Iron-based Superconductors in High Magnetic Fields

Iron-based superconductors are a recent discovery in the history of supercon-
ductivity. 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 supercon-
ducting 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. Its low Fermi energy sug-
gests the presence of fluctuations that might lead to the observation of the
vortex lattice melting over its entire phase diagram. This will be presented
in the first chapter of this thesis.
Specific heat was chosen as the main measurement technique of our 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 transi-
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tions, which is one of the core topics of this thesis. In the second chapter,
I will present the thermodynamic properties of interest to obtain our phase
diagram as well as the different set-ups used during our work.
The third chapter presents the measurements made during the PhD and
their analysis. A first part focuses on the pure FeSe compound. Our data
reveal an excess of specific heat before the superconducting transition at-
tributed to the presence of a transition between a solid and a vortex liq-
uid. Using a scaling approach 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 mag-
netic 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. The second part of this chapter focuses
on the FeSe0.88S0.12 doped compound. A comparative study of its phase di-
agram with that of FeSe is conducted, revealing a great similarity between
the two compounds. Moreover a new anomaly is detected in our specific
heat data, which is still under investigation.