Periodic Reporting for period 4 - StrongCoPhy4Energy (Strongly Correlated Physics and Materials for Energy Technology)
Période du rapport: 2021-10-01 au 2022-12-31
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 at ambient pressures, 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 project aims at contributing 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 have explored 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.
Superconductivity is at the moment found only at very low temperatures at ambient pressures, 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 project aims at contributing 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 have explored 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 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.
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.
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.
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 resistivity and thermoelectric power, or also the occurrence of magnetic phases have been achieved and have been instrumental to complete the project.
In particular we have:
- improved substantially the agreement of band-structures, thermodynamic and transport quantities of Fe-based superconductors (and more largely strongly-correlated materials) through the use of hybrid-functional-DFT + SSMF/DMFT
- devised a methodology to obtain precise transport quantities from exact-diagonalization DMFT
- extended the SSMF method to broken-symmetry phases and in particular magnetic ones
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 resistivity and thermoelectric power, or also the occurrence of magnetic phases have been achieved and have been instrumental to complete the project.
In particular we have:
- improved substantially the agreement of band-structures, thermodynamic and transport quantities of Fe-based superconductors (and more largely strongly-correlated materials) through the use of hybrid-functional-DFT + SSMF/DMFT
- devised a methodology to obtain precise transport quantities from exact-diagonalization DMFT
- extended the SSMF method to broken-symmetry phases and in particular magnetic ones