Fyzikální ústav Akademie věd ČR

Seminar Thursday, 10/11/2011 14:00 - 15:00

Speakers: Vlastimil Křápek
Place: Library of the Institute of Physics ASCR, Cukrovarnická 10, Prague 6
Organisers: Department of Magnetics and Superconductors

The interest in LaCoO₃ arises from its complex magnetic and transport properties. The low-temperature LaCoO₃ is a non-magnetic insulator. Above 100 K it transforms into a paramagnetic insulator and the metal-insulator crossover takes place around 500 K. The ionic state of Co in this compound is Co³⁺. The 3d orbitals, split to lower three t_2g and upper two e_g, accommodate six electrons. Depending on the number of e_g electrons we distinguish the low-spin (LS) state with a spin number S=0, the intermediate spin state (IS, S=1), and the high spin state (HS, S=2). Presently, there are no doubts about prevailing LS character of the low-temperature non-magnetic insulator and the HS character of the paramagnetic metal. Two probable scenarios for the paramagnetic insulator are the LS+HS statistical mixture or the IS. The possibility of the short-range orbitally ordered state with alternating LS+HS and LS sites has also been proposed.

We studied the properties of LaCoO₃ with a dynamical mean field theory (DMFT) combined with local density approximation for non-correlated part. We successfully reproduced the metal-insulator transition. The magnetic crossover cannot be modeled as the temperature is too low for employed DMFT expansion, but we were able to stabilize both non-magnetic and paramagnetic state adjusting the exchange interaction parameter J. Our results testify to LS+HS scenario of paramagnetic insulator, but we also observed strong hybridization with oxygen p-orbitals, which increases the mean number of electrons on Co atoms to around 6.7 in the non-magnetic and around 6.4 in the paramagnetic state.

Attachments:

krapek2011.pdf