[论文解读] All optical excitation of spin polarization in d-wave altermagnets

Marius Weber, Stephan Wust+14 more|ArXiv.org|Aug 9, 2024

Magneto-Optical Properties and Applications被引用 10

一句话总结

论文证明线性偏振的超快光脉冲可以在d-wave altermagnet (RuO2) 中诱导并控制自旋极化，得到ab initio理论与时间分辨磁光克尔测量的支持。

ABSTRACT

The recently discovered altermagnets exhibit collinear magnetic order with zero net magnetization but with unconventional spin-polarized d/g/i-wave band structures, expanding the known paradigms of ferromagnets and antiferromagnets. In addition to novel current-driven electronic transport effects, the unconventional time-reversal symmetry breaking in these systems also makes it possible to obtain a spin response to linearly polarized fields in the optical frequency domain. We show through ab-initio calculations of the prototypical d-wave altermagnet RuO$_2$, with $[C_2\|C_{4z}]$ symmetry combining twofold spin rotation with fourfold lattice rotation, that there is an optical analogue of a spin splitter effect, as the coupling to a linearly polarized exciting laser field makes the d-wave character of the altermagnet directly visible. By magneto-optical measurements on RuO$_2$ films of a few nanometer thickness, we demonstrate the predicted connection between the polarization of an ultrashort pump pulse and the sign and magnitude of a persistent optically excited electronic spin polarization. Our results point to the possibility of exciting and controlling the electronic spin polarization in altermagnets by such ultrashort optical pulses. In addition, the possibility of exciting an electronic spin polarization by linearly polarized optical fields in a compensated system is a unique consequence of the altermagnetic material properties, and our experimental results therefore present an indication for the existence of an altermagnetic phase in ultrathin RuO$_2$ films.

研究动机与目标

• 介绍并表征带有抵消磁化和d-wave自旋分裂的altermagnet。
• 证明线性偏振光能够在极短时间尺度在RuO2中诱导自旋极化。
• 显示所诱导的自旋极化依赖于泵浦偏振角并且可控。
• 提供实验证据支持超薄RuO2薄膜中的altermagnetic相。
• 概述对超快altermagnetic自旋电子学以及界面处自旋操控的影响。

提出的方法

• 对RuO2在altermagnetic相中进行包括自旋轨道耦合的 ab initio 计算，以获得自旋极化的能带结构。
• 通过动量分辨的偶极矩阵元以及针对线性偏振场的Fermi 的黄金法则框架计算光致激发的自旋密度。
• 用 S^z_exc = sum_{k,mu,epsilon_{k,mu}>=E_F} n_{k,mu} <s^z_{kmu}> 定义并从占据态评估激发自旋极化 S^z_exc。
• 使用时间依赖的光致激发模型，E(t) = E0 exp[-4 ln(2) t^2 / tau^2] e_phi，以捕捉极化角度依赖性。
• 在超薄RuO2薄膜上通过时间分辨偏极磁光克尔效应（time-resolved polar magneto-optical Kerr effect）实验验证预测，泵浦能量为1.5 eV，探测为3.1 eV，同时旋转泵浦偏振角度φ。
• 与参考的非磁性RuO2、TiO2和NiO进行比较，以确认该效应的altermagnetic起源。

实验结果

研究问题

• RQ1 线性偏振光致激发能否在d-wave altermagnets中产生自旋极化？
• RQ2 线性偏振泵浦的激发角度如何影响RuO2中光致自旋极化的符号和大小？
• RQ3 观测到的自旋极化是否是altermagnetic序的特征，以及它是否能作为超薄RuO2薄膜中altermagnetism的证据？
• RQ4 RuO2中光致自旋极化的时间尺度与衰减机制是什么，它们与传统磁体相比如何？

主要发现

• RuO2中的光致自旋极化对泵浦偏振角呈180°（两fold）周期性依赖，与d-wave altermagnetic能带结构相关。
• 激发后立即观察到显著的自旋极化，持续约250 fs，随后因自旋翻转散射而衰减。
• 非磁性RuO2、TiO2和NiO未显示角度相关的自旋极化，强调了该效应的altermagnetic起源。
• 实验结果与ab initio预测一致，证明在超薄RuO2薄膜中通过泵浦偏振方向来实现可控自旋极化。