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Metallic electronic transport in nickelate heterostructures can be induced and confined to two dimensions (2D) by controlling the structural parameters of the nickel-oxygen planes.

A number of intriguing properties emerge upon the formation of the epitaxial interface between the insulating oxides LaAlO(3) and SrTiO(3). These properties, which include a quasi two-dimensional conducting electron gas, low temperature superconductivity, and magnetism, are not present in the bulk materials, generating a great deal of interest in the fundamental physics of their origins. While it is generally accepted that the novel behavior arises as a result of a combination of electronic and atomic reconstructions and growth-induced defects, the complex interplay between these effects remains unclear. In this report, we review the progress that has been made towards unraveling the complete picture of the SrTiO(3)/LaAlO(3) interface, focusing primarily on present ab initio theoretical work and its relation to the experimental data. In the process, we highlight some key unresolved issues and discuss how they might be addressed by future experimental and theoretical studies.

The discovery of unconventional superconductivity in hole doped NdNiO2, similar to CaCuO2, has received enormous attention. However, different from CaCuO2, RNiO2 (R = Nd, La) has itinerant electrons in the rare-earth spacer layer. Previous studies show that the hybridization between Ni-$${d}_{{x}^{2}-{y}^{2}}$$ d x 2 − y 2 and rare-earth-d orbitals is very weak and thus RNiO2 is still a promising analog of CaCuO2. Here, we perform first-principles calculations to show that the hybridization between Ni-$${d}_{{x}^{2}-{y}^{2}}$$ d x 2 − y 2 orbital and itinerant electrons in RNiO2 is substantially stronger than previously thought. The dominant hybridization comes from an interstitial-s orbital rather than rare-earth-d orbitals, due to a large inter-cell hopping. Because of the hybridization, Ni local moment is screened by itinerant electrons and the critical UNi for long-range magnetic ordering is increased. Our work shows that the electronic structure of RNiO2 is distinct from CaCuO2, implying that the observed superconductivity in infinite-layer nickelates does not emerge from a doped Mott insulator.

We present an ab initio study of the (001) interfaces between two insulating perovskites, the polar LaAlO 3 and the non-polar SrTiO 3 . We observe an insulating-to-metallic transition above a critical LaAlO 3 thickness. We explain that the high conductivity observed at the TiO 2 /LaO interface, and the lack of similar conductivity at the SrO/AlO 2 interface, are inherent in the atomic geometry of the system. A large interfacial hopping matrix element between cations causes the formation of a bound electron state at the TiO 2 /LaO interface. This novel mechanism for the formation of interfacial bound states suggests a robust means for tuning conductivities at various oxide heterointerfaces.A wide variety of novel physical phenomena are observed at the hetero-epitaxial interfaces between perovskite oxides [1], such as high conductivity [3,4], external field doping [5,6], magnetism [7] and supercoductivity [8], all of which are lacking in the bulk constituents.Understanding the rich physics of these systems, and their potential applications, has been the focus of intensive research in the past decade.Recently, the discovery of many novel properties at the (001) epitaxial interface LaAlO 3 /SrTiO 3 (LAO/STO) has boosted considerable research [4,5,6,7,8,9,10,11,12].Since both LAO and STO are perovskite oxides, two types of interfaces can form along the (001) direction: TiO 2 /LaO (n-type) and SrO/AlO 2 (p-type) [4,9]. While experiments have found that the p-type interface is insulating, a high-mobility quasi two-dimensional electron gas (Q2-DEG) is formed at the n-type interface [4] when the LAO thickness exceeds a critical separation [5,10]. The origin of the Q2-DEG is the key to understanding the novelty of the LAO/STO interface, but it is still under heated debate in both theory [13,14,15,16,17,18,19,20,21] and experiment [9,22,23,24,25].Broadly, there are two representative schools of thought. The first suggests an intrinsic mechanism due to the polar discontinuity at the interface [4,9]. Since the nominal charges of the LaO and AlO 2 planes in (001) LAO are +1 and -1 (STO has neutral SrO and TiO 2 layers), there is a macroscropic electric field through the LAO. The voltage across the LAO is approximately proportional to its thickness and diverges with increasing thickness. To avoid the divergence, half an electron(hole) per two-dimensional unit cell must transfer across the n-type(p-type) interface, respectively [4,9]. Nevertheless, this symmetric description can not account for the drastic difference in electronic properties of the two interfaces. The second school of thought posits that extrinsic oxygen vacancies generated by the pulsed laser deposition create the dominating carriers responsible for the conductivity [9,22,23,24].However, both the existence of a lower limit in the carrier density of fully oxidized n-type interfaces [25] and the insulating property of p-type interfaces [4,9] strongly suggest that a more fundamental intrinsic asymmetry exists in the electronic reconstruction [26] between these two...

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We describe a general materials design approach that produces large orbital energy splittings (orbital polarization) in nickelate heterostructures, creating a two-dimensional single-band electronic surface at the Fermi energy. The resulting electronic structure mimics that of the high temperature cuprate superconductors. The two key ingredients are: (i) the construction of atomicscale distortions about the Ni site via charge transfer and internal electric fields, and (ii) the use of three component (tri-component) superlattices to break inversion symmetry. We use ab initio calculations to implement the approach, with experimental verification of the critical structural motif that enables the design to succeed.

The breaking of orbital degeneracy on a transition metal cation and the resulting unequal electronic occupations of these orbitals provide a powerful lever over electron density and spin