  Two-dimensional semiconducting systems, such as quantum wells and transition
metal dichalcogenides, are the foundations to investigate low dimensional
light-matter interactions. To date, the study of elementary photoexcitation,
namely the exciton, in 2D semiconductors with intrinsic magnetic order remains
a challenge due to the lack of suitable material platforms. Here, we report an
observation of excitons coupled to zigzag antiferromagnetic order in the
layered antiferromagnetic insulator NiPS3 using both photoluminescence (PL) and
optical reflection spectroscopy. The exciton exhibits a linewidth as narrow as
~350 ueV with near unity linear polarization in the PL spectrum. As the
thicknesses of samples is reduced from five layers to bilayers, the PL
intensity is drastically suppressed and eventually vanishes in monolayers,
consistent with the calculated bandgap being highly indirect for both bilayer
and monolayer. We observed strong linear dichroism (LD) over a broad spectra
range, which shares the same optical anisotropy axis, being locked to the
zigzag direction, as the exciton PL. Both LD and the degree of linear
polarization in the exciton PL decrease as the temperature increases and become
negligible above the Neel temperature. These observations suggest both optical
quantities are probes of the symmetry breaking magnetic order parameter. In
addition, a sharp resonance in the LD spectrum is observed with an energy near
the exciton PL. There exist over ten exciton-A1g phonon bound states on its
high energy side, which likely result from the strong modulation of the
ligand-to-metal charge transfer energy by strong electron-lattice interactions.
Our work establishes NiPS3 as a new 2D platform for exploring magneto-exciton
physics with strong correlations, as well as a building block for 2D
heterostructures for engineering physical phenomena with time reversal symmetry
breaking.
