Ferroelectric Control of Spin Textures in Layered Hybrid Perovskites

TL;DR

The paper investigates the layered hybrid perovskites (PA)2CsY2X7 (Y=Pb, Sn; X=I, Br) as a platform where strong spin–orbit coupling and ferroelectric polarization couple to stabilize spin textures. Using density functional theory with spin–orbit coupling, Berry-phase polarization analysis, and a symmetry-informed k dot p model, it shows indirect band gaps with extrema near the Gamma point, sizable spin splittings at the band edges, and robust in-plane ferroelectric polarization. Polarization switching reverses the spin orientation, enabling non-volatile electrical control of persistent spin textures and linking ferroelectric order directly to spin degrees of freedom. The results establish (PA)2CsY2X7 as a versatile design platform for spintronic functionality, including lead-free Sn-based variants and potential devices such as spin field-effect transistors, where spin textures can be controlled by an external electric field.

Abstract

Hybrid organic--inorganic perovskites with broken inversion symmetry provide a fertile ground for uncovering coupled spin-orbit and ferroelectric phenomena. Here, we investigate the layered family (PA)$_2$CsY$_2$X$_7$ (Y = Pb, Sn; X = I, Br) using density functional theory, Berry-phase polarization analysis, and effective $\boldsymbol{k \cdot p}$ modeling. Across all four members, we find indirect bandgaps with extrema near $Γ$, sizable spin splittings at both band edges, and robust in-plane ferroelectric polarization that stabilizes out-of-plane persistent spin textures (PSTs). Crucially, polarization reversal switches the spin orientation, enabling electrical control of PSTs and thereby non-volatile manipulation of spin states. These results establish (PA)$_2$CsY$_2$X$_7$ as a versatile materials platform where compositional design and ferroelectric switching jointly enable spintronic functionality.

Figures (7)

• Figure 1: Schematic representation of spin--orbit-induced band splitting and polarization-dependent spin texture reversal. (a) Parabolic band structure in the absence of spin--orbit coupling (SOC), showing spin-degenerate bands. (b) SOC-induced spin splitting with characteristic energy offset $E_{\mathrm{R/D}}$ and momentum shift $k_0$, illustrating the emergence of spin-split bands with out-of-plane spin orientation. (c) Depiction of spin textures in the $k_y$--$k_z$ plane under opposite ferroelectric polarizations ($\pm\mathbf{P}$). The red and blue loops correspond to spin textures with opposite $S_x$ orientations, indicating the formation of persistent spin textures (PSTs). Switching the in-plane polarization $\mathbf{P}$ reverses the spin texture, highlighting the intimate coupling between ferroelectricity and spin degrees of freedom.
• Figure 2: Side view of the relaxed crystal structures of bulk (left) and monolayer (right) (PA)$_2$CsSn$_2$Br$_7$. Sn-centered $\mathrm{SnBr}_6$ octahedra form the inorganic sheets separated by organic $\mathrm{PA}^+$ cations.
• Figure 3: Electronic band structures of bulk (a, c) and monolayer (b, d) (PA)$_2$CsSn$_2$Br$_7$. Panels (a, b) show results without spin--orbit coupling (SOC), while (c, d) include SOC. Insets illustrate the corresponding high-symmetry $k$-paths in the first Brillouin zone with coordinate axes.
• Figure 4: Bulk (PA)$_2$CsSn$_2$Br$_7$: unidirectional out-of-plane spin textures near $\Gamma$. Top row = VBM, bottom row = CBM. (a,d) DFT bands (black) and $\boldsymbol{k \cdot p}$ fits (red dashed); (b,e) DFT $S_x$ on constant-energy contours; (c,f) $S_x$ from the two-band $\boldsymbol{k \cdot p}$ model. Spin textures are dominated by the out-of-plane component $S_x$ (colorbar ranges differ between rows).
• Figure 5: Monolayer (PA)$_2$CsSn$_2$Br$_7$: unidirectional out-of-plane spin textures near $\Gamma$. Top row = VBM, bottom row = CBM. (a,d) DFT bands (black) and $\boldsymbol{k \cdot p}$ fits (red dashed); (b,e) DFT $S_x$ on constant-energy contours; (c,f) $S_x$ from the two-band $\boldsymbol{k \cdot p}$ model. Spin textures are dominated by the out-of-plane component $S_x$ (colorbar ranges differ between rows).