Sliding ferroelectricity and ferroelectricity-valley coupling in Janus RuXY (X, Y = Cl, Br, I) bilayers

Two-dimensional Janus transition-metal dihalides RuXY (X, Y = Cl, Br, I) constitute a class of van der Waals multiferroic materials with the coexistence of sliding ferroelectricity and interlayer antiferromagnetism. Here, we report the sliding ferroelectricity and ferroelectricity-valley coupling in Janus RuXY bilayers based on first-principles calculations. The out-of-plane ferroelectric polarization, which originates from vertical charge transfer induced by broken mirror symmetry, can be reversed through interlayer sliding. The relative electronegativity of the halogens strongly determines the enhancements or reductions of the polarizations of Janus RuXY bilayers. In addition, the valley polarization and valley splitting at the K/K' points in Janus RuXY bilayers are inverted upon ferroelectric switching due to interlayer sliding, demonstrating a potential ferroelectricity-valley coupling mechanism - the regulation of the valley state by the sliding ferroelectric polarization. Furthermore, atomically projected band structures illustrate that Ru atoms dominate the valley behavior, while contributions from halogen atoms are negligible. The coexistence of low switching energy and strong ferroelectricity-valley coupling makes Janus RuXY bilayers a promising platform for energy-efficient multiferroic and valleytronic devices.