Alterexciton: Single-Valley Dipolar Excitons in Noncentrosymmetric Monolayer

Dipolar excitons typically emerge in weakly coupled van der Waals heterostructures (vdWHs), where electrons and holes are confined in different layers. However, the tunability of these extrinsic interlayer dipolar excitons under external out-of-plane electric fields is constrained by built-in interfacial electric fields and significant nonradiative processes. Here, we propose a dipolar exciton in monolayer Hf₂SiCO₂, where vertically separated electrons and holes reside in a single layer of several atoms' thickness. The dipolar excitons in the two X valleys, connected by rotoreflection symmetry, possess alternating antiparallel out-of-plane electric dipole moments, which are termed an alterexciton. These dipolar excitons exhibit electrically tunable polarization in a single valley, which further leads to a single-valley excitonic insulator under an increasing electric field. Because of the optical selection rules, the layer-locked valley excitons exhibit linear dichroism and valley-dependent electrical tunability. Furthermore, under linearly polarized light, the Coulomb-bound electrons and holes of the excitons are simultaneously deflected by the Berry curvature in each layer-locked valley, giving rise to the exciton Hall effect. These results not only contribute to the valley-polarized manipulation of dipolar excitons but also facilitate the exploration of single-valley single-photon emitters.