Local Symmetry Breaking and Hidden Spin Polarization in 2D Hybrid Perovskites with Nonprimary Ammonium Cations

Two-dimensional hybrid perovskites (2DHPs) have attracted interest as potential platforms for spin-optoelectronic technologies, including spin filtering and polarized light-emitting diodes. When coupling 2DHPs with strong spin-orbit coupling effects, inversion symmetry breaking has often provided an effective indicator/descriptor for screening spintronic materials within the vast prospective organic-inorganic compositional space. Despite the importance of global symmetry, recent studies have revealed that hidden physical effects such as spin and valley polarization can arise due to local symmetry breaking within 2DHP structures that maintain global inversion symmetry. Since many experimental techniques (e.g., some optical probes) are sensitive to such local effects, a chemical understanding of the material design rules for local symmetry breaking is crucial for rationally developing functional spin-optoelectrotnic materials. In this perspective, we propose that such symmetry breaking in 2DHPs may naturally follow from the low symmetry of associated organic cation fragments and thus can be deliberately engineered. Specifically, we focus on 2DHP crystal structures comprising conformationally restricted nonprimary ammonium cations, wherein the local inversion symmetry is broken and spin polarization-related properties might be extracted. By comparing with inorganic materials, we propose a framework targeted at experimental and computational understanding of such physical effects in 2DHPs, for which the compositional flexibility may allow greater control over exotic spin and topological properties that have so far remained hidden.