Out-of-Plane Rashba Spin-Splitting and Structure-Property Relationships in Layered Cu₂AM^VSe₄ Chalcogenides (A = K, Rb, Cs; M^V = V, Nb, Ta)

Among 2D layered spintronic materials, persistent spin textures with out-of-plane spin polarization are especially desirable, originating from in-plane symmetry breaking. Some 2D materials such as transition metal dichalcogenides exhibit in-plane symmetry breaking within individual layers, but these layers stack in alternating polarities in the bulk, necessitating deposition as one or few atomic layers to express strong spin-splitting. To bypass this restriction, we investigated a family of nine globally non-centrosymmetric, layered Cu₂AM^VSe₄ (A = K, Rb, Cs; M^V = V, Nb, Ta) chalcogenides, incorporating M^V transition metal ions with varying spin-orbit coupling strengths to encourage and tune spin-splitting in the bulk. Compositions containing the heavy metal Ta were found to exhibit strong out-of-plane Rashba-type spin-splitting due to plane-parallel symmetry breaking in the 2D layer. Across the family, we explore the structural and compositional origins for the critical aspects of this spin-splitting, including magnitude, orientation, texture, and relative energetics in k-space. The evident out-of-plane spin-splitting creates a persistent spin texture within the 2D layer, and the intrinsic expression of this splitting in the bulk material eliminates the limitations otherwise imposed by ultra-thin conditions, improving scalability and further enabling applications in opto-spintronics, where effective light absorption is critical.