We have explored several Fe-based superconductors, which are a very much studied class of materials in the recent years, and shown all the hallmarks of Hund's coupling dominated physics.
Electronic compressibility (the sensitivity of electronic density in the metal to a shift in the chemical potential) is presently one of our main focuses because this quantity is strongly enhanced or even divergent at the frontier between a normal and a Hund's metal. This enhancement might favor superconductivity and other anomalous properties that are found in Fe-based superconductors and other Hund's metals.
We have thus explored its occurrence in realistic material simulation, for instance in the much studied FeSe. We concluded the FeSe lies well within the Hund's metal zone, and that an applied external pressure might bring it on the frontier with a more conventional metallic phase. Correspondingly one finds experimentally that superconductivity is enhanced at the corresponding pressures, which is a validation of our picture. Also the mono-layer version of FeSe, which holds the present record for the highest Tc for superconductivity in the family, was simulated and found a correspondingly enhanced compressibility compared to its bulk counterpart, thus again supporting our thesis. This work was also published in Phys. Rev. Lett. Furthermore, we have extended this analysis to more Fe-based superconductors: from the so-called "111" family, to LaFe2As2.
On the other hand we have isolated and studied the mechanism causing the compressibility enhancement in models of Hund's metals, and studied its dependency on key factors in materials like the crystal-field splitting of the Fe-orbital energies or the number of orbitals itself.
We have investigated in depth the relationship of the occurrence of enhanced compressibility and the Mott transition one finds in these models and materials at half-filling of the conduction bands. We have shown that the whole phenomenology is connected to the first-order nature of this Mott transition, and that the latter is caused, quite generally, by a small energy scale splitting the atomic ground state multiplet (such as Hund's coupling, crystal-field splitting etc). In a publication in Physical Review Letters early 2023 we have given ad thermodynamically consistent description of this connection, a general phase diagram, and pointed out the general occurrence of finite-doping "Mott" quantum critical points, potentially crucial for boosting superconductivity as already explored in the context of cuprate high-Tc superconductors.
We have also studied a corresponding idealized model for these materials (the single most studied family of unconventional superconductors, holding the record Tc among all materials at ambient pressure), in search of commonalities with our scenario. We are thus tracing a parallel and a common framework for high-Tc superconductivity with the Fe-based superconductors, in another article in preparation.
We have also explored magnetic phases in strongly correlated models and materials, and in particular their development as a function of Hund's coupling, and their role in the physics of Fe-based superconductors.
Finally we have outlined a systematic way to exploit Hund’s physics to find heavy-fermionic materials (a class showing rich and potentially technologically relevant physics, with quantum critical points, multiple phases in competition, possible electric control of magnetic degrees of freedom, exotic superconductivity, and more) not including rare-earth elements, which are often problematic in supply, handling and control. This breakthrough might open an entire new field of exploration.
More published work performed with various local (ESPCI) and national and international collaborators in Sorbonne Université, Ecole Polytechnique, CNRS Grenoble, University of Cagliari, SISSA-Trieste and Wuerzburg university has explored other aspects of the physics of Hund’s metals and Fe-based superconductors (and of correlated electron physics in general). Several tasks of the projet are in their final stage and will give rise to publications in the period following the end of the project.
Moreover some new lines of research have spurred during the project and are likely to be fruitful for the nest forthcoming years.
