By Claribel Dominguez, UNIGE
Based on article published in Nature Materials
Understanding the mechanisms that control the metal-to-insulator transition in correlated electron systems is one of important challenges in condensed matter physics. Remarkably, little is known about the characteristic lengths scale over which a metallic or insulating region can be established and the physics that sets this length scale. Is this length-scale controlled by propagation of lattice distortions or more subtle interfacial effects? Answering these questions is important both for understanding the fundamental physics of the metal-insulator transition and for obtaining the control that is essential for application in new generations of electronic devices.
In this work, we used experimental and theoretical methods to design and study superlattices of two distinct rare earth nickelate oxides SmNiO3 and NdNiO3, that, in bulk form, show a metal-to-insulator transition at two different temperatures – 400 K and 200 K respectively.
When the layers are thick enough, the two composing materials behave independently with separate metal-insulator transitions, which is not surprising. Interestingly, if the individual layers are made thin, around eight unit-cell thick – or less – then the whole structure behaves like a new unique material with a single metal-insulator transition at an intermediate temperature.
In these atomically-engineered structures, the atoms of the two distinct compounds register on top of each-other perfectly and the different levels of distortion are evolving smoothly at the interfaces. Up until now, this interfacial coupling of structural distortions was believed to uniquely define the electronic properties of the new artificial material.
Here, it could be shown using very advanced scanning transmission electron microscopy analyses and sophisticated calculations that the distortions do indeed evolve smoothly at the interfaces but, crucially, return to “bulk” values over one unit cell only. This means that distortion induced at interfaces cannot explain the unique metal-to-insulator transition observed when the layers are thicker than one or two unit cells, and certainly not up to eight unit cells. Somehow each material knows that it is very close to the interface with its neighbour but without being physically distorted.
So what is controlling this unusual coupling? It is shown that it is the interface between a metallic and an insulating region that is costing energy. If the layers are too thin, they decide to behave identically (both metallic or both insulating) and transition together. This is to avoid the energy cost of having a boundary between a metal and an insulator. If the layers are thick enough, they will “live” their own life and transition at the temperature expected for each material.
This exciting discovery published in Nature Materials, shows that two distinct compounds can “talk“ to each other beyond physically induced distortions. This gives researchers an additional knob to tune and control the properties of artificial electronic structures and pushes forward the field of materials by design.
Reference: Domínguez, C., Georgescu, A.B., Mundet, B. et al. Length scales of interfacial coupling between metal and insulator phases in oxides. Nat. Mater. 19, 1182–1187 (2020). //doi.org/10.1038/s41563-020-0757-x
Image: Scanning transmission electron micrscopy image of superlattice consisting of an alternating sequence of 5 atomic unit cells of NdNiO3 (blue) and 5 atomic unit cells of SmNiO3 (yellow). DFT-simulated structure for the same superlattice has been overlaid.