Multiferroic‐Enabled Magnetic‐Excitons in 2D Quantum‐Entangled Van der Waals Antiferromagnet NiI2

Matter–light interaction is at the center of diverse research fields from quantum optics to condensed matter physics, opening new fields like laser physics. A magnetic exciton is one such rare example found in magnetic insulators. However, it is relatively rare to observe that external variables control matter‐light interaction. Here, it is reported that the broken inversion symmetry of multiferroicity can act as an external knob enabling magnetic excitons in the van der Waals antiferromagnet NiI2. It is further discovered that this magnetic exciton arises from a transition between Zhang–Rice‐triplet and Zhang–Rice‐singlet fundamentally quantum‐entangled states. This quantum entanglement produces an ultrasharp optical exciton peak at 1.384 eV with a 5 meV linewidth. The work demonstrates that NiI2 is 2D magnetically ordered with an intrinsically quantum‐entangled ground state. Quantum‐entangled magnetic excitons develop in a triangular multiferroic NiI2 when it goes through a second phase transition at low temperatures. In this graph of optical absorption data, the bright regions indicate where two exciton‐related peaks appear below the second magnetic phase transition as indicated by the dashed line. The many‐body calculations show that this exciton arises from a transition between two quantum‐entangled states of Zhang–Rice triplet and Zhang–Rice singlet states.