Tuning Electronic Structure of Epitaxial Sr$_{2}$IrO$_{4}$ Thin Films via Strain

Recent research on Sr$_{2}$IrO$_{4}$ has shown that the energy scale associated with spin-orbit coupling is comparable to the crystal-field energy and the on-site Coulomb interaction. The strong competition between these fundamental interactions creates the potential for the emergence of novel electronic states. To understand the physics of Sr$_{2}$IrO$_{4}$ and to find a way of tuning its multiple competing interactions, we have investigated the transport, magnetic, and optical properties of $c$-axis oriented Sr$_{2}$IrO$_{4}$ epitaxial thin films grown on various oxide substrates. Under tensile (compressive) strain, increased (decreased) Ir-O-Ir bond-angles are expected to result in increased (decreased) electronic bandwidths. However, the films under various strains have little change in their transport properties. In optical spectroscopic measurements, we have observed that two optical absorption peaks near 0.5 eV and 1.0 eV are shifted to higher (lower) energies under tensile (compressive) strain, indicating that the electronic-correlation energy is affected by in-plane lattice-strain and interlayer-spacing. Our observations strongly suggest that not only the electronic bandwidth, but also the magnitude of the electronic correlation energy can be manipulated by lattice strain, which provides an important insight into the physics driven by the coexistence of strong spin-orbit coupling and electronic correlation.