Giant intrinsic circular dichroism of enantiomorphic flat Chern bands and flatband devices

Circular dichroism (CD) is generally observed in the optically active chiral molecules that originate from macroscopic electric and magnetic dipoles, which is usually quite small. In solid states, the so-called valley CD may arise microscopically from interband transitions between two chiral electronic valley bands of nonzero Berry curvatures at a given $k$ point. However, generally, two sets of $K$ and ${K}^{\ensuremath{'}}$ valleys coexist in the Brillouin zone with opposite chiral selectivities, so that the net CD is zero for the whole material. Here, we demonstrate a giant CD originating from photoexcitation between two chiral Chern flat bands of opposite Chern numbers, namely, the enantiomorphic flat Chern bands. The dissymmetry factor $g$ of such flat CD can reach the theoretical maximum value of 2 with the optimal spin-orbit coupling strength. Based on first-principles calculations, we identify that the Li intercalated bilayer $\ensuremath{\pi}$-conjugated nickel-bis(dithiolene) hosts a set of yin-yang kagome bands with an estimated large $g=0.74$ under magnetic field. Furthermore, based on the flat-CD mechanism, we propose two flatband devices of topological photodetectors and circularly polarized lasers.