First Principles Investigation of SrTiO$_3$ - NdNiO$_3$ Interfaces

O perovskites are a popular playground for both fundamental and applied research due to their rich variety of structural and magnetic phase transitions. These properties can be manipulated by interfacing different perovskite systems, where effects like epitaxial strain, mode decay, quantum confinement, magnetic frustration, and polar discontinuities emerge. In this study, we investigate the behaviour of SrTiO (STO) and NdNiO (NNO), two perovskites exhibiting important prototypical phase transitions with decreasing temperature that can be influenced by interfacial effects. STO is a quantum paraelectric in bulk, but becomes ferroelectric under strain, while NNO undergoes a metal-insulator transition (MIT) with decreasing temperature that can also be controlled with strain. Using first principles calculations based on density functional theory, we first determine how each material responds to strain in both [001]- and [111]-oriented systems. We demonstrate that strain can induce ferroelectricity in STO and modulate the MIT in NNO in either orientation. We then build and analyse [001]-oriented NNO-STO interfaced systems, where we observe surprisingly large differences in the STO band offsets depending on the interface termination, even when NNO is metallic and STO paraelectric. In a stoichiometric system, this leads to a sizeable field across STO, which amplifies and pins the polarisation in one direction. We also observe complex polar behaviour in both STO and NNO near the interfaces. Finally, we explore [111]-oriented interfaced systems of NNO and a wide range of substrates, discovering an unexpected polar discontinuity that arises from charge-ordering. This discontinuity offers a new mechanism for ferroelectricity via charge transfer, and combined with other interfacial effects, can result in the pinning of the breathing mode (which we propose amplifies the MIT temperature), the creation of two distinct types of two-dimensional electron gas, and the emergence of polar-metal phases.