We investigated the temperature-dependent evolution of the electronic structure of the $J_{\text{eff}}=\frac{1}{2}$ Mott insulator ${\text{Sr}}_2{\text{IrO}}_4$ using optical spectroscopy. The optical conductivity spectra $\sigma \left(\omega \right)$ of this compound has recently been found to exhibit two $d\text{−}d$ transitions associated with the transition between the $J_{\text{eff}}=\frac{1}{2}$ and $J_{\text{eff}}=\frac{3}{2}$ bands due to the cooperation of the electron correlation and spin-orbit coupling. As the temperature increases, the two peaks show significant changes resulting in a decrease in the Mott gap. The experimental observations are compared with the results of first-principles calculation in consideration of increasing bandwidth. We discuss the effect of the temperature change in the electronic structure of ${\text{Sr}}_2{\text{IrO}}_4$ in terms of local lattice distortion, excitonic effect, electron-phonon coupling, and magnetic ordering.

We report magnetotransport properties of heterointerfaces between the Mott insulator ${\text{LaTiO}}_3$ and the band insulator ${\text{SrTiO}}_3$ in a delta-doping geometry. At low temperatures, we have found a strong nonlinearity in the magnetic field dependence of the Hall resistivity, which can be effectively controlled by varying the temperature and the electric field. We attribute this effect to multichannel conduction of interfacial charges generated by an electronic reconstruction. In particular, the formation of a highly mobile conduction channel revealed by our data is explained by the greatly increased dielectric permeability of ${\text{SrTiO}}_3$ at low temperatures and its electric field dependence reflects the spatial distribution of the quasi-two-dimensional electron gas.

We report that a deionized water leaching and thermal annealing technique can be effective for preparing atomically flat and singly terminated surfaces of single crystalline SrTiO₃ substrates. After a two-step thermal-annealing and deionized-water leaching procedure, topography measured by atomic force microscopy shows the evolution of substrates from a rough to step-terraced surface structure. Lateral force microscopy confirms that the atomically flat surfaces are singly terminated. Moreover, this technique can be used to remove excessive strontium oxide or hydroxide composites segregated on the SrTiO₃ surface. This acid-etchant-free technique facilitates the preparation of atomically aligned SrTiO₃ substrates, which promotes studies on two-dimensional physics of complex oxide interfaces.

We have synthesized epitaxial Sr₂IrO₄ thin-films on various substrates and studied their electronic structure as a function of lattice-strain. Under tensile (compressive) strain, increased (decreased) Ir-O-Ir bond-angle is expected to result in increased (decreased) electronic bandwidth. However, we have observed that the 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 also affected by in-plane lattice-strain. The effective tuning of electronic structure under lattice-modification provides an important insight into the physics driven by the coexisting strong spin-orbit coupling and electronic correlation.

We have found that there is more than one type of conducting carriers generated in ${\text{LaAlO}}_3/{\text{SrTiO}}_3$ heterostructures by comparing the sheet carrier density and mobility from optical transmission spectroscopy with those from dc-transport measurements. When multiple types of carriers exist, optical characterization dominantly reflects the contribution from the high-density carriers whereas dc-transport measurements may exaggerate the contribution of the high-mobility carriers even though they are present at low density. Since the low-temperature mobility determined by dc-transport in the ${\text{LaAlO}}_3/{\text{SrTiO}}_3$ heterostructures is much higher than that extracted by optical method, we attribute the origin of high-mobility transport to the low-density conducting carriers.