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

Andri Darmawan, Markus E. Gruner+1 more|arXiv (Cornell University)|Feb 24, 2026

Multiferroics and related materials0 citations

TL;DR

The paper uses real-time TDDFT to show how optical excitation with linear and circular polarization induces site- and orbital-dependent charge dynamics in SrTiO3, leading to helicity-dependent transient orbital and spin magnetization without ionic motion, via angular-momentum transfer from light and SOC.

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.

Motivation & Objective

• Investigate carrier and magnetization dynamics in SrTiO3 under optical excitation with linearly and circularly polarized light.
• Identify how charge redistribution couples to orbital and spin angular momentum in a nonmagnetic band insulator.
• Elucidate the role of spin–orbit coupling in transferring angular momentum from light to electrons.
• Explore how laser frequency and pulse fluence influence transient magnetic states.

Proposed method

• Perform RT-TDDFT simulations using the Elk code with FP-LAPW basis.
• Use GGA-PBEsol exchange-correlation functional in a noncollinear, time-dependent Kohn–Sham framework.
• Model external laser fields via a vector potential in the dipole approximation for linear and circular polarization.
• Compute time-dependent spin and orbital angular momenta by projecting onto atomic spheres around SrTiO3 ions.
• Analyze time-dependent DOS and charge density redistribution to track orbital and spin dynamics.

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

Experimental results

Research questions

• RQ1 How do linearly versus circularly polarized light affect charge transfer between O 2p and Ti 3d states in SrTiO3?
• RQ2 Can circularly polarized light induce a measurable transient magnetization in a nonmagnetic insulator, and what are the roles of orbital and spin contributions?
• RQ3 What is the role of spin–orbit coupling in mediating spin magnetization from an optically induced orbital magnetization?
• RQ4 How do laser frequency and intensity influence the magnitude and temporal profile of induced magnetization?

Key findings

• Linearly polarized light drives out-of-phase, site-dependent O and Ti charge fluctuations resembling a soft TO mode, breaking inversion symmetry dynamically.
• Circularly polarized light induces a coherent rotation of charge dipoles around O, generating a helicity-dependent transient magnetization with opposite signs for O and Ti without ionic motion.
• The dominant mechanism is angular momentum transfer from light to electronic orbital angular momentum, with SOC enabling transfer from orbital to spin angular momentum.
• Orbital magnetization is an order of magnitude larger than the spin contribution during and after excitation, and turning off SOC quenches the spin moment while leaving the orbital moment intact.
• Transient magnetization scales with light intensity consistent with ΔM_IFE ∝ E × E*, but strong-field effects show deviations from linearity.
• For photon energies above the band gap (e.g., 2.5 eV), a persistent spin and orbital magnetization remains after the pulse; below the gap (e.g., 1.5 eV), only a transient orbital moment appears during the pulse and largely relaxes afterward.

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/positi