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

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.

Figures (7)

• Figure 1: Time evolution of the electronic charge $\Delta Q = Q(t)-Q(0)$ within the muffin-tin spheres of Ti, O and interstitial region following an excitation with (a) linearly and (b) circularly polarized light with a laser frequency of $\hbar\omega=2.5$ eV, laser peak intensity of $10^{11}$ W/cm$^2$ and a FWHM of 10 fs. The brown dashed line depicts the $z$-component of the electric field of LPL, while black and red dashed lines represent the $x$- and $y$-components of the electric field of CPL.
• Figure 2: Changes of the spin- and element-resolved time-dependentdensity of states (DOS) during the pulse ($t=17$ fs) for (a-b) linearly and (c-d) circularly polarized light with laser frequency of $\hbar\omega=2.5$ eV with peak intensity $10^{11}$ W/cm$^2$. Changes in occupation at negative/positive energies denote initial/final states.
• Figure 3: (a) Graphical illustration of linearly polarized light along the $z$-direction and circularly polarized light in the $xy$-plane driving the electronic excitation in SrTiO$_\text{3}$. Snapshots of the time-dependent electron density redistribution $\Delta \rho(\mathbf{r},t)$ during and after the laser excitation for (b) linearly and (c) circularly polarized pulse using laser frequency of $\hbar \omega= 2.5$ eV, FWHM = 10 fs, and laser peak intensity of S$_{\rm peak}$ = $10^{11}$ W/cm$^2$. Red and blue colors denote depletion and accumulation of charge, respectively. The isosurface level is $\pm$ 2.02 $\times 10^{-3} e_0/ \mathrm{\AA}^3$.
• Figure 4: Light-induced total (a) spin and (b) orbital magnetic moment for CPL with $\hbar\omega$ = 2.5 eV. The exclusion of SOC, denoted by a light blue line in (a) and (b), does not affect the orbital moment but quenches the spin moment. Element-resolved contributions to the laser-induced spin and orbital moment for (c and d) LH-CPL. Comparison of the transient (e) spin and (f) orbital magnetic moment induced by LH-CPL with different laser frequencies ranging from 1.0 eV to 3.0 eV, and S$_\mathrm{peak}$ = $10^{11}$ W/cm$^2$.
• Figure 5: (a) Electronic band structure along the high-symmetry directions and (b) projected density of states (PDOS) of SrTiO$_\text{3}$ obtained with the PBEsol exchange-correlation functional.