Breathing ferroelectricity induced topological valley states in kagome niobium halide monolayers

Recently, kagome lattices have garnered significant attention for their diverse properties in topology, magnetism, and electron correlations. However, the exploration of breathing kagome, which exhibit dynamic breathing behavior, remains relatively scarce. Structural breathing introduces an additional degree of freedom that is anticipated to fine-tune the exotic characteristic. In this study, we employ a combination of the k·p model and first-principles calculations to explore how breathing ferroelectricity modulate valley states within niobium halide monolayer. Through the interplay of magnetoelectric coupling and the lock-in between breathing and ferroelectric, we demonstrate that a breathing process can achieve valley polarization reversal and generate multiple valley states, including topologically nontrivial ones. These state transformations couple to circularly-polarized optical responses and various valley Hall effects. Our results suggest that breathing kagome represent promising platform for studying the interplay among structure, charge, spin and valley degrees of freedom, a crucial step toward developing multifunctional devices.