Disentangling the dynamics of transient spin and orbital magnetization in SrTiO$_3$ via the inverse Faraday effect from RT-TDDFT

TL;DR

This study uses RT-TDDFT to explore how linearly and circularly polarized light induce transient spin and orbital magnetization in SrTiO$_3$, revealing complex charge dynamics and angular momentum transfer mechanisms.

Contribution

It provides the first detailed real-time analysis of light-induced magnetization dynamics in SrTiO$_3$, highlighting the roles of orbital and spin angular momentum transfer.

Findings

Linearly polarized light causes out-of-phase charge fluctuations resembling phonon modes.

Circularly polarized light induces helicity-dependent transient magnetization.

Orbital angular momentum transfer dominates over spin angular momentum transfer.

Abstract

Light-matter interaction allows to achieve non-equilibrium states that are otherwise inaccessible. Motivated by recent experiments that report ferroelectricity -- and even multiferroicity -- in the prototypical diamagnetic band insulator SrTiO$_3$ induced by terahertz pulses, we investigate the carrier and magnetization dynamics of SrTiO$_3$ excited optically by linearly and circularly polarized light. Our real-time time-dependent density-functional theory (RT-TDDFT) results reveal a highly non-trivial, site- and orbital-dependent temporal evolution with charge transferred from O $2p$ to Ti $3d$ states. For linearly polarized light the orbitally polarized lobes of electron density at the oxygen and titanium sites fluctuate out-of-phase, resembling the soft transverse optical phonon mode, dynamically breaking inversion symmetry. In contrast, circularly polarized pulses induce a coherent rotation of the charge dipoles around O. This induces a helicity-dependent finite transient magnetization with opposite sign for oxygen and Ti even without ionic motion. Detailed analysis reveals that the dominant mechanism is the transfer of angular momentum of light to the electronic orbital angular momentum, while spin-orbit coupling plays a key role in the transfer from orbital to spin angular momentum, the former being an order of magnitude larger than the latter.