Breathing ferroelectricity induced topological valley states in kagome niobium halide monolayers

In recent years, kagome lattices have garnered significant attention for their diverse properties in topology, magnetism, and electron correlations. However, the exploration of breathing kagome lattices, which exhibit dynamic breathing behavior, remains relatively scarce. Structural breathing introduces an additional degree of freedom that is anticipated to fine-tune the electronic structure, potentially leading to exotic properties within the system. In this study, we employ a combination of the kp model and first-principles calculations to explore how breathing ferroelectricity can modulate valley states within a monolayer of niobium halide with breathing kagome lattice. Through the interplay of magnetoelectric coupling and the lock-in between breathing and ferroelectric states, we demonstrate that a dynamically breathing process, when controlled by an appropriately applied electric field, can achieve valley polarization reversal and generate multiple valley states, including those that are topologically nontrivial. These state transformations may couple to distinctive properties in circularly-polarized optical responses and various valley Hall effects. Consequently, our results suggest that materials featuring breathing kagome lattices represent promising platforms for studying the interplay among structure, charge, spin, and valley degrees of freedom, a crucial step toward developing multifunctional devices.