Rare earth nickelates: from the 113 to the 112 phase
Materials Science Seminar
Rare earth nickelates: from the 113 to the 112 phase
Lucia Varbaro
Department of Quantum Matter Physics
University of Geneva, Geneva, Switzerland
Abstract: Rare-earth nickelates (RENiO3) belong to the wider family of perovskite oxides. These materials display a temperature-dependent metal-to-insulator transition (MIT) accompanied by a symmetry-lowering breathing distortion of the NiO6 octahedra units at a characteristic temperature (TMI) that depends on the size of the rare earth cation, RE [1,2]. High-quality solid solutions and superlattices of these compounds can be grown via RF off-axis magnetron sputtering. In this seminar, I will present work on SmₓNd₁₋ₓNiO₃ solid solutions, where we explored the interplay between the MIT and the paramagnetic-to-antiferromagnetic transition as a function of x. Our study demonstrates that films undergoing a first-order resistive phase transition can display decoupled transitions (TMI > TN) [3]. Furthermore, using nickelate superlattices we recently studied the characteristic length scale over which a metallic or an insulating phase can be established and the underlying mechanisms governing it [4,5]. By growing SmNiO3/NdNiO3 we showed that this length scale depends on the interplay between the energy cost of the boundary between metallic and insulating phases and the energy gain of the bulk phases [4]. The phase boundary cost of maintaining this interface can be exploited as a parameter to tune the electronic properties of these materials, specifically alternating metallic layers (La0.3Nd0.7NiO3) and layers displaying an MIT (SmNiO3) within a superlattice, with the aim of pushing a metal into an insulating state by increasing the interfaces density [6]. Finally, I will outline our path toward the 112 infinite-layer nickelate phase, first discovered by D. Li through topotactic reduction of perovskite nickelates using CaH₂ or NaH to selectively removing the apical oxygens and obtain a square planar geometry [7,8]. Following W. Wei's approach at Yale, we use the proposed solid-state reduction method: 2Al + 3NdNiO3 → Al2O3 + 3NdNiO2 to obtain the 112 phase [9, 10]. The films presented were all synthesized via RF off-axis magnetron sputtering, with the aluminum layer sputtered in situ on-axis. Therefore, we leverage our expertise in growing high-quality 113 films and heterostructures to achieve the 112 phase in neodymium and samarium nickelate solid solution thin films on LSAT substrates.
Bio: Lucia Varbaro is a PhD student in the Department of Quantum Matter Physics at the University of Geneva, Switzerland, where she has been part of Prof. Jean-Marc Triscone's group since February 2021. She obtained her bachelor's and master's degrees at the University of Genova, Italy, where she contributed to the European project OXiNEMS, working on the design and characterization of micro-electromechanical sensors for magnetometric measurements in collaboration with universities, research centers, and industry partners. Her doctoral research in Geneva focuses on the growth and characterization of nickelate-based thin films and heterostructures using Radio-Frequency Magnetron Sputtering, employing techniques such as X-ray diffraction, atomic force microscopy, transport measurements, and nanofabrication. Over the past three years, her work has led to multiple publications in journals such as Advanced Electronic Materials and APL Materials and has fostered international collaborations. Her research has been presented through invited and contributed talks at scientific conferences and workshops worldwide, particularly in the field of oxide electronics. More recently, she has been engaged in a new experimental project on superconductivity, specifically investigating the infinite layer phase of rare-earth nickelates. A key factor in achieving these results has been the Tremplin grant, which she was awarded in 2023. This support has allowed her to dedicate more time to her research by reducing teaching responsibilities, enabling her to focus on writing articles, finalizing her thesis, and strengthening international collaborations.
Wednesday, April 2
3 - 4 p.m.
Becton C031
Host: Prof. Charles Ahn