The interplay of magnetism and chirality with valley excitons

Most biological molecules are chiral and exist in one of the two mirror-image enantiomers. The so-called homochirality of life is one of the most fundamental mysteries for human beings. Theory and experiments show that the magneto-chiral (MCh) effect albeit small may play a role in the molecular enantioselectivity of prehistoric life by simultaneously breaking both the parity-inversion and time-reversal symmetry. For the first time, we observe an excitonic MCh effect in artificially arranged van der Waals (vdW) crystals with an unprecedented giant MCh anisotropy more than 30 times larger than that in natural materials. Moreover, the observed excitonic MCh effect possesses a spectral splitting that is more than 50 times larger than that induced by the Zeeman effect. Such an excitonic MCh effect creates a link among magnetism, chirality, spin, and excitons, which has long been explored ever since the discovery of the natural optical activity by Arago and magnetically induced optical activity by Faraday in the early 19th century. Furthermore, the electronic nature of spins in two-dimensional vdW semiconductors makes the manipulation of enantioselectivity in magneto-chiral systems attractive for photochemical reactions, asymmetric synthesis, and chiral quantum optics.