Atomic-Scale Observation of Stacking Domain Structures in a Semiconducting Van Der Waals Ferrovalley Material VAgP2Se6

Abstract The van der Waals (vdW) ferrovalley materials show spontaneous valley polarization without external tuning by simultaneously possessing the valley degree of freedom and the broken time-reversal symmetry but are extremely rare. The ferrovalley crystals with spin-orbital coupling are predicted to generate novel quantum phenomena in two-dimensional thin layers, such as the anomalous valley Hall effect. However, due to the reduced symmetry, a potential issue in vdW ferrovalley materials is their complex domain structures. Here, we report domain structure studies in a semiconducting vdW ferrovalley material VAgP2Se6. Our studies reveal clear evidence of second harmonic generation (SHG) response and magnetic hysteresis loop in the bulk VAgP2Se6 crystals suggesting spontaneously broken inversion and time-reversal symmetry. We image its domain structures directly at atomic resolution using aberration-corrected scanning transmission electron microscopy (AC-STEM). Six variants of domains stacking along the c-axis are observed and can be well understood via group theory analysis. The domain structures of vdW materials can create naturally occurring heterostructures that could alter the electronic structures and may lead to topological interfacial states but have not been explored extensively. Our findings open the possibilities for observing and understanding exotic quantum phenomena in vdW ferrovalley materials by engineering the domain structures.