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Transport d'entropie, thermoélectricité dans les supraconducteurs non conventionnels

Abstract : During these last fifteen years, Nernst effect has attracted much attention. This was particularly due to its behavior in high Tc superconducting hole doped cuprates. Indeed, it has been reported in these compounds that a finite Nernst effect persists, in magnetic field and in temperature, outside of the superconducting phase, although Nernst effet is supposed to be, in the case of simple metals, nonzero only in the vortex phase. This result was of importance because it supported experimentally, among other results, a scenario of preformed pairs and vortex-like excitations in the normal phase of the hole doped cuprates. However, the interpretation of the Nernst effect suffer from a lack of experimental datas in other strongly correlated electrons systems. This has initiated the work of this thesis, which contains results obtained from 2001 to 2004 in the Laboratoire de Physique Quantique - ESPCI in Paris, and which consists mostly in measurements of Nernst effect in another class of strongly correlated electrons compounds : the heavy fermions. More precisely, we performed measurements of Nernst effet at zero pressure for temperatures lying between 1.5K and 50K and for magnetic fields below 12T, in three different compounds : CeCoIn5, URu2Si2 and CeRu2Si2. These measurements are, nowadays, the sole results of Nernst effect available on heavy fermions compounds. \vspace(12pt) In the three cases, we discovered in the metallic phase the emergence of a giant Nernst effect which amplitude (some µV/KT) is by several order of magnitude higher than what is expected in a simple metal, and which reaches the characteristic size of the vortex signal in the mixed phase of cuprates. More precisely, CeCoIn5 hosts a very large \emph(negative) Nernst effect in the non Fermi liquid part of its phase diagram associated to the presence of a quantum critical point at B_c=5T. However, the Nernst effet does not show any anomaly at B_c, but much more in the B=0 limit where the Nernst coefficient reaches 1µV/KT. The emergence of a strong positive Nernst effect in URu2Si2 in concomitant to the entrance in the so called hidden order phase. Here again, the Nernst coefficient reaches its maximum of 4µV/KT at low field, and is the largest Nernst effect ever measured in a metallic phase. Finally, the Nernst effect seems to be strongly affected by the metamagnetism in CeRu2Si2. Indeed, it presents, at the metamagnetic transition, a change of sign which coincides with the maximum in specific heat. Very strikingly, in the three compounds, the emergence of a large Nernst effect coincide with the presence of a large Hall effect ; moreover, especially in CeCoIn5 and CeRu2Si2, the structure of Hall and Nernst effect are quite similar. The existence of this signal still remains intriguing, particularly in absence of systematic measurements in other heavy fermions compounds. However, but in a very simple way, we showed that the mass renormalisation might be an ingredient at the origin of this giant Nernst effect. Moreover, for each compound, several scenarios can be proposed to explain this emergence : proximity to a quantum critical point, antiferromagnetic fluctuations, density waves... We have discussed at the end of this thesis several scenarios and proposed possible measurements to confirm or infirm this propositions.
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Submitted on : Friday, January 7, 2005 - 1:25:58 PM
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Romain Bel. Transport d'entropie, thermoélectricité dans les supraconducteurs non conventionnels. Matière Condensée [cond-mat]. Université Paris-Diderot - Paris VII, 2004. Français. ⟨tel-00007955⟩

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