THEORY OF ELECTRONIC CORRELATIONS AND COLLECTIVE PHENOMENA

Group of Jan Kuneš

Cobaltites

LDA+DMFT band structure of 0.2 hole doped LaCoO3 in FM state: majority spin (upper panel), minority spin (lower panel)

LaCoO3 is a prototypical system with competing spin states. A small gap non-magnetic insulator at low temperature exhibits Curie-Weiss susceptibility above 100 K followed by disappearance of the charge gap between 450 and 600 K. While the low spin (singlet) nature of the ground state is generally accepted, the character of the excited state (intermediate or high spin) is a source of controversy. We have used the LDA+DMFT approach to study the Opens external link in new windowthermal and Opens external link in new windowdoping effects. Recently, we started to explore the possibility of LaCoO3 being a system close to excitonic condensation. We have found a condensate ground state in Opens external link in new windowPr0.5Ca0.5CoO3 with LDA+U approach. Treating a simplified model with DMFT we have show the transition in high magnetic field carries characteristics of Opens external link in new windowfield-induced excitonic condesation.

Charge transfer materials

Generalized band structure of paramagnetic NiO

Charge-transfer insulators are a subset of strongly correlated (Mott) insulator in which the the charge gap is not defined by the Hubbard bands due to the presence of ligand states (O 2p in oxides). The characteristic feature of charge-transfer insulators is electron-hole asymmetry when it comes to doping. For proper description of charge-transfer materials the p-d hybridization and d-d correlations has to be treated simultaneously (p-d model). We have studied some prototypical charge-transfer materials such as Opens external link in new windowNiO or Opens external link in new windowSrCoO3.

Metal-insulator transition in NiS2-xSex

Pyrite crystal structure of NiS2

The NiS2-xSex solid solution provides a model system where the transition between metallic phase and the Mott insulator can be tuned by Se content, pressure or temperature. Using Opens external link in new windowLDA+DMFT we show that, contrary to common claims, the transition is essentially controlled by varying hybridization gap  within the S(Se) p-band related to the bond length of the S-S (Se-Se) dimer.

Superconductivity of compressed fcc Li

Nesting function of compressed fcc Li

Discovery of higher temperature superconductivity in MgB2 brought about  a renewed interest in strongly coupled conventional superconductors.  Surprisingly  Li, one of the simplest possible  solids,  turns out to be  ~20 K superconductor when compressed.  We use  density functional electronic structure and linear response theory to investigate the origin of Li superconductivity and lattice dynamics and to link the effects to the Fermi surface topology.