Conductivity and Local Structure of LaNiO₃ Thin Films

Abstract

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...

Counter-thermal flow of holes in high-mobility LaNiO3 thin films

“…Over the entire field range, the Hall response is essentially linear with non-discernible higher-order terms, which supports a dominant single-band in transport. The sign of the Hall coefficient R H is positive suggesting predominantly hole-like charge carriers, in agreement with previous studies 11,31 . Despite the fact that LNO is a metal, we observed an increase in R H by about a factor of three upon cooling, which is shown in Figs.…”

Rare-earth nickelatesRNiO3: thin films and heterostructures

“…LaNiO 3 seems less affected by these limitations with the exception of the formation of cracks at very large thickness (> 300 nm), and therefore appears to be a versatile electrode material. However, the conductive properties of LaNiO 3 can be altered in ultrathin epitaxial films, as previously discussed 240,241 . Finally, we note that LaNiO 3 has the added advantage that it is inexpensive compared to other oxides such as SrRuO 3 .…”

“…38 This behavior may have a largely structural origin. Hardening of the phonon modes associated with oxygen octahedral distortions, concomitant with the dimensionality-induced electronic localization, has recently been reported 143 . In the same study it was found that the film structure differentiates into three different local structures -one found on the surface, one towards the interface with the substrate and the third in the interior layers where the boundary effects are minimal.…”

Negative magnetoresistance in epitaxial films of neodymium nickelate

“…Such a symmetry misfit is typical for the epitaxy of orthorhombic perovskites on square surfaces of versatile substrates [10][11][12][13]. The symmetry misfit relaxes within a few nanometers near the film-substrate interface, as was evidenced by the evolution of tilts towards bulklike patterns in nickelates [11,[14][15][16].…”