Crossover between Mott and Efros-Shklovskii variable-range hopping in Sr₂IrO₄ epitaxial thin films by misfit strain and isovalent doping

We report three-dimensional Mott variable-range hopping (3D Mott-VRH) transport of Sr₂IrO₄ epitaxial thin films, with a transition to Efros-Shklovskii variable-range hopping (ES-VRH) with increased misfit strain or isovalent doping. We have observed that the characteristic temperature of 3D Mott-VRH transport decreases under increased misfit strain, implying that the density of states near the Fermi energy is reconstructed. With further increased misfit strain (or doping with Ca or Ba ions), a crossover from the 3D Mott-VRH to ES-VRH transport takes place due to increased carrier localization by disorder, opening a Coulomb gap by increasing the effective electron-correlation. The results of magnetoresistance measurements also confirm that the disorder caused by misfit strain or isovalent doping plays an important role in the electronic transport of these Sr₂IrO₄ thin films. Our experimental observations propose that subtle external stimuli such as structural modifications can modulate the electronic properties of the relativistic Mott insulator, suggesting an unprecedented pathway for electronic device applications.