Dynamical conductivity of strongly correlated electron systems at oxide interfaces

Abstract

The Mott metal-insulator transition (MIT) in transition-metal complex oxides results from strong electron-electron interactions and is accompanied by a rich spectrum of phenomena, including magnetic, charge, and orbital ordering, superconductivity, structural distortions, polarons, and very high-density 2-dimensional interface electron liquids. Recent advances in oxide heteroepitaxy allow interface control as a promising new approach to tuning the exotic properties of materials near the quantum critical point, with potential application to technologies including phase-change electronics, high power transistors, and sensors. The dynamical conductivity of oxide heterostructures is measured using a combination of terahertz time-domain spectroscopy, Fourier transform infrared spectroscopy, and dc magnetotransport. The rare-earth nickelates RNiO₃ (R = La, Nd...) exhibit a temperature and bandwidth controlled MIT in bulk. Measurements of the Drude response in epitaxial thin films provide quantification of the strain-dependent mass enhancement in the metallic phase due to strong correlations. Reduction of LaNiO ₃ film thickness leads to additional mass renormalization attributed to structural distortions at the heteroepitaxial interface, and an MIT is observed depending on the interfacing materials in coherent perovskite heterostructures. The rare-earth titanates RTiO₃ exhibit a bandwidth and band filling controlled Mott MIT. Furthermore, the heterointerface between Mott insulating GdTiO₃ and band insulating SrTiO₃ exhibits a 2-dimensional itinerant electron liquid, with extremely high sheet densities of 3 × 10¹⁴ cm^-2. The dynamical conductivity of the interface electrons is analyzed in terms of subband-dependent electron mobility and the established large polaron dynamics in bulk SrTiO₃. Additional confinement of the electron liquids is achieved by decreasing the SrTiO₃ layer thickness, with attendant increase in the dynamical mass. Taking the confinement to its extreme limit, a single (GdO) ⁺ plane in Mott insulating GdTiO₃ is replaced with a (SrO) ⁰ plane. This is equivalent to "delta-doping" the Mott insulator with an extremely high density sheet of holes. The transport and absorption in the resulting two-dimensional insulator are consistent with a simple model of small polaron hopping. A comparison is made to similar features in the conductivity of randomly doped Sr_1-xGd_xTiO₃ films.