Physics of ultrathin films and heterostructures of rare earth nickelates (1606.09291v1)

Abstract: The electronic structure of transition metal oxides featuring correlated electrons can be rationalized within the Zaanen-Sawatzky-Allen framework. Following a brief description of the present paradigms of electronic behavior, we focus on the physics of rare earth nickelates as an archetype of complexity emerging within the charge transfer regime. The intriguing prospect of realizing the physics of high $T_c$ cuprates through heterostructuring resulted in a massive endeavor to epitaxially stabilize these materials in ultra-thin form. A plethora of new phenomena unfolded in such artificial structures due to the effect of epitaxial strain, quantum confinement, and interfacial charge transfer. Here we review the present status of artificial rare-earth nickelates in an effort to uncover the interconnection between the electronic and magnetic behavior and the underlying crystal structure. We conclude by discussing future directions to disentangle the puzzle regarding the origin of the metal-insulator transition, the role of oxygen holes, and the true nature of the antiferromagnetic spin configuration in the ultra-thin limit.

• The paper reveals that ultrathin rare earth nickelate films exhibit complex metal-insulator transitions driven by intertwined electronic and magnetic ordering per the Zaanen-Sawatzky-Allen framework.
• It employs epitaxial strain and heterostructuring techniques to tune electronic phases and distinguish between metal-insulator and magnetic transition temperatures.
• The study uses DMFT calculations and experimental analyses to uncover novel quantum behaviors, suggesting pathways toward high Tc superconductivity and advanced computing applications.

Physics of Ultrathin Films and Heterostructures of Rare Earth Nickelates: An Analytical Overview

The paper under review discusses the physics underlying ultrathin films and heterostructures of rare earth nickelates, particularly emphasizing their electronic and magnetic properties within the charge transfer regime. The authors utilize the Zaanen-Sawatzky-Allen framework to rationalize the electronic structure of transition metal oxides characterized by correlated electrons.

Key Focus and Results

The research brings to light complex phenomena such as metal-insulator transitions (MIT), charge ordering, and magnetic ordering inherent in rare earth nickelates with the chemical formula $RE$NiO$_3$ where $RE$ represents rare-earth elements like La, Pr, Nd, etc. The investigation into LaNiO$_3$ and other nickelates reveals their underlying complexity and potential for high $T_c$ superconducting properties through strategic heterostructuring.

• The paper reviews the status of artificial rare-earth nickelates, uncovering the interplay between electronic and magnetic behavior driven by a complex crystal structure.
• It provides insights into the physics of the MIT driven by electronic or magnetic ordering.
• Strong numerical results, such as the separation of MIT temperatures ($T_{\mathrm{MIT}}$) from magnetic transition temperatures ($T_N$) due to varying $RE$ ions are documented, highlighting the role of crystallography in these physical properties.
• The paper also explores the effects of epitaxial strain, quantum confinement, and interfacial charge transfer on these materials, showcasing a rich tapestry of discoverable phenomena facilitated by modern thin-film growth techniques.

Emerging Phenomena Through Heteroepitaxy

The authors emphasize the utility of thin films and heterostructures in expanding the understanding and control over these oxide materials, which has been largely facilitated by advances in atomic precision oxide growth methodologies. Epitaxial strain, in particular, emerges as a powerful tool for tuning electronic phases and provides vital insight into novel quantum states achievable in confined dimensions.

• Experimental evidence suggests that strain affects nickelate phases significantly, allowing controlled studies of structure-property relations. For instance, while typically insulating in bulk under compression, films can become metallic, highlighting a vital avenue for experimental exploration.
• This strain-dependent manipulation also offers potential insights into orbital engineering, significant in pursuits of realizing functional similarities to high $T_c$ cuprates.

Theoretical Perspectives and Future Developments

The theoretical framework assumes a quasiparticle viewpoint, tackling electronic configuration constraints and exploring possibilities beyond conventional Mott-Hubbard physics. Recent dynamical mean-field theory (DMFT) calculations validate these materials as functioning within a domain characterized by negative charge-transfer energy parameters—demanding reevaluation of canonical models in explaining ground-state behavior.

The paper speculates that continued exploration of strong electron-lattice interactions will pave the way for applications in neuromorphic computing, synaptic devices, and further pragmatic technology integrations. Researchers are encouraged to explore richer lattice architectures and alternative oxidation states for deeper understanding.

Conclusion

This review is instrumental in setting the groundwork for further exploration into the complex interplay of strain, dimensionality, and emergent quantum phenomena in rare earth nickelates. As methodologies develop and experimental probes deliver more precise data, the role of heterostructures in unveiling new physics in strongly correlated materials continues to offer promising pathways for innovation in condensed matter physics.