Breathing Tuned Functionality in Janus Kagome Monolayers

Flat bands exhibit an extremely heavy electron effective mass and quenched kinetic energy. In contrast, valley states typically occur at band extrema and serve as information carriers with high tunability. These two properties are mutually exclusive in conventional materials. Here, in the breathing kagome Nb₃X₈ (X = Cl, Br, I), we introduce a layer degree of freedom to construct a Janus monolayer Nb₃Cl₄I₄ to combine these two aspects. The intrinsic breathing ferroelectricity can be regarded as a reversible sublayer electric field that switches the system between Nb₃I₈-like valley properties and Nb₃Cl₈-like flat-band correlations. At room temperature, this polarization reversal yields a stable on/off ratio in transport. As the temperature decreases and long-range magnetic ordering sets in, the same reversal drives a transition between spin-valley-locked multivalley states and a strong correlation effect driven by flat-band effects. Through this breathing-tuned coexistence of two aspects of electronic behavior, our work expands the landscape of the breathing process.