Abstract
Heteroepitaxy offers a new type of control mechanism for the crystal structure, the electronic correlations, and thus the functional properties of transition-metal oxides. Here, we combine electrical transport measurements, high-resolution scanning transmission electron microscopy (STEM), and density functional theory (DFT) to investigate the evolution of the metal-to-insulator transition (MIT) in NdNiO$_3$ films as a function of film thickness and NdGaO$_3$ substrate crystallographic orientation. We find that for two different substrate facets, orthorhombic (101) and (011), modifications of the NiO$_6$ octahedral network are key for tuning the transition temperature $T_{\text{MIT}}$ over a wide temperature range. A comparison of films of identical thickness reveals that growth on [101]-oriented substrates generally results in a higher $T_{\text{MIT}}$, which can be attributed to an enhanced bond-disproportionation as revealed by the DFT+$U$ calculations, and a tendency of [011]-oriented films to formation of structural defects and stabilization of non-equilibrium phases. Our results provide insights into the structure-property relationship of a correlated electron system and its evolution at microscopic length scales and give new perspectives for the epitaxial control of macroscopic phases in metal-oxide heterostructures.
Highlights
The strong coupling between spin, charge, orbital, and structural degrees of freedom gives rise to exotic ground states in transition-metal oxides (TMOs), such as unusual magnetism, multiferroicity, enhanced thermoelectricity, and high-temperature superconductivity [1]
A comparison of films of identical thickness reveals that growth on [101]-oriented substrates generally results in a higher TMIT, which can be attributed to an enhanced bond disproportionation as revealed by the density functional theory (DFT)+U calculations, and a tendency of [011]-oriented films to formation of structural defects and stabilization of nonequilibrium phases
Within our set of samples, we find the highest value of TMIT = 319 K for the 106 Å thick NdNiO3 film on (101) NdGaO3, which is substantially higher than the TMIT of 189 K of the corresponding film on (011) NdGaO3, and 150 K reported for films on (001) NdGaO3 [30]
Summary
The strong coupling between spin, charge, orbital, and structural degrees of freedom gives rise to exotic ground states in transition-metal oxides (TMOs), such as unusual magnetism, multiferroicity, enhanced thermoelectricity, and high-temperature superconductivity [1]. The prospect of realizing new, technologically accessible and robust functionality is based on a profound understanding of the mechanisms underlying these phenomena. Along these lines, the structureproperty relationship has been considered key in bulk TMOs, yet only few studies have addressed its role in heterostructures. The structural modification at heterointerfaces and their relation to the emergence of novel phases has been investigated in detail [2,3]. This new focus on interfaces is mostly related to tremendous progress in techniques that can obtain detailed structural information about
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