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Lien visio : https://univ-grenoble-alpes-fr.zoom.us/j/92780394998?pwd=M2xyT0NCSnhBYVBLVWl6Y2RqMXhoUT09
The presentation will be in English.
Iron-based superconductors are a recent discovery in the history of superconductivity. Discovered about ten years ago, their phase doping-temperature diagrams are close to some materials like cuprates. When cuprates show schematically a Mott insulator phase, a pseudogap phase and a superconducting phase, iron-based superconductors mainly show three transitions: a nematic transition, a magnetic transition and a superconducting transition. However, the simplest of the iron-based superconductors, FeSe, does not show a magnetic phase at ambient conditions and therefore a slightly different phase diagram. Its low Fermi energy suggests the presence of fluctuations that might lead to the observation of the vortex lattice melting over its entire phase diagram.
Specific heat was chosen as the main measurement technique of our FeSe samples. Two set-ups were used: the Dual Slope Method and alternative Calorimetry. The alternative calorimetry device allows access to high magnetic fields up to 35T and thus gives the opportunity to study the complete phase diagram of our compounds. It is also a particularly sensitive probe to phase transitions, which is one of the core topics of this thesis.
I will therefore present the measurements made during the PhD and their analysis. The presentation focuses on the pure FeSe compound. Our data reveal an excess of specific heat before the superconducting transition attributed to the presence of a transition between a solid and a vortex liquid. Using a scaling approach, proving the existence of sizable Gaussian fluctuations, we deduce the fictitious upper critical field line. The temperature dependence of this field shows an important effect of Cooper pair breaking by paramagnetic effects, mainly when the magnetic field is oriented along FeSe layers. The melting of the vortex lattice also shows that beyond thermal fluctuations, possible quantum fluctuations could be present. Finally we study the possible appearance of a Fulde-Ferrell-Larkin-Ovchinnikov phase.
Supervisors :
Thierry Klein
Matthieu Le Tacon
Christophe Marcenat
Frédéric Hardy
Jury’s Members :
Amalia Coldea
Alain Pautrat
Klaus Hasselbach
Jörg Schmalian
