M4S (CPHT+LSI+PMC)

Orator: Prof. Yuriy Mokrousov (Forschungszentrum Juelich, Germany) 

Title: Orbitronics: Hype, Hope or the Next Frontier 

Abstract: Orbitronics proposes a light-element, non-relativistic alternative to spin-based functionalities, offering a route toward more sustainable and cost-effective device concepts. I will begin by introducing the basic concepts of orbital angular momentum (OAM) and orbital transport. I will then turn to the theoretical challenges that even the orbital dipole moment alone presents: a properly gauge-invariant OAM operator cannot be constructed naively, non-local contributions frequently dominate over local approximations, and the orbital Hall effect exhibits behavior that departs markedly from simplified treatments. I will show that this complexity extends further, as OAM does not exist in isolation but constitutes the lowest member of a hierarchy of orbital multipoles, coupled to quadrupolar, octupolar, and higher-order degrees of freedom. I will briefly outline a new theoretical framework for orbital transport developed from first principles, encompassing a properly defined OAM operator via Wannier functions, non-local corrections to orbital conductivity and renormalizations of the g-tensor in metals. We will further discuss CoO as a representative "orbital antiferromagnet," in which orbital quadrupoles govern magnetic anisotropy and multipolar spin-orbit excitations are efficiently addressed by orbital exchange fields alone, illustrating that the dipole moment does not capture the full orbital response. In doing so, I will highlight how this framework extends orbitronics toward classes of materials — including correlated oxides and other complex magnetic systems — not yet systematically considered within the field. I will discuss how accounting for this multipolar structure — rather than reducing orbitronics to a simple analogue of spin-orbital conversion — is essential for a complete understanding of orbital phenomena and may define promising directions for future research in the field.