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Fluctuations et point quantique critique nématiques dans le supraconducteur au fer FeSe1-xSx

Abstract : Discovered in 2008, Iron-based superconductors have opened up a completely new field of research for high critical temperature superconductivity whose mechanism remains debated. In addition to a superconducting phase, these compounds develop a tetragonal / orthorhombic structural phase transition of electronic origin, an electron nematic transition, below the transition temperature Ts. The main subject of this thesis is the study of the nematic phase of iron-based superconductors FeSe1-xSx by polarization-resolved Raman spectroscopy.In the sulfur-doped iron selenides FeSe1-xSx, the structural transition temperature Ts decreases to zero close to a Quantum Critical Point (QCP) for a critical sulfur doping x = 0.17. In order to reveal the influence of nematic QCP on superconductivity, we have followed the evolution of electronic nematic fluctuations as a function of temperature and doping. The analysis of the nematic susceptibility allowed us to highlight the substantial effect of nemato – elastic coupling on the marginal behavior of superconductivity near the nematic QCP in these systems.On the other hand, we have identified the presence of two distinct components in the nematic fluctuations spectrum, the low-energy one ( <15meV ) has a critical character with respect to the nematic transition while the high-energy one ( ≈ 50meV ) is essentially non-critical. This observation is attributed to two possible sources of nematic fluctuations, which can be explained by the duality of the itinerant and localized character of the charge carriers in these systems.
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Soumaya Chibani. Fluctuations et point quantique critique nématiques dans le supraconducteur au fer FeSe1-xSx. Physique [physics]. Université Paris Cité; Université de Tunis El Manar, 2020. Français. ⟨NNT : 2020UNIP7197⟩. ⟨tel-03337906⟩

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