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Strongly Correlated Physics and Materials for Energy Technology

Periodic Reporting for period 2 - StrongCoPhy4Energy (Strongly Correlated Physics and Materials for Energy Technology)

Reporting period: 2018-10-01 to 2020-03-31

At the heart of the StrongCoPhy4Energy Project is the physics of the “Hund’s metals”(materials where the intra-atomic exchange crucially shapes the metallicity properties, inducing e.g. large fluctuating local magnetic moments in the paramagnetic metal, and orbitally-selective strong correlations and Mott localization) and its relationship with interesting properties (and useful for potential energy-technology applications) of the wider class of “strongly correlated” materials, including unconventional superconductivity, thermoelectricity and other.
Superconductivity is at the moment found only at very low temperatures and thus unavailable for everyday life appliances where cryogenic fluids are not usable. A common goal of the international community is to synthesize new materials which superconduct at higher temperature and this projects hopefully will contribute in the understanding of the Hund's metals which are a potentially key class of materials in this respect. The same Hund’s physics at play in these materials could yield substantial advances to thermoelectric cooling or heat harvesting which is essential for a sustainable approach to refrigeration and optimization of energy waste.
In this Project we explore several universal features of this physics, that is theoretically realized in both idealized models and realistic material simulations, and many signs of which have been experimentally verified.We model known Hund’s metals (like Fe-based superconductors) and compare our modeling with experiments so to validate both our methodology and our understanding of their physics, including superconductivity. We compare these realistic simulations with simpler models in order to isolate the universal features of Hund’s physics. We finally aim at, and study within our collaboration with experimentalists, new materials which are potential candidates for improved performances in superconductivity, thermoelectricity and other fundamental and technological properties of interest.
In the beginning of the project the group was created and through the first half of the Project consisted mainly of two postdocs and two PhD students (one already at the end of this doctoral study, one at the beginning of it), along with the PI.

We have explored several Fe-based superconductors, which are a very much studied class of materials in the recent years, and show 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 of a recent finding from the PI (published in Physical Review Letters just before the beginning of the Project) that 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.

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. This and other features are the subject of several articles in preparation.

We have also studied a corresponding idealized model for cuprate superconductors (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.

More published work performed with various local (ESPCI) and national and international collaborators in Sorbonne Université, Ecole Polytechnique, CNRS Grenoble, University of Cagliari and SISSA-Trieste explored other aspects of the physics of Hund’s metals and Fe-based superconductors, and paved the ground for our ongoing research.
On the technical level several progresses in the algorithms implementing the two main techniques used to theoretically explore the physics of strongly correlated materials, and Hund’s metals in particular, Dynamical Mean-Field Theory (DMFT) and Slave-Spin Mean Field (SSMF), have been implemented.
Further extensions of these methods to address or improve the calculations of specific features of the materials, such as the electronic structure of Fe-based superconductors or transport quantities like thermoelectric power, are under scrutiny at the moment.

Until the end of the project we expect, among the main goals:
- to have a full understanding of the mechanism behind the compressibility enhancement present in Hund’s metals, its relationship to Mott and orbitally-selective Mott physics, its outcome on superconductivity and on other properties of the Hund’s metals and of unconventional superconductors in general, along with its possible experimental validation
- to have a quantitative understanding of the orbitally-selective Mott physics related to the Hund’s metal phase, and in particular of the fate of the most correlated electrons under the change of several material factors
- to unify this understanding with the analogous phenomenology found in cuprates superconductors and possibly establish commonalities in the superconductive mechanism
- to explore other properties of this class of materials like its thermoelectric and thermomagnetic power