Highly efficient carrier doping into the spin-orbit coupled Mott insulator ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ is achieved by low-energy hydrogen ion beam irradiation at low temperature. We demonstrate that heavy doping of hydrogen into a ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ epitaxial thin film induces a large increase in conductivity by band-filling control via electron doping, which is confirmed by Hall effect measurements. The introduction of a large amount of hydrogen and its distribution along the depth direction are clarified by nuclear reaction analysis. The doped interstitial and substitutional hydrogens act as electron donors with minimum perturbation to the lattice, as evidenced by crystal structural analysis and first-principles calculations of the defect formation energy for doped hydrogen. The hydrogen-doping method offers a strategy toward realization of novel quantum phases in strongly correlated spin-orbit entangled systems.

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• Electrical conductivity
• First-principles calculations
• Irradiation effects
• Spin-orbit coupling
• Mott insulators
• Thin films