Bulletin of the American Physical Society
APS March Meeting 2024
Monday–Friday, March 4–8, 2024; Minneapolis & Virtual
Session Z24: Recent Advances in OrbitronicsInvited

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Sponsoring Units: GMAG Chair: Vivek Amin, IUPUI Room: 101DE 
Friday, March 8, 2024 11:30AM  12:06PM 
Z24.00001: Optical observation of the orbital Hall effect in a light metal Ti, Mn, and Cu Invited Speaker: GyungMin Choi Orbitronics aims for control of the orbital current in nonmagnetic materials. Among various techniques to measure orbital, the optical method employing the magnetooptical Kerr effect has the advantages of high sensitivity and vectorandtime resolution. I will present the magnetooptical observation of the orbital Hall effect in a light metal Ti. The Kerr rotation by the orbital magnetic moment accumulated at Ti surfaces due to the orbital Hall current is measured, whose result agrees with theoretical calculations quantitatively and is supported by the orbital torque measurement in Tibased magnetic heterostructures. In addition, I will present the optical detection of the orbital Hall effect in other light metals of Mn and Cu. The direct detection of orbital accumulation on the surfaces of light metals provides important information for orbital generation, transport, and relaxation. 
Friday, March 8, 2024 12:06PM  12:42PM 
Z24.00002: On the reciprocal relation between the direct and inverse orbital Hall effects Invited Speaker: Dongwook Go The reciprocal relation is a fundamental manifestation of the fluctuationdissipation theorem for nearequilibrium transport phenomena. In orbitronics, the orbital Hall effect plays a pivotal role as it can be used to generate orbital currents. Its reciprocal counterpart, the inverse orbital Hall effect refers to a generation of charge current by gradient of orbitaldependent chemical potential, namely “orbital voltage”, which can serve as a mechanism to electrically detect orbital currents in orbitronic devices. In recent years, the orbitaltocharge conversion has been experimentally detected [15]. However, the theoretical description of the reciprocity between the direct and inverse orbital Hall effects is far from trivial because orbital currents are illdefined and the orbital angular momentum is not conserved. We show that the reciprocal relation between charge and orbital transport can be rigorously established by adopting the definition of “proper” current proposed by Shi et al. [6], in which the nonconserving effect is considered. Based on the implementation in our firstprinciples codes, we present a detailed analysis of the direct and inverse orbital Hall effects in Pt(111) and W(110) thin films, prototypical systems used in experiments. We demonstrate that the reciprocal relation is exactly satisfied for the “total” responses. However, we find interesting features in “local” responses, especially near the surface, where the direct and inverse effects can significantly deviate from each other. We find its origin in the orbital Rashba coupling at the surface, in which the angular momentum transfer between the lattice and orbital degrees of freedom is strongly pronounced. We believe this explains the large inverse orbital RashbaEdelstein effect measured in a recent THz spectroscopy experiment [1]. We discuss further implications of our finding and its relevance in various experimental setups. 
Friday, March 8, 2024 12:42PM  1:18PM 
Z24.00003: Orbital Rashba effects and lightinduced Orbital Current in Teraherz Emission Experiments. Invited Speaker: Henri Jaffres Orbitronics is based on the use of orbital currents as information carriers. These may be generated by the application of an electric field, and inversely, by reciprocity, they could be converted back to charge or even spin currents. In this presentation, I will start my presentation by describing experiments showing the strong enhancement of the currentinduced torques on the Co layer of a Pt/Co bilayer by the addition of an Al layer on Co (Al protected from oxidation). The enhancement is predominant for the FL torque, up to factor of 9. The interpretation [2] comes from abinitio calculations showing large Co orbital moments in the interfacial Co layer with a helical texture similar to the spin texture on the surface of topological insulators. The calculation of the resulting torques leads to a good agreement between the calculated and enhanced experimental torques. 
Friday, March 8, 2024 1:18PM  1:54PM 
Z24.00004: Giant Orbital Hall Effect and OrbitaltoSpin Conversion in 3d, 5d and 4f Metallic Heterostructures Invited Speaker: Pietro Gambardella Recent theories have shown that an electric field can induce a transverse flow of orbital angular momenta in elemental metals, even if crystal field and band structure effects completely quench the orbital magnetism at equilibrium [13]. In particular, electric currents in 3d elements can generate a substantial nonequilibrium orbital accumulation that is comparable to or even larger than the spin accumulation caused by the spin Hall effect and the RashbaEdelstein effect in the 5d elements [4,5]. The generation of orbital currents plays a pivotal role in inducing spinorbit torques in ferromagnets [69], which opens new avenues for the realization of spintronic devices for memory and logic applications [10]. In this talk I will discuss the emergence of strong orbital Hall in 3d metals, focusing on the crucial role of orbitaltospin conversion in the generation of spinorbit torques [9] and orbital magnetoresistance [11,12]. I will further show how the inclusion of rareearths as spacer layers or as alloyed species can be used to boost the orbitaltospin conversion and the torque efficiency in magnetic heterostructures and discuss recent results involving the orbital RashbaEdelstein effect. 
Friday, March 8, 2024 1:54PM  2:30PM 
Z24.00005: Orbital Hall effect in twodimensional materials Invited Speaker: Tatiana G Rappoport I will explore orbital effects in twodimensional (2D) materials and the potential implications they hold. A central theme of my presentation will be the orbital Hall effect (OHE), a phenomenon closely related to the spin Hall effect (SHE). The OHE, much like the SHE, gives rise to a transverse flow of angular momentum due to a longitudinally applied electric field. However, what sets it apart is its distinct origin, emerging from the interplay between orbital attributes and crystal symmetries, free from reliance on spinorbit coupling. 
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