Direct x-ray detection of current-induced orbital accumulation in transition metals

The possibility to generate orbital angular momentum currents from charge currents is attracting a lot of interest in the research community. Several emerging phenomena associated to orbital currents have been investigated in the last few years, but experiments employing different techniques and materials have produced a widespread of results in terms of generation efficiency and diffusion lengths. Here we aim at employing x-ray synchrotron techniques to track changes in the electronic orbital occupation induced by electric currents in transition metals (TMs) such as Ti and Mn. This proposal builds up on our previous studies based on Cu/oxide interfaces where we detected, for the first time, the emergence of spectral shifts along the L3,2 absorption edges of Cu due to orbital angular momentum accumulation by XMCD-PEEM (experiments 2023027537 and 2024028319). However, the amplitude of the spectral shifts detected were smaller than expected according to our spin-orbit torques (SOTs) analysis and literature reports. In this proposal we aim at investigating Ti and Mn, two of the TMs most studied in the literature by diverse techniques (MOKE, EELS, SOTs) and compare with our results in Cu/oxide. We aim at clarifying the origin of the apparent discrepancies reported in the literature and confirm the orbital nature of the angular momentum detected in these materials upon the application of electric currents.

从电荷电流生成轨道角动量流的可能性，正受到国际学界的广泛关注。近数年来，学界已对多种与轨道电流相关的新兴现象开展研究，但采用不同技术与材料完成的实验，在轨道电流产生效率与扩散长度维度得到了纷繁多样的结果。本研究方案拟借助同步辐射X射线技术，追踪过渡金属（transition metals, TMs）如钛（Ti）、锰（Mn）中电流诱导的电子轨道占据态变化。本方案基于我们此前针对铜/氧化物（Cu/oxide）界面的研究工作：在过往研究中，我们首次通过X射线磁圆二色光电子发射电子显微镜（XMCD-PEEM）观测到，轨道角动量积累可导致铜的L3、2吸收边出现光谱位移，相关实验编号为2023027537与2024028319。但根据我们的自旋轨道转矩（spin-orbit torques, SOTs）分析及已有文献报道，此次观测到的光谱位移幅度低于预期值。在本研究方案中，我们拟针对钛与锰展开研究——这两种过渡金属是当前文献中被磁光克尔效应（Magneto-Optical Kerr Effect, MOKE）、电子能量损失谱（Electron Energy Loss Spectroscopy, EELS）、自旋轨道转矩SOTs等多种技术广泛研究的对象——并将所得结果与我们在铜/氧化物界面体系中得到的结果进行对比。我们旨在阐明文献中报道的表观差异的根源，并证实：在外加电流作用下，这些材料中观测到的角动量具有轨道起源属性。