Spin-momentum locking and its derivative effects in quantum materials

Spin-polarized quantum materials exhibiting strong spin-orbital coupling (SOC) and space inversion breaking have led to novel spintronic effects, such as spin-orbital torque and spin field effect transistors, through the mechanism of spin-momentum locking (e. g. Rashba and Dresselhaus spin-splitting). Recently, researchers have made significant discoveries regarding spin-momentum locking effects by introducing additional degrees of freedom. These include the hidden spin polarization effect and the antiferromagnetic spin splitting effect, which break the limit of inversion symmetry and the constraint of SOC, respectively. These findings extend the scope of available spintronic materials. We first offer a concise overview of the fundamental principles and current developments in spin-momentum locking. Next, we focus on the hidden spin polarization as a spin-momentum-layer locking effect, and antiferromagnetic spin splitting without the assistance of spin-orbit coupling. We also discuss emerging spintronic effects associated with these phenomena. Lastly, we provide insights into the future progress of multi-degree-of-freedom coupling in spin-momentum locking, and also potential applications for the next generation of spin-polarized materials.