Electronic and optical properties of La-doped Sr3Ir2O7 epitaxial thin films

We have investigated structural, transport, and optical properties of tensile strained ${(\mathrm{S}{\mathrm{r}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_x)}₃\mathrm{I}{\mathrm{r}}₂{\mathrm{O}}₇$ ($x=0$, 0.025, 0.05) epitaxial thin films. While high-${T}_{\mathrm{c}}$ superconductivity is predicted theoretically in the system, we have observed that all of the samples remain insulating with finite optical gap energies and Mott variable-range hopping characteristics in transport. Cross-sectional scanning transmission electron microscopy indicates that structural defects such as stacking faults appear in this system. The insulating behavior of the La-doped $\mathrm{S}{\mathrm{r}}₃\mathrm{I}{\mathrm{r}}₂{\mathrm{O}}₇$ thin films is presumably due to disorder-induced localization and ineffective electron doping of La, which brings to light the intriguing difference between epitaxial thin films and bulk single crystals of the iridates.