Orbitronics: Harnessing Orbital Angular Momentum

• Orbitronics is an emerging field in condensed matter physics focused on generating, manipulating, and detecting orbital angular momentum as an independent degree of freedom.
• It utilizes intrinsic Berry curvature, symmetry breaking, and external fields to create and control orbital currents without relying on strong spin–orbit coupling.
• Applications include low-power logic, terahertz emission, and memory devices, with potential extension to topological and bosonic orbital phenomena.

Orbitronics is an area of condensed matter physics concerned with the generation, manipulation, and detection of the orbital angular momentum (OAM) of electrons—or, in some contexts, bosonic quasiparticles—as an independent degree of freedom for information transport and device function. In contrast to spintronics, which exploits the electron spin, orbitronics focuses on OAM arising from the spatial structure of Bloch wavefunctions in crystals and is not fundamentally dependent on spin–orbit coupling (SOC). OAM-driven currents and textures, often termed “orbital currents,” can be produced and controlled through crystal symmetry, electric fields, magnetic order, light, or even lattice rotations, opening unique functionalities for information processing, storage, low-power logic, terahertz emission, and the manipulation of topological states.

1. Fundamental Principles and Theoretical Foundations

In periodic solids, electrons inherit OAM from atomic orbitals, which is modified by crystal symmetry and hybridization, yielding “orbital textures” in momentum space. The orbital current operator takes the generic form: $$j^L_\alpha = \frac{1}{2}(v_\alpha L_\beta + L_\beta v_\alpha)$$ where $v_\alpha$ is the velocity operator and $L_\beta$ is the orbital angular momentum operator (Go et al., 2021). A prototypical physical manifestation is the orbital Hall effect (OHE): the generation of a transverse OAM current in response to an applied electric field, analogous to the spin Hall effect but not requiring strong SOC (Cysne et al., 17 Feb 2025).

The modern theoretical framework describes the OHE and related phenomena via the Berry phase theory, where the orbital Berry curvature $\Omega^{(L)}(\mathbf{k})$ acts as an effective momentum-space “magnetic field” for OAM. The intrinsic OHE conductivity is given by: $$\sigma^{L_z}_{xy} = e \sum_n \int_\mathrm{BZ} \frac{d^2k}{(2\pi)^2} f(\varepsilon_{n\mathbf{k}}) \Omega^{(L_z)}_{n,xy}(\mathbf{k})$$ where $\Omega^{(L_z)}_{n,xy}(\mathbf{k})$ denotes the orbital Berry curvature of band $n$ (Cysne et al., 17 Feb 2025). The orbital Rashba–Edelstein effect (OREE) further allows for non-equilibrium orbital accumulation at interfaces in noncentrosymmetric crystals under applied electric fields.

A key distinction from spin is that orbital quenching is ubiquitous in static crystals due to symmetry and crystal field splitting, yet orbitronic effects arise when systems are driven out of equilibrium (by fields, optical excitation, symmetry breaking, etc.), or have appropriate orbital-hybridizing symmetry (Go et al., 2021, Fukami et al., 28 Sep 2025). Theoretical studies have clarified that both intra-atomic (atomic-site) and inter-atomic ("intercellular") contributions to orbital currents are relevant; the intra-atomic component is more robust to disorder (Pezo et al., 2023).

2. Mechanisms for Generation and Control of Orbital Currents

Multiple intrinsic and extrinsic mechanisms generate and modulate orbital currents:

