Jan Kuneš - Research

Our research focuses on electronic correlations in models and real materials including effects such as magnetism, phase transitions and lattice instabilities. Our primary tool are large scale calculations using dynamical mean-field theory and density functional theory. Our main output are phase diagrams and spectral functions both on one- and two-particle level.

Highlights

X-ray circular dichroism observed in altermagnets

Altermagnets is a new magnetic species, which split off from antiferromagnets recently. They are distinguished by spin polarized bands and other effects such as magnetic dichroism (different absorption of right and left circularly polarized light depending on the orientation of the magnetic moments). MnTe is a Mott insulator and an altermagnet with g-wave spin texture (dictated by its crystallographic space group). In a joint theory&experiment work, we have predicted and out colleagues measured the x-ray magnetic circular dichorism (XMCD) in MnTe. This is the observation of XMCD in a collinear compensated (no net magnetization) magnet. Interestingly the effect takes place for light arriving perpendicular to the magnetic moments (a common geometry for XMCD is parallel). Another interesting feature is that one has to include core-valence exchange interaction to get any effect (this is not the case for XMCD in ferromagnets.)

Magnons and spin gap with DMFT: SrRu2O6

Calculation of dynamical susceptibilities within DMFT requires solution of Bethe-Salpeter equation (BSE). This is numerically difficult task as it requires working with objects that depend of three frequencies and four spin/orbital indices. Here we have calculated dynamical spin susceptibility (magnon spectrum) for a model with 2 atoms (3 orbitals each) of SrRu2O6. The material has an easy-axis single-ion anisotropy determined by the crystal field and spin-orbit coupling, which is responsible for the spin gap. Comparison to the experimental RIXS data (grey dots), shows that we were able to reproduce the magnon spectra very accurately (the only adjustable parameter is the interaction strength, which is strongly constrained by other physical properties).

Valence skipping, internal dopiing, and site-selective Mott transition in PbCoO3 under presssure

PbCoO3 is a material which combines a number phenomena found in transition-metal oxides: site-selective Mott transition, spin-state crossover, valence skipping and internal doping -- valence change of Co due to transfer of electrons from Pb. In addition these phenomena are closely coupled to structural modifications -- tilting of the oxygen octahedra. We present a comprehensive theoretical study, which shows how these phenomena arise from correlation effects on the Co-site and the location of Pb-O antibonding states in the vicinity of Fermi level.