Orbitronics, an emerging frontier at the intersection of condensed matter physics and information technology, focuses on harnessing the orbital angular momentum flow (orbital current) of electrons as a novel carrier for information and energy transfer. Contrary to the long-held notion of orbital quenching in solids, recent breakthroughs have established that orbital currents can be efficiently generated and detected via mechanisms such as the orbital Hall effect (OHE) and the orbital Rashba-Edelstein effect (OREE), without requiring strong spin-orbit coupling (SOC). This review systematically outlines the foundational framework of orbitronics, encompassing its physical mechanisms, distinguishing characteristics, material platforms, and device prospects.
The core of orbitronics lies in the electrical generation and detection of orbital currents. The OHE, a bulk-mediated transport phenomenon analogous to the spin Hall effect (SHE), generates a transverse orbital current in response to an applied electric field, with the orbital polarization, current flow, and field direction mutually orthogonal. The OREE, originating from inversion symmetry breaking at surfaces or interfaces, creates a non-equilibrium orbital accumulation through chiral orbital textures. Their inverse effects (IOHE and IOREE) enable the conversion of orbital currents back into detectable charge signals. Experimental verification of these effects has been achieved in diverse materials, including light 3d metals (Ti, Cr, Mn), 4d/5d transition metals (Zr, Nb, Ta, W), and heterostructures, employing techniques such as the magneto-optical Kerr effect, harmonic voltage analysis, spin-torque ferromagnetic resonance, and terahertz emission spectroscopy.
Orbital currents exhibit several distinctive features that fundamentally differentiate them from spin currents. First, they demonstrate long-range, non-oscillatory propagation in ferromagnets, with reported orbital diffusion lengths (λ_OHE) reaching up to tens of nanometers, far exceeding the typical spin dephasing length (<1 nm). This is attributed to momentum-space "hot spots" of near-degenerate orbital states that suppress rapid dephasing. Second, the resulting orbital torque (OT) exhibits a critical dependence on the SOC within the adjacent ferromagnetic layer, as the orbital angular momentum must first be converted into spin angular momentum via SOC before coupling to the magnetization via exchange interaction. This contrasts with conventional spin-orbit torque (SOT), which relies on strong SOC in the non-magnetic layer. Third, orbital responses are intrinsically coupled to atomic arrangement and crystalline symmetry, making them highly sensitive to structural order, interface quality, and crystal orientation, thereby offering additional knobs for performance engineering.
The material landscape for orbitronics is remarkably broad. Significant orbital Hall conductivities (σ_OH), often surpassing the spin Hall conductivities (σ_SH) of heavy metals such as Pt, have been predicted and observed in weakly spin-orbit coupled materials including Ti, Cr, Zr, and even silicon. This material versatility alleviates the reliance on scarce and costly heavy elements and opens avenues for using abundant, lightweight materials in spintronic applications. Key material platforms include "pure orbital" materials (e.g., 3d/4d metals), "orbital-to-spin converter" materials (e.g., 5d metals and certain rare-earth elements), "orbitally active" interfaces (e.g., Cu/oxide), and low-dimensional systems such as two-dimensional transition metal dichalcogenides.
Looking forward, orbitronics presents a compelling pathway toward next-generation, low-power, and high-performance electronic devices. Potential applications include ultra-low-current magnetization switching in magnetic random-access memories (MRAM), high-frequency terahertz emitters, non-reciprocal logic devices, and novel architectures for neuromorphic and quantum information processing. Future research efforts should prioritize the development of direct orbital probing techniques, the discovery of efficient orbital-to-spin conversion materials, the design of functional orbital device prototypes, and the integration of orbital functionalities with existing semiconductor and spintronic technologies. By leveraging the unique properties of orbital angular momentum, orbitronics promises to expand the horizons of solid-state physics and enable a new class of energy-efficient information technologies.