Quantum Coherence of Excitons Strongly Bound to the Magnetic Order

Excitons in NiPS3 have shown very strong coupling to the antiferromagnetic order and exceedingly sharp photoluminescence line width, indicating very long quantum coherence. A long excitonic coherence has been inferred so far but never measured directly. Here, we start by investigating the substructure of the excitonic transition using high magnetic fields and confirm the nature of the different spectral components observed and strong coupling to the magnetic order. We conclude that the subcomponents of the excitonic transition originate from the three equivalent orientations of the Neel vectors. Surprisingly, these transitions are shifted energetically and are independent of each other. We perform four-wave mixing experiments and measure the excitonic dephasing rate at ∼20 ps, which corresponds to a homogeneous line width of <20 μeV. To further investigate the coherence time observed, the radiative lifetime of the excitons is measured. The radiative lifetime is half that of the excitonic coherence and is thought to be the limiting factor in setting the upper limit of the coherence time.