  The unique discovery of magnetic exciton in van der Waals antiferromagnet
NiPS3 arises between two quantum many-body states of a Zhang-Rice singlet
excited state and a Zhang-Rice triplet ground state. Simultaneously, the
spectral width of photoluminescence originating from this exciton is
exceedingly narrow as 0.4 meV. These extraordinary properties, including the
extreme coherence of the magnetic exciton in NiPS3, beg many questions. We
studied doping effects using Ni1-xCdxPS3 using two experimental techniques and
theoretical studies. Our experimental results show that the magnetic exciton is
drastically suppressed upon a few % Cd doping. All these happen while the width
of the exciton only gradually increases, and the antiferromagnetic ground state
is robust. These results highlight the lattice uniformity's hidden importance
as a prerequisite for coherent magnetic exciton. Finally, an exciting scenario
emerges: the broken charge transfer forbids the otherwise uniform formation of
the coherent magnetic exciton in (Ni,Cd)PS3.
