  Non-volatile manipulation of valley polarization in solids has long been
desired for valleytronics applications but remains challenging. Here, we
propose a novel strategy for non-volatile manipulating valleys through bilayer
stacking, which enables spontaneous valley polarization without breaking
time-reversal symmetry. We call this noval physics as bilayer stacking
ferrovalley (BSFV). The group theory analysis reveals that the two-dimensional
(2D) valley materials with hexagonal and square lattices can host BSFV. By
searching the 2D material database, we discovered 14 monolayer 2D materials
with direct gaps that are candidates for realizing BSFV. Further
first-principles calculations demonstrate that BSFV exists in RhCl3 and InI
bilayers. The bilayer stacking breaks their three- and four-fold rotation
symmetry, resulting in 39 and 326 meV valley polarization, respectively. More
interestingly, the valley polarization in our systems can be switched by
interlayer sliding. Our study opens up a new direction for designing
ferrovalley materials and thus greatly enriches the platform for the research
of valleytronics.
