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Ab-Initio Real-Time Magnon Dynamics in Ferromagnetic and Ferrimagnetic Systems
2020, Singh, Nisha, Elliott, Peter, Dewhurst, J. Kay, Gross, E.K.U.
Magnonics—an emerging field of physics—is based on the collective excitations of ordered spins called spin waves. These low-energy excitations carry pure spin currents, paving the way for future technological devices working at low energies and on ultrafast timescales. The traditional ab-initio approach to predict these spin-wave energies is based on linear-response time-dependent density functional theory (LR-TDDFT) in the momentum and frequency regime. Herein, the simulation of magnon dynamics using real-time time-dependent density functional theory is demonstrated, thus extending the domain of ab-initio magnonic studies. Unlike LR-TDDFT, this enables us to observe atom-resolved dynamics of individual magnon modes and, using a supercell approach, the dynamics of several magnon modes can be observed simultaneously. The energies of these magnon modes are concurrent with those found using LR-TDDFT. Next, the complex dynamics of the superposition of magnon modes is studied, before finally studying the element-resolved modes in multisublattice magnetic systems.
Ab initio theory of plasmonic superconductivity within the Eliashberg and density-functional formalisms
2020, Davydov, A., Sanna, A., Pellegrini, C., Dewhurst, J.K., Sharma, S., Gross, E.K.U.
We extend the two leading methods for the ab initio computational description of phonon-mediated superconductors, namely Eliashberg theory and density-functional theory for superconductors (SCDFT), to include plasmonic effects. Furthermore, we introduce a hybrid formalism in which the Eliashberg approximation for the electron-phonon coupling is combined with the SCDFT treatment of the dynamically screened Coulomb interaction. The methods have been tested on a set of well-known conventional superconductors by studying how the plasmon contribution affects the phononic mechanism in determining the critical temperature (TC). Our simulations show that plasmonic SCDFT leads to a good agreement between predicted and measured TC's, whereas Eliashberg theory considerably overestimates the plasmon-mediated pairing and, therefore, TC. The hybrid approach, on the other hand, gives results close to SCDFT and overall in excellent agreement with experiments.