Electronic Structure Engineering Through Atomic-Scale Strain Control in Complex Oxide Heterostructures

In complex oxide systems, the coupling of local atomic configurations and electronic degrees of freedom play a fundamental role in understanding exotic phenomena such as formation of charge ordering, metal-insulator transition and colossal magnetoresistance. Atomic-scale thin film syntheses enable to disentangle these competing interactions and tune novel ground states of materials, which do not exist in bulk crystals. Modifying strain by depositing epitaxial thin films on substrates with different lattice spacing results in a precise control of the local physical behavior. Scanning transmission electron microscopy (STEM) combined with electron energy-loss spectroscopy (EELS) not only provides information on the local atomic structure, but also on the site-specific chemical composition and electronic structure of materials and interfaces. Such direct real-space observations are important to unveil the microscopic origins of macroscopic properties in complex oxides and are essential for possible applications in relevant electronic and spintronic devices.<br/><br/>Here, I demonstrate how strain locally alters physical properties in two different complex oxide systems using STEM-EELS. I will first describe our observation of epitaxial LiV₂O₄ thin films grown on SrTiO₃ and MgO substrates. I find that the V-3<i>d</i> electrons redistribute in the lattice and become confined or delocalize depending on the epitaxial strain, hereby generating ordered or disordered patterns. I reveal two competing behaviors of the thin films on the two different substrates, a metallic charge-disordered phase on SrTiO₃ and an insulating charge-ordered phase on MgO. These findings evidence that charge fluctuations can freeze into an insulating “charge-glass”-like state, when epitaxial strain is applied, which relieves the frustration in LiV₂O₄.<br/><br/>The second part focuses on La_0.5Sr_0.5MnO₃-La₂CuO₄ heterostructures grown on three different substrates, SrTiO₃, (LaAlO₃)_0.3-(Sr₂AlTaO₆)_0.7 and LaSrAlO₄ [1]. I find that charge rearrangements in these systems give rise to two different magnetic phases, an interfacial antiferromagnetic layer and an enhanced ferromagnetic metallic region away from the interfaces. Furthermore, the epitaxial strain controls the magnitude of charge redistribution and further influences the macroscopic electronic and magnetic structures in these heterostructures differently from the strain affects reported on single-phase films. These results provide a route for understanding, controlling and designing local novel phases in complex oxides heterostructures [2].<br/><br/>References:<br/>[1] Y.-M. Wu, Y. Suyolcu, G. Kim, G. Christiani, Y. Wang, B. Keimer, G. Logvenov, P.A. van Aken: Atomic-Scale Tuning of the Charge Distribution by Strain Engineering in Oxide Heterostructures, ACS Nano 15 (2021) 16228-16235.<br/>[2] This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 823717 – ESTEEM3.