[论文解读] 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 materials被引用 0

一句话总结

论文使用实时TDDFT展示线性和圆偏振的光学激发如何在SrTiO3中引发与位点和轨道相关的电荷动力学，从而在没有离子运动的情况下，通过光的角动量转移和自旋轨道耦合实现手性相关的瞬态轨道与自旋磁化。

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.

研究动机与目标

• 在SrTiO3受线性和圆偏振光照射时研究载流子与磁化动态。
• 识别电荷重分布如何在非磁性带绝缘体中与轨道角动量和自旋角动量耦合。
• 阐明自旋–轨道耦合在将角动量从光传递给电子中的作用。
• 探索激光频率与脉冲通量如何影响瞬态磁态。

提出的方法

• 使用Elk代码进行RT-TDDFT模拟，基于FP-LAPW基组。
• 在非共线、时变Kohn–Sham框架中使用GGA-PBEsol交换相关泛函。
• 通过|偶极近似的矢量势模拟线性和圆偏振的外场激光。
• 通过投影到SrTiO3离子周围的原子球来计算时变自旋和轨道角动量。
• 分析时变态DOS和电荷密度重新分布以跟踪轨道和自旋动力学。

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

实验结果

研究问题

• RQ1 线性偏振光与圆偏振光在SrTiO3中对O 2p和Ti 3d态之间的电荷转移有何影响？
• RQ2 圆偏振光是否能在非磁性绝缘体中诱导可测的瞬态磁化，以及轨道和自旋贡献的作用？
• RQ3 自旋–轨道耦合在将光诱导的轨道磁化中的自旋磁化传递中起到何种作用？
• RQ4 激光频率与强度如何影响诱导磁化的幅度和时间轮廓？

主要发现

• 线偏振光驱动O和Ti电荷振荡呈现相位差、位点相关的振荡，类似软的TO模，动态打破反演对称性。
• 圆偏振光诱导O周围电荷偶极的相干旋转，产生手性相关的瞬态磁化，对于O和Ti符号相反且无离子运动。
• 主导机制是光向电子轨道角动量的角动量转移，自旋–轨道耦合使轨道角动量向自旋角动量转移成为可能。
• 轨道磁化在激发期间及之后都比自旋贡献大一个数量级，关闭SOC会淬灭自旋矩但保持轨道矩。
• 瞬态磁化与光强度成比例，与ΔM_IFE ∝ E × E*一致，但强场效应显示非线性偏离。
• 当光子能量高于带隙（如2.5 eV）时，脉冲后仍存在持续的自旋和轨道磁化；低于带隙（如1.5 eV）时，脉冲期间仅出现瞬态轨道矩，大体在之后迅速衰减。

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