Orbitronics: Electron orbital angular momentum dynamics in solids*

This talk introduces recent developments in orbitronics [1,2], the study of electron angular momentum dynamics in solids. In contrast to the widely spread belief that the electron orbital angular momentum is quenched in solids, it will be illustrated that the electron orbital angular momentum degree of freedom is not quenched and exhibits rich dynamics even without the spin-orbit coupling [3]. We then discuss recent efforts to probe the orbital dynamics experimentally [4,5]. Orbital torque measurement [4] and the orbital accumulation measurement [5] will be discussed. The orbital torque measurement is based on the theoretical prediction [6] that an orbital current induces torque when an orbital current is injected into a ferromagnet. The orbital accumulation measurement is based on the reasoning that an orbital Hall current results in an orbital accumulation in side surfaces. If time allows, formal aspects of the orbital dynamics will also be discussed, which imply important differences between the orbital and the spin dynamics.

[1] Dongwook Go, Daegeun Jo, Changyoung Kim, and Hyun-Woo Lee, Intrinsic spin and orbital Hall effects from orbital texture, Physical Review Letters 121, 086602 (2018).

[2] Dongwook Go, Daegeun Jo, Hyun-Woo Lee, Mathias Klaui, and Yuriy Mokrousov, Orbitronics: Orbital currents in solids, Europhysics Letters (Perspective) 135, 37001 (2021).

[3] Seungyun Han, Hyun-Woo Lee, and Kyoung-Whan Kim, Orbital dynamics in centrosymmetric systems, Physical Review Letters 128, 176601 (2022).

[4] Dongjoon Lee, Dongwook Go, Hyeon-Jong Park, Wonmin Jeong, Hye-Won Ko, Deokhyun Yun, Daegeun Jo, Soogil Lee, Gyungchoon Go, Jung Hyun Oh, Kab-Jin Kim, Byong-Guk Park, Byoung-Chul Min, Hyun Cheol Koo, Hyun-Woo Lee, OukJae Lee, and Kyung-Jin Lee, Orbital torque in magnetic bilayers, Nature Communications 12, 6710 (2021).

[5] Young-Gwan Choi, Daegeun Jo, Kyung-Hun Ko, Dongwook Go, Kyung-Han Kim, Hee Gyum Park, Changyoung Kim, Byoung-Chul Min, Gyung-Min Choi, and Hyun-Woo Lee, Observation of the orbital Hall effect in a light metal Ti, Nature 619, 52 (2023).

[6] Dongwook Go, and Hyun-Woo Lee, Orbital torque: Torque generation by orbital current injection, Physical Review Research 2, 013177 (2020).