Spin-momentum locking in locally noncentrosymmetric quantum materials

The primary focus of spintronics is the investigation of novel spin splitting effects and related spin-polarized quantum materials, which have been extensively pursued for their potential applications. The structural inversion asymmetric Rashba splitting, bulk inversion asymmetric Dresselhaus splitting, and ferromagnetic spin polarization derived from Zeeman splitting constitute the foundation of traditional spintronics. From a symmetry perspective, ferromagnets achieve spin splitting through the breaking of time-reversal symmetry. However, in time-reversal symmetric and inversion symmetric materials with spin-orbit coupling, unexpected forms of spin-splitting can also arise by breaking local inversion symmetry, known as hidden spin-momentum locking (SML), bringing infinite vitality to fundamental research and future applications. This review first highlights notable advancements in spin-splitting within centrosymmetric systems, then examines the influence of hidden SM locking on superconducting and topological behaviors, concluding with a discussion on prospective opportunities in this emerging field. Given the rapid progress in non-relativistic spin splittings-particularly within altermagnetism-we develop appropriately scaled extensions to advance this emerging field. This review seeks to enhance our understanding of the 'hidden effect' in fundamental research while uncovering additional quantum phenomena that emerge from introducing extra degrees of freedom-an aspect that underscores the unique appeal of quantum materials capable of continuously demonstrating novel effects.