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EXMAG Report Summary

Project ID: 646807
Funded under: H2020-EU.1.1.

Periodic Reporting for period 1 - EXMAG (Excitonic Magnetism in Strongly Correlated Materials)

Reporting period: 2015-06-01 to 2016-08-31

Summary of the context and overall objectives of the project

Spontaneous symmetry breaking leading to states of matter with long-range order is one of the central topics in condensed matter physics. Common types of order, such as ferro- and anti-ferromagnetic, are characterized by spin or charge densities modulated on inter-atomic scale, therefore well studied thanks to various scattering experiments. Order parameters that are not of this type are much more difficult to detect, giving rise to names such as hidden order or electronic nematicity. Their impact on transport or thermodynamic properties may, nevertheless, be substantial. In the EXMAG project we will investigate excitonic condensation in systems with strongly correlated electrons as a new mechanism leading to unconventional ordered states. The objective of the project is to characterize the physical properties of various excitonic phases and to find their realization in real materials. We will focus on intermediate coupling strength and doped systems where the interaction between the excitonic order and the charge carriers is expected to lead to new physics. In particular, we want to explore the potential of the excitonic order to induce instabilities, e.g. magnetic or superconducting, that are not present in the normal phase. We will also address the possibility of topologically non-trivial quasi-particle band-structures in the excitonic phase. Our main tool will be numerical simulations based on the dynamical mean-field theory and ab initio band-structure methods. We will pursue two main lines of research: investigation of simple models allowing access to many physical observables and studies of real materials capturing the chemical complexities at the cost of more severe approximations. Ultimately, we want to understand in detail the properties of the excitonic magnets and their potential functionalities, and to identify the main control parameters and promising materials.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

We want to point out two main results achieved in the first reporting period. The first one concerns general understanding of the phenomenon of excitonic magnetism. We have found a possibility to generate spin textures in the reciprocal space. These are states of matter characterized by absence of spin polarized density, i.e., time reversal invariant, which nevertheless carry macroscopic spin polarized currents. Such states were observed previously in non-centrosymmetric materials with spin-orbit coupling and found applications in spintronics. We have shown that spin textures can arise also without spin-orbit coupling as a result of spontaneous symmetry breaking. Besides numerical simulations we have provided an analytic model of generalized double-exchange mechanism which allows to understand how the spin textures and other observed phases arise.

The second result concerns an old problem of LaCoO3. Motivated by recent high magnetic field experiments we have performed model calculations in order to test the hypothesis that the observed meta-magnetic transition is a field-induced exciton condensation. We found that the temperature dependence of the critical field is indeed compatible with the excitonic scenario, while it excludes the scenario of field-induced spin-state ordering. Furthermore, we have performed analytic calculation supporting the picture of LaCoO3 as a material close to excitonic condensation. The key observation is that the atomic multiplets should not be viewed as states bound to a given atom, but rather as excitons and bi-excitons which can move through the material.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Spontaneous spin-orbit coupling has been proposed phenomenologically a decade ago. As a side product of our investigation of the two-band Hubbard model we found the realization of the concept in a relatively simple lattice model with local interaction. The spontaneous spin-orbit coupling give rise to reciprocal space spin textures. Such texture found, in other context, application in spin--charge-current control. We are not by far not in the position to propose material specific realizations, but our result opens a new direction. Taken from the other side the result reveals some of the richness of the two-band Hubbard model, which only recently has been systematically investigated by us and a few other groups. We also find it important that we provide simple analytic explanation of our results in the form of generalized double-exchange.

LaCoO3 has been studied by many groups for more than half-a-century. The standard interpretation of the experimental results operates with atomic states associated with individual atoms. The unfinished debate is then centered around the ordering of these states on the energy scale, i.e., which is the first excited state, and a possible spatial ordering of these states, so called spin-state order and the lack of experimental evidence for it. The interpretation of our results brings, in our opinion, a substantial change of the paradigm on LaCoO3. While we operate with the same atomic states, we show that some of them become mobile excitons when the atoms form a crystalline lattice. This allows possibilities that were not considered so far, e.g. that the lowest excited state of and atom has a different character than the lowest excited state on the lattice problem. Our proposal has already motivated experimental effort by several groups to verify it. If confirmed this general picture could settle the debate on LaCoO3 and provide a general framework for understanding of other materials.

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