Emerging Phenomena in Ferroeletric and Anti-Ferromagnetic 2D vdW Semiconductors

Materials in dual- or multi-functional space are attractive for a number of reasons. The presence of two distinct functionalities may allow the control or sensing of one degree-of-freedom (DOF) by the other. New and desirable properties may emerge from coupling of different DOFs. Here we present two new 2D vdW semiconductors, CrSBr and NbOI₂. The former is an A-type anti-ferromagnet and latter a ferroelectric. In antiferromagnetic semiconductor CrSBr, we discovered that the excitonic transition is strongly coupled to the magnetic order and this coupling can be attributed to the spin-dependent interlayer electron hopping integral. This coupling has allowed the sensitive detection of magnons in the 0.1 meV energy range by excitons in the 1 eV energy range. In the ferroelectric semiconductor NbOI₂, we discovered giant optical rectification and THz emission, with efficiency more than one order-magnitude larger than current state-of-the-art material, ZnTe. We show that electronic transition in this semiconductor is strongly coupled to the ferroelectric order. We also present experimental evidence for the coherent ferron, i.e., the collective oscillation of the ferroelectric order.