• Intrinsic OHE: Electric field-induced hybridization of orbitals in Bloch bands, leading to opposite OAM flows in different momentum directions, with Berry curvature playing a central role. The effect is pronounced in materials with strong orbital hybridization and nontrivial symmetry (Go et al., 2021, Chen et al., 11 Apr 2024).
• Optically-induced Orbitronics: Circularly-polarized light can resonantly drive transitions in the orbital sector, breaking time-reversal symmetry and opening topological gaps, as demonstrated for the L=1 multiplet on a triangular lattice (Phong et al., 2018). Femtosecond pulses in Ni induce direct orbit currents, dominating over spin channels in some cases; their propagation velocity and conversion into charge currents can be probed by terahertz emission (Xu et al., 2023).
• Magnon–Orbitronics Coupling: In magnetically ordered systems, magnons (spin waves) with finite scalar spin chirality can induce a “topological orbital moment” in the electronic system, providing a magnon-driven mechanism for imprinting and transporting orbital magnetism (Zhang et al., 2020).
• Symmetry-induced Orbital Transport: Chirality, inversion/mirror symmetry breaking, and rotation allow for fine control of OAM; noncentrosymmetric stacking in layered systems (e.g., AA vs AB stacking in TMDCs) and crystal-handedness directly alter which valleys or layers host localized orbital polarization (Ji et al., 13 Oct 2025).
• Ferro-rotational Order and Higher Multipoles: Novel mechanisms rooted in ferro-rotational (FR) order and the associated electric hexadecapole moment create unconventional (longitudinal or non-orthogonal) orbital currents, extending the orbitronic palette beyond the standard OHE (Jo et al., 7 May 2025).

3. Topological and Material Aspects

Orbitronics is deeply entwined with modern band topology, where OAM textures are both signatures and drivers of topological phenomena:

• Topological Phase Transitions: Band inversion leads to a switch in dominant orbital character near the Fermi level, sharply altering the sign and magnitude of the OHE. This enables topology-engineered OAM currents, as in Janus RuBrCl and MnBi₂Te₄, where the OHE is tuned by strain or composition (Chen et al., 11 Apr 2024).
• Chiral and Multifold Fermions: In chiral semimetals like CoSi, the OAM is locked almost isotropically along the momentum direction (L·k), resulting in monopole-like OAM textures in the Brillouin zone, which are directly visualized in circular dichroism–ARPES (Hagiwara et al., 27 Oct 2024, Yen et al., 2023). The polarity of OAM monopoles can be controlled by switching the structural chirality.
• Giant OAM and Valley Orbitronics: In rhombohedral graphene multilayers, inter-atomic cycloid motion can generate giant OAM (exceeding ±30 µB) and massive orbital Hall responses, highly tunable by gating, magnetic field, or periodic driving (Floquet engineering) (Mu et al., 21 Mar 2025).
• Phonon and Magnon Orbitronics: Quantum geometric effects enable the transfer of OAM between bosonic quasiparticles (e.g., between magnons and phonons), unifying the concept of OAM in hybrid bosonic systems, and allowing for its electrical detection via induced edge voltages (To et al., 30 Sep 2025).

An overview of representative material platforms:

System	Orbitronic Effect	Key Features
TMD monolayers/bilayers	OHE, valley polarization	Valley-contrasting OAM, topological
RuBrCl, MnBi₂Te₄	Topology-tuned OHE	Strain/phase controlled OAM
Rhombohedral graphene multilayers	Giant OMM, OHE, valley control	Intrinsic, spin-free platform
Chiral semimetals (CoSi, PtGa, PdGa)	Monopole OAM, topological response	Chirality, OAM-momentum locking
Ni-based heterostructures	Optically-induced orbit currents	Terahertz emission, fast dynamics

4. Experimental Detection, Propagation, and Toroques

The measurement and utilization of orbital currents present unique challenges due to OAM's indirect coupling to external probes:

• Detection Techniques: Magneto-optical Kerr effect (MOKE), x-ray magnetic circular dichroism (XMCD), and CD-ARPES provide direct probes of OAM accumulation and textures (Go et al., 2021, Yen et al., 2023, Hagiwara et al., 27 Oct 2024). Conversion of orbit currents to charge (or spin) signals is possible via inverse orbital Rashba-Edelstein or inverse orbital Hall effects, and the detection of terahertz emission (Xu et al., 2023, Guan et al., 22 Apr 2025).
• Propagation Lengths: Experimental evidence indicates orbital currents can propagate over distances of ~100 nm in oxidized Cu channels at room temperature (Gao et al., 16 Feb 2025), but in heavy metals, orbital mean free paths can be sub-nanometer, less than spin transport lengths, due to lattice symmetry constraints (Guan et al., 22 Apr 2025). There are unresolved discrepancies between predicted and observed orbital diffusion lengths, with literature reporting both long-range (via "hotspots") and ultrashort orbital transport (Fukami et al., 28 Sep 2025).
• Reciprocity and Conversion: Onsager reciprocity between orbital-charge interconversion has been experimentally established, forming a basis for directional device architectures (Gao et al., 16 Feb 2025). The efficiency of orbital-to-spin or orbital-to-charge conversion (often parameterized by a dimensionless angle ξ) is highly material and interface-dependent (Fukami et al., 28 Sep 2025, Hayashi et al., 2022).
• Torque Mechanisms: Orbital torques, generated by the injection of OAM into a ferromagnet and subsequently converted via SOC, enable control of magnetization dynamics with potentially higher efficiencies and longer propagation than conventional spin torques (Hayashi et al., 2022, Go et al., 2021). Theoretical models identify scenarios (e.g., in systems with nontrivial quantum metrics) where electric fields induce intrinsic OAM torques, determined by the quantum geometric tensor and sensitive to the symmetry of the band structure (Atencia et al., 2023).

