Wang, Jiaou | Search NASA

DOE OSTI2023

Orbital control of metal-to-insulator transition in high-entropy nickelates

High-entropy materials have emerged as a promising platform for exploring unique electronic and structural properties. In this study, we investigated the orbital control of the metal-to-insulator transition in thin films of high-entropy nickelates, which are composed of five rare-earth elements in equi-atomic concentrations. By manipulating the levels of misfit strain through substrate choice and film thickness, we modulated the electronic properties and examined the intricate interplay between strain, orbital interactions, and the metal-to-insulator transition. Compared to other nickelates thin films like NdNi O 3 and SmNi O 3 , thin films of high-entropy nickelates exhibited remarkable resilience in maintaining control over their transport properties and orbital polarization, even in the presence of considerable A-site disorder. Further, this finding suggests that the electronic properties of A-site disordered high-entropy nickelates are still governed by the electronic bandwidth, primarily influenced by the Ni-O bonding geometry. This study provides valuable insights into the complex interplay among composition, structure, and electronic properties in perovskite oxides. These insights have the potential to guide the development of perovskite oxide materials with tailored electronic properties for a wide range of applications.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Atomically engineered cobaltite layers for robust ferromagnetism

Emergent phenomena at heterointerfaces are directly associated with the bonding geometry of adjacent layers. Effective control of accessible parameters, such as the bond length and bonding angles, offers an elegant method to tailor competing energies of the electronic and magnetic ground states. In this study, we construct unit-thick syntactic layers of cobaltites within a strongly tilted octahedral matrix via atomically precise synthesis. The octahedral tilt patterns of adjacent layers propagate into cobaltites, leading to a continuation of octahedral tilting while maintaining substantial misfit tensile strain. These effects induce severe rumpling within an atomic plane of neighboring layers, further triggering the electronic reconstruction between the splitting orbitals. First-principles calculations reveal that the cobalt ions transit to a higher spin state level upon octahedral tilting, resulting in robust ferromagnetism in ultrathin cobaltites. This work demonstrates a design methodology for fine-tuning the lattice and spin degrees of freedom in correlated quantum heterostructures by exploiting epitaxial geometric engineering.

36 MATERIALS SCIENCE↗