Development of methods for transient dynamics in strongly correlated electron systems
Abstract
Many exotic physical phenomena, such as metal-to-insulator transitions, magnetism and superconductivity, occur when strong coupling between electronic, vibrational and magnetic degrees of freedom is present. The highly non-linear behaviour of these materials makes minute perturbations cause major changes to the materials properties. The characteristic timescales of the microscopic order, and the mechanisms governing the transitions between these phases, are 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.
Within this project, we develop methods and software to describe these phenomena. We apply our developments to increase the understanding of strongly correlated materials, and aim for example to improve control of a materials conductivity by identifying the optimum conditions for switching between a materials phases.
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 an 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.
To be able to describe transient phenomena in correlated materials, such as the interaction with electromagnetic fields, or injection of charge through leads, we need to develop the theory further. We are currently focusing on the development of real-time propagation time-dependent DFT (TD-DFT), where we incorporate elements of DFT+DMFT to extend the ability of the method to adress strongly correlated electrons. A benefit with the formulation is that non-adiabatic molecular dynamics can be cheaply implemented, allowing us to study for example emergent phenomena in materials where ionic relaxation occurs in a metal-insulator transition.
The project is funded by the Swedish Research Council (VR) and Stiftelsen för Strategisk Forskning (SSF) through the grants "Coherent Control of Materials Properties" (VR), Modeling Correlated Materials for Future Devices (SSF) and Multiscale Modeling for Transformer Optimization (SSF). With the increase in funds we expect to hire additional personell during the allocation-period.
Through these projects, we have a number of work-packages that we adress.
1) Development of orbital and time-dependent exchange-correlation potentials for TD-DFT.
2) Software improvements in order to adress lattice reconstructions involving 1000s of atoms.
3) Identification of optimal control fields for state-to-states switching in reduced dimensions (2D-materials)
4) Physics of Transition-metal-oxide based memristors.
Due to the focus on development, we expect that the load on the project can be uneven during the period.
