Séminaire - 01/12/2022 - Olag JANSON - Magnetic modeling using first-principles calculations: from cuprates to cobaltates

Invitation : Roland Hayn (Département PHANO, Equipe MQT).

Diffusion : IM2NP, CINaM, Irphe, LP3 (via N. Sannier), Madirel (via R. Denoyel), PIIM (via T. Angot), CPT (T. Martin), Fédération de Chimie (via S. Viel), CP2M

SEMINAIRE Jeudi 01 Décembre 2022 à 11h00

Salle des séminaires de l'Im2np, campus de Saint-Jérôme, 1er étage Bâtiment Poincaré

Olag JANSON

 Junior Research Group "Computational Methods for Correlated Materials"

Leibniz Institute for Solid State and Materials Research

Dresden, Germany

Magnetic modeling using first-principles calculations: from cuprates to cobaltates

Quantum magnets exhibit a variety of unusual behaviors, such as magnetization plateaus, bound magnon states or the magnetocaloric effect. Particularly interesting are spin liquids: states evading long-range magnetic ordering despite sizable correlations between the constituent magnetic moments.  Considerable efforts are put into finding and studying such exotic behaviors in real materials, and there is a growing body of experimental data covering more and more material classes. Yet, interpretation of such experimental observations is rarely straightforward and generally requires reliable information on their spin model --- the set of exchange interactions that underlie the magnetism on the microscopic level.

In this talk, I will demonstrate that first-principles calculations are a powerful tool to reliably obtain magnetic exchange interactions in different classes of magnetic insulators. This numerical approach involves several steps, starting from density-functional-theory (DFT) band-structure calculations, the identification of magnetically relevant states and the construction of the effective low-energy model, DFT+U total energy calculations, and finally, simulations of the resulting spin model.

The computational effort and even the scope of this numerical analysis depend on the relevance of orbital degrees of freedom and the spin-orbit coupling.  Undoped cuprates with Cu2+ featuring a single hole in the d-shell are excellent one-orbital systems that can be modeled efficiently and accurately.  At the other pole are Co2+ cobaltates with the d7 electronic configuration, for which a DFT-based analysis is much more challenging. Nonetheless, even in this case, DFT calculations can provide useful insights. I will show several examples for both material families and discuss the performance and accuracy of the methods and outline future directions.