Atomic-level polarization reversal in sliding ferroelectric semiconductors.

Intriguing "slidetronics" has been reported in van der Waals (vdW) layered non-centrosymmetric materials and newly-emerging artificially-tuned twisted moiré superlattices, but correlative experiments that spatially track the interlayer sliding dynamics at atomic-level remain elusive. Here, we address the decisive challenge to in-situ trace the atomic-level interlayer sliding and the induced polarization reversal in vdW-layered yttrium-doped γ-InSe, step by step and atom by atom. We directly observe the real-time interlayer sliding by a 1/3-unit cell along the armchair direction, corresponding to vertical polarization reversal. The sliding driven only by low energetic electron-beam illumination suggests rather low switching barriers. Additionally, we propose a new sliding mechanism that supports the observed reversal pathway, i.e., two bilayer units slide towards each other simultaneously. Our insights into the polarization reversal via the atomic-scale interlayer sliding provide a momentous initial progress for the ongoing and future research on sliding ferroelectrics towards non-volatile storages or ferroelectric field-effect transistors.

Optical properties of ferroic Fe2O(SeO3)2 and Fe2(SeO3)3·3H2O.

Ferroic compounds Fe2O(SeO3)2 (FSO) and Fe2(SeO3)3·3H2O (FSOH) prepared by the hydrothermal method are characterized and their optical properties are investigated by combining with first-principles calculations. The results show that (i) FSO is antiferromagnetic below ∼110 K and becomes ferromagnetic at elevated temperatures, while FSOH is antiferromagnetic at low temperatures probably due to a change in the spin state from Fe3+ (S = 5/2) to Fe2+ (S = 2); (ii) the optical bandgap is determined to be ∼2.83 eV for FSO and ∼2.15 eV for FSOH, consistent with the theoretical calculation; and (iii) the angle-resolved polarized Raman spectroscopy results of both crystals demonstrate the strong anisotropic light absorption and birefringence effects, and the unconventional symmetricity of some Raman modes is observed, which can be interpreted from the variation of Raman scattering elements. This work can provide not only an understanding of the structure and physical properties of iron selenites, but also a strategy for exploring the anomalous Raman behaviors in anisotropic crystals, facilitating the design and engineering of novel functional devices with low-symmetry ferroic materials.