approaches to controlling the MIT have been made, for example, by electric field effects [9] and through optical means. [10] Today, RNOs retain a strong focus, with recent work striving to understand their physics. [11][12][13][14][15] The R = La compound is the only RNO that does not have an MIT in bulk; it is metallic and paramagnetic at all temperatures. LaNiO 3 (LNO) may prove an ideal candidate as a base for engineering functional oxide heterostructures. For instance, it was suggested that specially engineered superlattices, based on single unit cells (u.c.) of LNO, may support superconductivity, [16] and it has been shown that this material is orbitally polarizable in specifically designed heterostructures. [17,18] Necessary to fine-tune the functionalities of LNO is a full understanding of the effects of heterostructuring on an atomic level, and the implications that the local structure, at this scale, has on the electronic properties. A close examination of the thin film structure at the boundaries with the substrate and the vacuum, as well as the effects of reducing the dimensionality on coexistence and, ultimately, competition between these local structures, is required.In reducing dimensionality, three conductivity regimes have previously been observed; thicker metallic films, intermediate thicknesses with a resistivity upturn, and insulating films under the ultrathin limit, which can be 3-6 u.c., depending upon the substrate. [19][20][21] In line with this, photoemission studies found drastic changes to the LNO Fermi surface as the thickness approaches a few u.c., indicating that there is a fundamental change in the electronic structure. [22,23] Here we report an intriguing thickness-dependent transport behavior in high-quality LNO films grown on a (001) LaAlO 3 (LAO) substrate, whereby conductivity is enhanced in films of 6-11 u.c. (2.3-4.3 nm). A maximum conductivity is also observed in ab initio calculations (for a thickness of 6-8 u.c.). In agreement with scanning transmission electron microscopy (STEM), the simulations further indicate that there are three characteristic local structures in the depth of the films. A three-element model of parallel conductors reproduces the thickness-dependent transport behavior well, and implies that conductivity enhancement derives from a struggle for dominance between the local structure of the surface and of the heterointerface.Both LNO and LAO are rhombohedral (R-3c) in bulk. LNO (pseudocubic lattice parameter 3.84 Å) deposited on LAO (pseudocubic lattice parameter 3.79 Å) is compressively strained by −1.3%.A marked conductivity enhancement is reported in 6-11 unit cell LaNiO 3 thin films. A maximal conductivity is also observed in ab initio calculations for films of the same thickness. In agreement with results from state of the art scanning transmission electron microscopy, the calculations also reveal a differentiated film structure comprising characteristic surface, interior, and heterointerface structures. Based on this observation, a three-element para...

The transport properties of CaCuO2/SrTiO3 single interfaces are studied by resistance versus temperature measurements in external magnetic fields. The superconducting anisotropy where and are the superconducting coherence lengths parallel and perpendicular to the interface, respectively, shows values higher than that previously obtained for CaCuO2/SrTiO3 superlattices deposited in the same conditions. The larger anisotropy, observed for the single interfaces, indicates that the charge carriers are confined inside a thin superconducting layer next to the interface rather than spread throughout the whole CaCuO2 block. The activation energy and the irreversibility line confirm this hypothesis, suggesting that quasi two-dimensional transport is dominant in this system. The interpretation of the experimental data in the framework of the Berezinskii–Kosterlitz–Thouless theory confirms that the thickness of the superconducting sheet layer is about 1 nm, corresponding roughly to two CaCuO2 unit cells.

Realization of heterostructures containing multiple two-dimensional electron liquids requires a fine control of the fabrication process. Here, we report a structural and spectroscopy study of LaAlO3/SrTiO3/LaAlO3 trilayers grown on the SrTiO3 substrate by pulsed-laser deposition. Scanning transmission electron microscopy with the help of ab initio calculations reveals that antisite defects associated with oxygen vacancies are primarily present in the SrTiO3 film (STO-f) close to the p-type interface (STO-f/LaAlO3), while oxygen vacancies prevail close to the top n-type interface (LaAlO3/STO-f). At the same interface, misfit dislocations relax the tensile strain of the top LaAlO3 layer. Combining x-ray absorption spectroscopy, x-ray linear dichroism, resonant photoemission spectroscopy, and electron energy loss spectroscopy, we observe that the 3d orbital reconstruction at the interface between LaAlO3 and the SrTiO3 substrate is confined over a few interfacial Ti planes while, at the top n-type interface (LaAlO3/STO-f), the absence of a dichroic signal can be related to the blurring of the interfacial orbital reconstruction due to the heterogeneity of defects.