THEORY OF ELECTRONIC CORRELATIONS AND COLLECTIVE PHENOMENA

Group of Jan Kuneš

Exciton condensation

Excitonic phases in two-band Hubbard model around half filling

Materials formed from atoms with singlet (LS, low spin) ground state followed by low-lying (HS, high spin) excitations tend to relax the high degeneracy by formation of long-range order. The nature of the ordered state depends on the inter-atomic interactions: HS-HS repulsion favors Opens external link in new windowclassical spin-state order, while HS-LS exchange favors formation of Opens external link in new windowspinful superfluid (exciton condensate), which can be viewed as Bose-Einstein condesation of HS (bosonic) excitons above LS vacuum. A spinful condensate can exist in several Opens external link in new windowdistinct thermodynamic phases with strikingly different physical properties. Recently, we have demonstrated the formation of Opens external link in new windowspontaneous spin textures in two-band Hubbard model under suitable conditions. 

Spin-state transition

Closing of the charge gap with pressure in MnO

Spin-state transition can be described as abrupt change of magnitude or complete disappearance of local magnetic moment. It can take place both in paramagnetic or magnetically ordered state. While its origin is essentially atomic - competition between crystal field and Hund's coupling - it has interesting consequences for periodic systems such as metal-insulator transition, volume collapse or various ordering instabilities are subject of our interest. We have used LDA+DMFT appraoch to study pressure-driven spin-state transitions in Opens external link in new windowMnO and Opens external link in new windowFe2OOpens external link in new window3, or Opens external link in new windowtemperature and Opens external link in new windowdoping driven spin-state transition in LaCoO3.