Rashba Spin Splitting in Non-Centrosymmetric Hybrid Metal Halide Perovskites Through Cation Tuning

Metal halide perovskites are a class of hybrid semiconductors that offer extraordinary structural tunability. Non-centrosymmetric low dimensional perovskites with strong spin-orbit coupling introduced by the heavy metal and halide atoms can exhibit useful properties such as Rashba and/or Dresselhaus spin splitting, ferroelectricity, and optical nonlinearity. Here, we discuss approaches to tune the structural symmetry of low-dimensional halide perovskites via the spacer, A-site, and metal cations and the consequence on spin splitting. Symmetry breaking in complex layered systems can occur across different length scales and local distortions do not always lead to bulk non-centrosymmetry. “Hidden” (monolayer) non-centrosymmetry is relatively common for <i>n</i> &gt; 1 2D perovskites – including some phases with excellent excitonic properties – and can result in a hidden Rashba spin splitting along multiple paths in <i>k</i>-space that can be brought out in bulk via external perturbation such as applied pressure. Further cation engineering by modifying the metal site to increase stereochemical lone pair activity, increasing quantum well thickness (<i>n</i>), and incorporating an asymmetrically substituted spacer cation can break global (space group) centrosymmetry, but this doesn’t always increase the magnitude of spin splitting and typically limits the splitting to one path in <i>k</i>-space. However, global <i>C</i>_2<i>v</i> polar symmetry leads to unidirectional spin splitting in k, i.e.,<i> </i>persistent spin texture, which promises to increase spin relaxation time. We further show that the metal halide composition influences the type of structural distortion that results from incorporation of asymmetric spacer cations (<i>e.g., </i>tilting, octahedral distortion, and/or dimensional reduction). This work provides structural insights for designing new, low-dimensional non-centrosymmetric perovskites for spin-orbitronic and quantum applications.