Mottronics: Technology in transition

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  • Published: Mar 1, 2014
  • Author: David Bradley
  • Channels: X-ray Spectrometry
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Mott under strain

Epitaxial mismatches in the lattices of nickelate ultra-thin films can be used to tune the energetic landscape of Mott materials and thereby control conductor/insulator transitions.

Resonant X-ray scattering has been used to investigate the conducting and insulating phases of ultra-thin films of Mott materials by applying an epitaxial strain to the crystal lattice. This is an important step on the road to a new technology based on the transition from metal to insulator known as "Mottronics".

"Mottronics" is an emerging field named for 1977 Nobel physics laureate Sir Nevill Francis Mott who worked on the electronic structure of magnetic and disordered systems, with a particular focus on amorphous semiconductors. As such, mottronics involves materials, generally metal oxides, that can be induced to switch between an electrically conductive and an insulating phase. If a way to control this phase transition, then Mott materials will hold great promise for a future generation of transistors and memory units for faster computing devices that are also more energy efficient than the chips in current devices.

Ultra-thin

Now, a team of researchers at Berkeley Lab's Advanced Light Source (ALS) have demonstrated how they can control the conducting/insulating phase transition in an ultra-thin film of a Mott material using an epitaxial strain applied to the crystal lattice.

"Our work shows how an epitaxial mismatch in the lattice can be used as a knot to tune the energetic landscape of Mott materials and thereby control conductor/insulator transitions," explains Jian Liu, lead author of the research published in the journal Nature Communications. Liu worked alongside Jak Chakhalian of the University of Arkansas and Mehdi Kargarian, Mikhail Kareev, Ben Gray, Phil Ryan, Alejandro Cruz, Nadeem Tahir, Yi-De Chuang, Jinghua Guo, James Rondinelli, John Freeland and Gregory Fiete.

"Through epitaxial strain, we forced nickelate - NdNiO3 - films containing only a few atomic layers into different phases with dramatically different electronic and magnetic properties. While some of these phases are not obtainable in conventional ways, we were able to produce them in a form that is ready for device development," he explains.

Rare earth nickelates

Nickel-based rare-earth perovskite oxides, or "nickelates" are thought of as an ideal model for the study of Mott materials because they display strongly correlated electron systems that give rise to unique electronic and magnetic properties. The team studied their nickelates on ALS beamline 8.0.1, a high flux undulator beamline that produces X-ray beams optimized for the study of nanoscale materials and strongly correlated physics. The beamline provides the high photon flux and energy range that are critical when dealing with nanoscale samples," Liu explains. "The state-of-the-art Resonant X-ray Scattering endstation has a high-speed, high-sensitivity CCD camera that makes it feasible to find and track diffraction peaks off a thin film that was only six nanometres thick."

The Mott transition between conducting and insulating phases in nickelates is determined by various microscopic interactions, there is a balance between those that favour the conducting phase and those that favour the insulating phase. This energy balance ultimately determines how easily electrons can move between the nickel and oxygen ions. The team found that by applying a sufficient epitaxial strain to alter the space between these ions, it could tune the balance and so control the transition. Moreover, they also showed that strain could be used to control the magnetic properties of the nickelate.

"Magnetism is another hallmark of Mott materials that often goes hand-in-hand with the insulating state and is used to distinguish Mott insulators," adds Liu. "The challenge is that most Mott insulators, including nickelates, are antiferromagnets that macroscopically behave as non-magnetic materials." The researchers were able to track directly the magnetic evolution of the thin films while tuning the transition. "Our findings give us a better understanding of the physics behind the magnetic properties of these nickelate films and point to potential applications for this magnetism in novel Mottronics devices," he adds.

"Our next step would be to build devices based on the different states we can achieve with epitaxial strain," Liu told SpectroscopyNOW. "Meanwhile, given the strain-control revealed in our work, we will select the suitable strain state and synthesize artificial crystals of nickelates on top of it. This will afford a whole dimension to the property engineering and yield nanomaterials that nature would never provide us."

Related Links

Nature Commun, vol 4, Article 2714: "Heterointerface engineered electronic and magnetic phases of NdNiO3 thin films"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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