  Surprising properties of doped Mott insulators are at the heart of many
quantum materials, including transition metal oxides and organic materials. The
key to unraveling complex phenomena observed in these systems lies in
understanding the interplay of spin and charge degrees of freedom. One of the
most debated questions concerns the nature of charge carriers in a background
of fluctuating spins. To shed new light on this problem, we suggest a
simplified model with mixed dimensionality, where holes move through a Mott
insulator unidirectionally while spin exchange interactions are two
dimensional. By studying individual holes in this system, we find direct
evidence for the formation of mesonic bound states of holons and spinons,
connected by a string of displaced spins -- a precursor of the spin-charge
separation obtained in the 1D limit of the model. Our predictions can be tested
using ultracold atoms in a quantum gas microscope, allowing to directly image
spinons and holons, and reveal the short-range hidden string order which we
predict in this model.
