Periodic Reporting for period 4 - EXMAG (Excitonic Magnetism in Strongly Correlated Materials)
Reporting period: 2019-09-01 to 2020-05-31
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.
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.
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.
We have uncovered a number of distinct spin-triplet excitonic condensates, described their properties and the mechanism of their formation. In particular, the results obtained in doped strongly-coupled systems are not accessible by other methods.
We have studied for the first time the dynamical susceptibilities in phases with spontaneously broken symmetry using the dynamical mean-field theory. We have established the properties of collective modes which appear in these phases. We have studied the crossover - spectral weight transfer - between gapped Goldstone modes and amplitude (Higgs) modes in systems with weakly broken continuous symmetry.
We have predicted and experimentally demonstrated the mobility of intermediate spin excitation in LaCoO3. We have developed a new method to treat non-canonical bosons with hard-core constraints within the dynamical mean field theory and applied it to explain the temperature dependence of the resonant inelastic x-ray scattering spectra of LaCoO3. We have derived a low-energy effective model of LaCoO3 as a gas of strongly interacting excitons and used it to explain the properties of strained LaCoO3 films.
We have developed a tool for calculation of various core-level spectra based on configuration interaction solver of Anderson impurity model. We have demonstrated that combination of dynamical mean-field calculations with the newly developed tool provides
core-level photoemission, x-ray absorption and resonant inelastic x-ray scattering spectra, which compare very well to the experimental observations. We have performed several studies to establish this for a broad range of transition-metal oxides before proceeding
to some materials of current interest.
We have uncovered the unusual nature of the recently reported antiferromagnetism in RuO2. We have explained the origin of the magnetic instability in this material as well as its unusual spin polarized band structure. As a results, a transverse spin current is generated by a charge current flowing along a specific crystalographic direction, as pointed out by others.
We have studied for the first time the dynamical susceptibilities in phases with spontaneously broken symmetry using the dynamical mean-field theory. We have established the properties of collective modes which appear in these phases. We have studied the crossover - spectral weight transfer - between gapped Goldstone modes and amplitude (Higgs) modes in systems with weakly broken continuous symmetry.
We have predicted and experimentally demonstrated the mobility of intermediate spin excitation in LaCoO3. We have developed a new method to treat non-canonical bosons with hard-core constraints within the dynamical mean field theory and applied it to explain the temperature dependence of the resonant inelastic x-ray scattering spectra of LaCoO3. We have derived a low-energy effective model of LaCoO3 as a gas of strongly interacting excitons and used it to explain the properties of strained LaCoO3 films.
We have developed a tool for calculation of various core-level spectra based on configuration interaction solver of Anderson impurity model. We have demonstrated that combination of dynamical mean-field calculations with the newly developed tool provides
core-level photoemission, x-ray absorption and resonant inelastic x-ray scattering spectra, which compare very well to the experimental observations. We have performed several studies to establish this for a broad range of transition-metal oxides before proceeding
to some materials of current interest.
We have uncovered the unusual nature of the recently reported antiferromagnetism in RuO2. We have explained the origin of the magnetic instability in this material as well as its unusual spin polarized band structure. As a results, a transverse spin current is generated by a charge current flowing along a specific crystalographic direction, as pointed out by others.