This review stands in the larger framework of functional materials by focussing on heterostructures of rare-earth nickelates, described by the chemical formula RNiO where R is a trivalent rare-earth R  =  La, Pr, Nd, Sm, …, Lu. Nickelates are characterized by a rich phase diagram of structural and physical properties and serve as a benchmark for the physics of phase transitions in correlated oxides where electron-lattice coupling plays a key role. Much of the recent interest in nickelates concerns heterostructures, that is single layers of thin film, multilayers or superlattices, with the general objective of modulating their physical properties through strain control, confinement or interface effects. We will discuss the extensive studies on nickelate heterostructures as well as outline different approaches to tuning and controlling their physical properties and, finally, review application concepts for future devices.

ABSTRACT. The functional properties of oxide heterostructures ultimately rely on how the electronic and structural mismatches occurring at interfaces are accommodated by the chosen

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...

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In this study, we analyze how the octahedral tilts and rotations of thin films of LaNiO3 and LaAlO3 grown on different substrates, determined using synchrotron X-ray diffraction-measured half-integer Bragg peaks, depend upon the total film thickness. We find a striking difference between films grown on SrTiO3 and LaAlO3 substrates which appears to stem not only from the difference in epitaxial strain state but also from the level of continuity at the heterointerface. In particular, the chemically and structurally discontinuous LaNiO3/SrTiO3 and LaAlO3/SrTiO3 interfaces cause a large variation in the octahedral network as a function of film thickness whereas the rather continuous LaNiO3/LaAlO3 interface seems to allow from just a few unit cells the formation of a stable octahedral pattern corresponding to that expected only given the applied biaxial strain.

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Collective rotations and tilts of oxygen polyhedra play a crucial role in the physical properties of complex oxides such as magnetism and conductivity. Such rotations can be tuned by preparing thin films in which dimensionality, strain, and interface effects come into play. However, little is known of the tilt and rotational distortions in films a few unit cells thick including the question of if coherent tilt patterns survive at all in this ultrathin limit. Here, a series of films of perovskite LaNiO3 is studied and it is shown that the phonon mode related to oxygen octahedral tilts can be followed by Raman spectroscopy down to a film thickness of three pseudocubic perovskite unit cells (∼1.2 nm). To push the limits of resolution to the ultrathin regime, a statistical analysis method is introduced to separate the Raman signals of the film and substrate. Most interestingly, these analyses reveal a pronounced hardening of the tilt vibrational mode in the thinnest films. A comparison between the experimental results, first principles simulations of the atomic structure, and the standing wave model, which accounts for size effects on the phononic properties, reveals that in the ultrathin regime, the Raman spectra are a hybrid entity of both the bulk and surface phononic behavior. These results showcase Raman spectroscopy as a powerful tool to probe the behavior of perovskite films down to the ultrathin limit.

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