电流诱导轨道四极矩积累的观测
Observation of current-induced orbital quadrupole accumulation
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中文总结 AI 辅助
本研究通过偏振分辨克尔显微镜观测到Ti与Pt表面的电流诱导轨道四极矩积累，发现其四极响应超过磁偶极响应，揭示了不依赖自旋-轨道耦合的电荷-轨道转换通道，拓展了电流诱导现象的多极研究范畴。
中文摘要 AI 辅助
自旋电子学与轨道电子学依赖于电流诱导的磁偶极子（自旋与轨道角动量）积累。然而，电子轨道本质上携带偶极子之外的多极矩，其中秩为2的轨道四极矩是主导项。本研究利用偏振分辨克尔显微镜，在自旋-轨道耦合强度显著不同的金属钛（Ti）与铂（Pt）表面观测电流诱导的轨道四极矩积累。通过分离非对角光学电导率的对称与反对称分量，本研究将时间反演偶极的四极响应与传统时间反演奇极的磁偶极响应区分开来，发现两种金属中的四极光学响应均超过偶极响应。对测量响应的第一性原理分析表明，尽管两种金属的自旋-轨道耦合强度差异巨大，其四极矩积累处于同一量级，这与一种此前未被发现的、不依赖自旋-轨道耦合的电荷-轨道转换通道相符。本研究的发现确立了电流诱导的轨道极化在本质上是多极的，将电流诱导现象从偶极范畴扩展至多极范畴，并为轨道有序相的电学调控开辟了新途径。
英文摘要
Spintronics and orbitronics rely on current-induced accumulations of magnetic dipoles: spin and orbital angular momentum. However, electronic orbitals inherently carry multipoles beyond the dipole, with the rank-2 orbital quadrupole as the leading term. Here we use polarization-resolved Kerr microscopy to observe current-induced orbital-quadrupole accumulation at the surfaces of Ti and Pt, metals with markedly different spin--orbit-coupling strengths. By separating the symmetric and antisymmetric components of the off-diagonal optical conductivity, we isolate the time-reversal-even quadrupolar response from the conventional time-reversal-odd magnetic-dipolar one, and find that the quadrupolar optical response exceeds the dipolar one in both metals. First-principles analysis of the measured responses indicates that the quadrupole accumulations are of the same order of magnitude in the two metals despite their widely different spin--orbit-coupling strengths, consistent with a previously unidentified channel of charge-to-orbital conversion that does not require spin--orbit coupling. Our findings establish that current-induced orbital polarization is fundamentally multipolar, expanding current-induced phenomena from the dipolar to the multipolar regime and opening a route to electrical control of orbital-ordered phases.
