We have performed extensive calculations on the two-band Hubbard model in the parameter range where excitonic condensation is expected. A number of distinct excitonic phases were found and their properties described. Particularly interesting are the phases, which give rise
to spin textures in the reciprocal space with curious magnetic properties, which allow, for example, generation of a transverse spin current by a charge current. We have also investigated ordered phases, which compete with the excitonic condensation.
Using dynamical susceptibilities we have studied the collective modes appearing due to spontaneous symmetry breaking and their behavior at the phase boundaries. This lead to observation of spectral weight transfer between the Goldstone and amplitude (Higgs) modes in systems with approximate continuous symmetry. We have found that coupling between excitonic and spin susceptibilities in the excitonic condensate makes the excitonic modes visible to inelastic neutron scattering.
We have performed material specific calculations for a number of materials, potential hosts of excitonic magnetism. We have predicted mobile spinful excitons in LaCoO3, later confirmed by resonant inelastic x-ray scattering experiments. We have developed a new theoretical approach, dynamical mean-field theory for hard-core bosons, which allowed us to capture the effect of exciton-exciton interaction important in LaCoO3 at elevated temperatures. Using a model of strongly interacting excitons for LaCoO3 we have explained the atomic-scale disproportionation and insulating ferromagnetic state observed in strained LaCoO3 films. Among other materials we have studied an unusual metallic antiferromagnet RuO2 and predicted its spin polarized band structure similar to d-wave spin texture found in some excitonic condensates.
Finally, we have worked on techniques for detection of excitonic condensates. We have developed a new numerical tool for calculation of core-level spectra and demonstrated its capabilities by systematic comparison to the available core-level photoemission, x-ray absorption and
resonant inelastic x-ray scattering data for a broad range of transition metal oxides. These experimental techniques see and increased use in solid state physics, chemistry and materials science. Our calculations exhibit a very good quantitative match with the experimental spectra and provide their interpretation in terms of microscopic parameters of the studied materials. This is particularly important for the resonant inelastic x-ray scattering, where direct interpretation without theoretical modelling is not possible due to the complex excitation process involved.
The results of the project were published in 24 peer reviewed articles.
