat]. Université Pierre et Marie Curie - Paris VI, 2007. Français. ⟨tel-00443883⟩

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