The mechanism that leads to the high temperature superconductivity in a family of layered copper oxides has been an active area of research since its discovery in 1986. Recently, it has been predicted theoretically that the electronic structure in the high temperature superconducting cuprates can be realized in lanthanum nickelate when it is sandwiched between insulating oxide such as lanthanum aluminate in the so-called superlattices. Both the lanthanum nickelate and lanthanum aluminate layers need to be very thin, just two atomic layers. Proving or disproving this prediction can not only help understand the mechanism of high temperature superconductivity, but potentially lead to discovery of new high temperature superconductors in cuprates, nickelates, and other material systems. The goal of this project is to fabricate the nickelate superlattices by atomic layer-by-layer growth using laser molecular beam epitaxy and measure their electronic structure properties and superconductivity to test the theoretical prediction. Success of the project can significantly advance the knowledge in the areas of strongly correlated transition metal oxides, materials by design, and nanoscale engineering of oxide heterostructures. The project supports a female graduate student for her Ph.D. degree, thus directly broadens the participation of underrepresented groups.
TECHNICAL DETAILS: This EAGER grant focuses on tuning orbital order in nickelate superlattices by atomic layer-by-layer growth using laser molecular beam epitaxy. Recently, it has been predicted theoretically that using reduced dimensionality and epitaxial strain the electronic structure in the high-Tc superconducting cuprates can be realized in lanthanum nickelate by sandwiching it between insulating oxide such as lanthanum aluminate in superlattices where each period contains one unit cell of each materials. Doping could then induce superconductivity. No experimental proof has been reported despite numerous efforts and the validity of the theoretical prediction has been questioned. This project uses a new film deposition technique, laser molecular beam epitaxy from separate oxide targets, to achieve the atomic layer-by-layer growth of the nickelate superlattices. This approach is more appropriate than the growth techniques that have been attempted in tuning the orbital order and inducing superconductivity in the nickelate superlattices. The success of the project can significantly advance the knowledge in the areas of strongly correlated transition metal oxides, materials design, and nanoscale engineering of oxide heterostructures. The project provides multidisciplinary training for a female graduate student, directly broadening the participation of an underrepresented group.
Intellectual merit: The possibility of controlling the properties of materials in the limit of only a few atomic layers would promote a new path in the design of functional materials for next generation of electronic devices. Here the main challenge is to grow materials with atomic layer control with structure as perfect as possible and to predict the properties of the materials in such a small scale. In this work, we used atomic layer-by-layer laser molecular beam epitaxy to grow LaNiO3 thin films of only a few atomic layers in thickness and study their properties such as metal insulator transition and possibly induced superconductivity in LaNiO3 heterostructures. The results significantly impact the understanding of the nature of metal insulator transition in thin LaNiO3 films. The demonstration of atomic layering and stoichiometry control represents an important advance in materials design and nanoscale engineering of oxide heterostructures. Broader Impact: The project has provided multidisciplinary training for a graduate student, Ms. Maryam Golalikhani, as part of her Ph. D. thesis work. This work requires close collaborations with scientists in different fields and groups, an excellent opportunity for Ms. Golalikhani to build teamwork and collaboration skills for her future career as a scientist. She has participated in a program designed to encourage under-represented local high shool students, mainly female African American, to choose science for their future career, serving as a role model to them. Understanding of the metal insulator transition in metal oxides can lead to new sciences and new devices with potentially large impact on the society in general. In this project, we have successfully demonstrated atomic layer by layer growth of LaAlO3 from La2O3 and Al2O3 targets and LaNiO3 from La2O3 and NiO targets. In the work on LaNiO3 films on Al2O3-terminated LaAlO3 substrate, we have found that the deposition of a few unit cells of homoepitaxial LaAlO3 is necessary for the initial LaNiO3 layers to grow with high quality. Using the intensity oscillation of the reflection high energy electron diffraction (RHEED) to monitor and control the growth, we grew the films by alternately putting down one atomic layer at a time. We can thus measure the transport properties and electronic structures of the LaNiO3 films with only a few atomic layers in thickness with nominal terminations of either LaO or NiO2. We found that the LaNiO3 film is metallic at high temperatures even when it is as thin as 2 unit cells with LaO termination. This is the smallest thickness reported to show metallic behavior, owning to the high crystalline quality, cation stoichiometry, and full oxygenation of the films. An oxygen K edge feature in the x-ray absorption spectra is clearly linked to the transition to the insulating phase as well as oxygen vacancies. We conclude that dimensionality and strain are not sufficient to induce the MIT without the contribution of oxygen vacancies even in the best quality LaNiO3 ultrathin films. Dimensionality, strain, crystallinity, and oxygen vacancies are all indispensable ingredients in a true control of the electronic properties of nanoscale strong correlated materials.
