PGI Colloquium: Prof. Dr. Aurélien Manchon, Aix-Marseille Univ, CNRS, CINaM, Marseille, France

Please note: You will receive the link to the online talk in the e-mail invitation, usually sent out a few days before the lecture takes place. It is also available on request from the contact person below.

Theory of orbital generation, diffusion, and detection

Recent progress in the physics of spin-charge interconversion mediated by spin-orbit coupling has shed new light on the orbital angular momentum degree of freedom. Indeed, while the orbital ordering driven by the crystal-field potential governs the interplay between crystal structure and electronic properties of strongly correlated materials such as Mott insulators, the possibility of transporting the orbital information in these materials has remained an open question so far. In the context of metallic spintronics though, it has been progressively realized that the orbital angular momentum can be generated out of equilibrium, transported, and detected, rather similarly to the spin angular momentum.

The interconversion between charge and orbital currents, via orbital Hall and orbital Rashba effects for instance, might be much more efficient than its spin counterpart because it arises from the orbital texture imposed by the crystal-field potential rather than from spin-orbit coupling. Therefore, corresponding phenomena such as orbital torque and orbital magnetoresistance have been proposed and experimentally reported. Recent phenomenological models of orbital diffusion have been recently proposed but lack quantitative predictability by overlooking microscopic details. Understanding the way orbital currents and densities propagate in metals and accumulate at interfaces requires determining transport coefficients such as orbital conductivity or diffusivity, as well as the ability to interconvert spin currents into orbital currents via spin-orbit coupling.

In this presentation, I intend to provide a discussion of the theoretical aspects of orbital generation, transport, and detection. After introducing general ideas about orbital ordering and orbital angular momentum, I will first discuss the orbital-charge interconversion mechanisms, orbital Hall, and orbital Edelstein effects. I will insist on the distinction between intra-atomic and inter-atomic orbital contributions in these two effects [1]. Then, I will present a quantum theory of orbital diffusion and uncover several mechanisms governing orbital torque and magnetoresistance phenomena, including orbital diffusivity, spin-orbit polarization, orbital swapping, and orbital mixing conductance [2,3]. These new concepts are crucial to the understanding of experimental results and can be computed from first principles. Finally, I will discuss the possibility of detecting orbital densities optically using the orbital Kerr effect. I will particularly emphasize the central role played by the second-order Hall effect, which is a companion phenomenon to the orbital Edelstein effect.

[1] Pezo et al., Orbital Hall effect in crystals: inter-atomic versus intra-atomic contributions, Physical Review B 106, 104414 (2021); Orbital Hall physics in two-dimensional Dirac materials, Physical Review B 108, 075427 (2023).
[2] Aurélien Manchon, A. Pezo, Kyoung-Whan Kim, Kyung-Jin Lee, Orbital diffusion, polarization and swapping in centrosymmetric metals, arXiv:2310.04763
[3] Ning et al., unpublished