5. Challenges, Open Questions, and Applications

Orbitronics faces significant theoretical, experimental, and engineering challenges:

• Relaxation and Quenching: The relaxation and dephasing mechanisms for OAM (orbital quenching, mixing, scattering) are not fully understood, with open questions remaining about why orbital transport appears long-ranged in some experiments but ultrashort in crystalline heavy metals (Fukami et al., 28 Sep 2025, Guan et al., 22 Apr 2025).
• Material and Interface Engineering: The magnitude of orbital currents and their conversion to spin/charge is extremely sensitive to crystal symmetry, stacking, interface roughness, and composition. Reproducing high conversion efficiencies comparable to or better than traditional spin–orbit materials remains an open field of research (Fukami et al., 28 Sep 2025, Ji et al., 13 Oct 2025).
• Device Integration: Applications in nonvolatile memory (MRAM), neuromorphic circuits, and low-power interconnects rely on achieving high torque efficiency, scalable diffusion lengths, and compatibility with established device architectures. Devices exploiting orbital torques could use lighter elements, reducing energy dissipation, but must overcome integration challenges relating to TMR, endurance, and interfacing (Fukami et al., 28 Sep 2025).
• Future Prospects: Theoretical and experimental efforts are underway to develop detection schemes for pure orbital currents, disentangle orbital and spin contributions, optimize heterostructures and topology-driven OHE, harness chiral OAM textures, and extend orbitronic concepts to bosonic and mechanical platforms (Cysne et al., 17 Feb 2025, Chen et al., 11 Apr 2024, To et al., 30 Sep 2025).

6. Extensions: Bosonic Orbitronics, Topological and Chiral Phenomena

Orbitronics is not confined to electronic systems but extends to magnonic and phononic quasiparticles:

• Magnon Orbitronics: Magnons with nonzero scalar spin chirality can drive large electronic orbital magnetizations and thermal orbital Nernst effects, integrating magnonics and orbital control for energy-efficient hybrid devices (Zhang et al., 2020, Go et al., 2023).
• Phonon Orbitronics: Magnon-phonon hybridization, mediated by symmetry-breaking magnetoelastic coupling, allows for the transfer of OAM between magnons and phonons, making possible the electrical control of both vibrational energy and magnetization via quantum-geometric effects (To et al., 30 Sep 2025).
• Chiral Semimetals and Monopoles: The discovery of OAM monopoles and robust chirality-linked orbital textures in materials like PdGa, CoSi, and PtGa paves the way for highly tunable orbitronic functionalities where the OAM current is dictated by structural handedness (Hagiwara et al., 27 Oct 2024, Yen et al., 2023).

7. Summary and Outlook

Orbitronics, through the exploitation of orbital angular momentum in solids, offers a new axis for information transport and device operation, supplementing or superseding conventional spintronic mechanisms. The diverse generation mechanisms, robust theoretical underpinnings, and expanding material platforms—including 2D crystals, chiral semimetals, and ferro-rotational compounds—have revealed unique topological, geometric, and dynamical phenomena. Critical open questions concern the mechanisms of orbital relaxation, conversion efficiency at interfaces, and optimal design principles for device applications ranging from MRAM to terahertz emitters. The integration of quantum geometric concepts and the extension to bosonic systems herald a new domain where control over orbital degrees of freedom underpins both fundamental physics and technological advances (Fukami et al., 28 Sep 2025, Cysne et al., 17 Feb 2025, Ji et al., 13 Oct 2025).