  As a hallmark of electronic correlation, spin-charge interplay underlies many
emergent phenomena in doped Mott insulators, such as high-temperature
superconductivity, whereas the half-filled parent state is usually
electronically frozen with an antiferromagnetic order that resists external
control. We report on the observation of a new positive magnetoresistance that
probes the staggered susceptibility of a pseudospin-half square-lattice Mott
insulator built as an artificial SrIrO3/SrTiO3 superlattice. Its size is
particularly large in the high-temperature insulating paramagnetic phase near
the N\'eel transition. This novel magnetoresistance originates from a
collective charge response to the large longitudinal spin fluctuations under a
linear coupling between the external magnetic field and the staggered
magnetization enabled by strong spin-orbit interaction. Our results demonstrate
a magnetic control of the binding energy of the fluctuating particle-hole pairs
in the Slater-Mott crossover regime analogous to the BCS-to-Bose-Einstein
condensation crossover of ultracold-superfluids.
