Magnetism or no magnetism? The influence of substrates on electronic interactions

A new study at Monash University illustrates how substrates affect strong electronic interactions in two-dimensional metal-organic frameworks.

Materials with strong electronic interactions may have applications in energy-efficient electronics. When these materials are placed on a substrate, their electronic properties are modified by charge transfer, deformation and hybridization.

The study also shows that electric fields and applied stress could be used to ‘turn on’ and off interactive phases such as magnetism, enabling potential applications in energy-efficient electronics.

ACTIVATE AND DEACTIVATE MAGNETISM WITH SUBSTRATES

Strong interactions between electrons in materials give rise to effects such as magnetism and superconductivity. These effects have uses in magnetic memory, spintronics, and quantum computing, making them attractive for emerging technologies.

Last year, another study at Monash found strong electronic interactions in a 2D metal-organic framework. Researchers have found signatures of magnetism in this material. They showed that this magnetism was due to strong interactions that were only present when the non-magnetic components were brought together.

This material was developed on a metal substrate. The substrate was important for the growth and measurement of the material.

Explainer: metallo-organic framework

A crystalline material where organic molecules are joined by metallic atoms. Organometallic frameworks can exhibit many different properties by modifying metal molecules or atoms. Understanding quasiparticle excitations and their interactions is crucial for efforts to control complex materials (such as high-temperature superconductors and topological insulators) that could form the basis of low-energy electronics and computational processing. quantum information.

“We observed this effect when the material was grown on silver, but not when it was grown on copper, although they are very similar,” explains Bernard Field (Monash), co-author of the previous study and lead author of the current study. .

“So that raised the question: why did the material behave so differently on different substrates? »

The researchers simulated the metal-organic structure on many different substrates to determine under what conditions magnetism could emerge.

They also created a simple model that accurately describes physical phenomena in their atomic-scale simulations. This model allowed the team to quickly and easily explore a wider range of systems with precise control of important parameters.

Three key variables were found to determine the effect of substrates on electronic interactions: charge transfer, strain, and substrate hybridization.

  • Load transfer it is when a substrate donates or takes electrons from the 2D material. The effect of interactions was strongest when the material had one free electron per molecule.
  • Stump it is when a substrate stretches or compresses the 2D material. When the material is stretched, electrons find it difficult to move between molecules and atoms, so they are more strongly subjected to local interactions.
  • Hybridization it is when the electronic character of the substrate and the 2D material are mixed due to the coupling between them. Metal substrates often have strong hybridization, which can suppress magnetism. But insulating substrates, such as atomically thin hexagonal boron nitride, have very low hybridization and preserve electronic interactions in the material.

With this understanding of what key variables are, it is possible to envision how to manipulate these variables to control electronic interactions.

The study showed that an electric field could turn magnetism on and off by altering charge transfer.

Electric fields are how existing transistors work. The electrical control of the magnetic phases is vital for the use of these materials in electronic devices.

The study also showed that applied stress could turn magnetism on and off. This could be achieved by using piezoelectric materials. This is also an important consideration for flexible electronics.

“The team continues to study strong interactions in 2D metal-organic frameworks, which provide a rich platform to explore new quantum physics applied to energy-efficient electronic devices,” says corresponding author Prof. Nikhil Medhekar (Monash Materials Science and Engineering Department). , who led the study, “We are investigating more advanced methods to simulate strong interactions between electrons. »

“This work provides quantitative predictions, using various theoretical formalisms, of the electronic properties of low-dimensional nanomaterials over a wide range of substrates and conditions,” says co-author A/Prof Agustin Schiffrin (Monash School of Physics and Astronomy), which conducts experimental research on these materials, “This can guide future experiments in the real world, which is extremely valuable for experimental researchers. »

This study was supported by the Australian Research Council (Centre of Excellence and Future Fellowship programs). Resources for the numerical calculations were provided by the National Computing Infrastructure (NCI) and the Pawsey Supercomputing Center.

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