Development of methods for transient dynamics in strongly correlated electron systems
Materials exhibiting strong correlation attract much attention due to the occurrence of many prominent emergent phenomena. A source of both emergence and complexity is the many coupled degrees of freedom, making minute perturbations cause major changes to the material. The characteristic timescales of the mechanisms governing the transitions between these phases span several orders of magnitude, but are all ultrafast, from sub femtosecond for the absorption of light, to several picoseconds for transferring energy and momentum from the electronic to the ionic degrees of freedom.
Traditionally computational materials science employs so called density functional theory (DFT). DFT works with an effective semi-local mean field treatment for describing the electron-electron interaction, available functionals are inadequate for treating compounds with strong electron correlation. For this we have developed an implementation of the so called DFT+Dynamic Mean Field Theory (DMFT), where we have a explicit treatment of the subset of electrons which exhibits strong correlation. The DFT+DMFT methodology has proven to be very efficient to describe the electronic structure of compounds with strong electron correlation, including magnetism and optical properties.
The DMFT implementation is currently running efficiently in the Full-Potential Linearized Muffin-Tin Orbital DFT code RSPt. Recent experimental advances allows for investigations of transient phenomena on ultrashort timescales, typically femtoseconds. We are working on recent implementation of real-time propagation time-dependent DFT, where we incorporate elements of DFT+DMFT to extend the ability of the method to adress strongly correlated electrons.
A class of material which exhibits strong electron correlation is transition-metal oxides, specifically where the transition-metal ion is in a tetrahedral or octahedral environment, such as Perovskites or the Perovskite-like Ruddlesden-Popper phases. Examples are SrRuO3 is an itinerant ferromagnet, Sr2RuO4 is a ferromagnetic superconductor and Sr3Ru2O7 is metamagnetic. We indend to investigate how the crystal structure of these materials influence the electronic structure, including local dynamical electronic excitations using DMFT, while mapping long range interaction on a phenomenological model to treat spin-wave excitations. Using the implementation for transient dynamics, we can study how external electromagnetic fields influence the material. The goal is to develop computational tools able to describe the transient processes involved in resistive switching.
Running DFT+DMFT on these classes of materials are extremely demanding, in order to parametrize the so called self-energy, and map it on to a functional suitable for real-time TDDFT we have to perform a lot of calculations. Hence, development of efficient codes is essential.
All implementations will be shared with other users of the code, hence the outcome will be of greater benefit for the Swedish electronic structure community.