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...

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.

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.

The interfaces between complex oxides can generate fascinating properties that are not observed in the single compounds. A significant example is the high‐mobility 2‐dimensional electron liquid (2DEL) detected at the interface between two good band‐gap insulators, a LaAlO 3 (LAO) thin film grown epitaxially on (001) TiO 2 ‐terminated SrTiO 3 (STO) single crystal [1]. The 2DEL formation is understood in the framework of the polar catastrophe scenario for which electrons are transferred at the interface in order to minimize the built‐in potential generated by the contact between the polar planes of LAO and the neutral ones of STO. According this model a fraction of Ti 3+ , with 3d 1 configuration, should be stabilized in proximity of the interface. LaAlO 3 /SrTiO 3 bi‐Interfaces, here discussed, are multilayer structures with a STO film and a second LAO thin film subsequently grown on the top of the first LAO thin film. Such system displays three inequivalent interfaces ‐ two of which are conducting: LAO / STO substrate and LAO / STO film, for STO thickness ≥ 8 nm [2,3]. Our work is driven by the effort to understand the 2DEL formation at the LAO / STO film interface. For this purpose bi‐interfaces with thick (12 nm ≈ 30 unit cells (uc)) and thin (6 nm) STO film were investigated and discussed in parallel. High‐angle annular dark‐field (HAADF) imaging as well as electron energy‐loss spectroscopy (EELS) were performed in an aberration corrected Nion UltraSTEM TM Scanning Transmission Electron Microscope (STEM). The possibility to combine HAADF, an incoherent and Z‐sensible technique ideal to investigate distortions and defects, and EELS, a spectroscopy capable to probe valence states with atomic spatial resolution, makes STEM a powerful tool to understand interfaces. Specifically in STO the hybridization between the 3d band of Ti and the 2p of O results in a features‐rich spectroscopy. According the HAADF images collected in STO thick bi‐Interfaces, coherent growth, with no obvious defects or dislocations, was observed at the bottom and the middle interface whereas a periodic network of edge dislocations were identified at the top interface pointing out to a relaxed LAO / STO film and to a strained LAO / STO substrate interface. Ti fine structure corroborates the HAADF observations since evidences of orbital reconstruction i.e. a shift of ≈ 60 meV towards higher energy of the orbital‐field edge L 3 ‐e g , are observed at the LAO / STO substrate and not at the LAO / STO film interface. Generally Ti‐L 2,3 fine structure is known to be a spectroscopic fingerprint of the strain state of the interfaces. Besides strain, roughness and polarity of the interfaces are key features. In order to determine the termination plane sequences, a large energy range (1.9 keV) for the EELS data was used collecting simultaneously all the meaningful edges from Ti‐L 2,3 (at ca. 450 eV) to Sr‐L 2,3 (at ca. 1950 eV). These atomically resolved elemental maps show that the insulating interface(s) is(are) the sharpest, indicating that the cation intermixing may play a role in the response of the system to the occurrence of the 2DEL.