Anisotropic Electronic Properties of $a$-Axis Oriented Sr$_{2}$IrO$_{4}$ Epitaxial Thin Films

We have successfully synthesized $a$-axis oriented Sr$_{2}$IrO$_{4}$ epitaxial thin films on LaSrGaO$_{4}$ (100) substrates by pulsed laser deposition. The tetragonal structure of the substrate allows for the film to grow with compressive strain along both in-plane directions ($b$- and $c$-axes). This results in the $c$-axis of the film being in-plane. We will present the anisotropic structural, electronic, and optical properties of these $a$-axis oriented thin films along both the $b$- and $c$-axes. X-ray diffraction confirms these films are of high quality and are fully strained along the $c$-axis while the $b$-axis undergoes strain relaxation. The $c$-axis resistivity is approximately one order of magnitude larger than that of the \textit{ab}-plane. Optical absorption spectra with E$\bot c$ polarization show both Ir 5$d$ intersite transitions and charge-transfer transitions (O 2$p$ to Ir 5$d)$, while E//$c$ spectra show only the latter. The structural anisotropy created by biaxial strain in $a$-axis-oriented thin-films also changes the electronic structure and gap energy. These $a$-axis-oriented, epitaxial thin-films provide a powerful tool to investigate the highly anisotropic electronic properties of Sr$_{2}$IrO$_{4}$.