Magnons from real-space real-time time-dependent density functional theory

In the last couple of years, the first studies investigating magnetization dynamics from first principles in real time have emerged. We recently developed an efficient and non-perturbative scheme to compute magnetic excitations for extended systems employing the framework of real-time time-dependent density functional theory, which is an alternative to the linear-response TDDFT formulation.

To investigate magnons, we drive the system out of equilibrium using an ultrashort magnetic kick perpendicular to the ground-state magnetization of the material. The dynamical properties of the system are obtained by propagating the time-dependent Kohn-Sham equations in real time, and the analysis of the time-dependent magnetization reveals the transverse magnetic excitation spectrum of the magnet.

In the limit of weak magnetic kicks, we recover the results obtained using the linear-response formulation of time-dependent density functional theory.

Interestingly, our approach does not rely on the assumptions of small perturbations and can be used to investigate nonlinear phenomena induced by a strong magnetic field as well as out-of-equilibrium situations where the system is kicked from an excited state. In this talk, we will present results for nonlinear magnetic excitations from first principles in iron and nickel oxide.

Using the real-space method, we benefit from a favorable scaling for large systems, such as supercells needed for describing small momenta. Moreover, using the flexibility of choosing boundary conditions that are employed, we also implemented a method using "twisted boundary conditions" employing the generalized Bloch theorem, allowing for studying magnons employing only primitive cells instead of supercells.