Power in patterns

Materials World magazine
,
1 May 2018

A basket-weaving pattern has led to the discovery of a new quantum electronic material for computing and zero-loss energy devices. Ellis Davies reports.  

The Japanese kagome basket-weaving pattern (above) has piqued the interest of physicists for decades because of its network of corner sharing triangles that has symmetries expected to give rise to exotic electronic states – such as the Quantum Hall effect (electrons flowing through a two-dimensional material bend into tight, circular paths and flow along edges without losing energy). Now, collaboration between Massachusetts Institute of Technology (MIT), Harvard University, and Lawrence Berkeley National Laboratory, USA, has developed the first kagome metal – an electrically conducting crystal, made from layers of iron and tin atoms. Each atomic layer is arranged in the repeating pattern of a kagome lattice, and could be used in zero energy loss devices, such as dissipationless (does not lose energy) power lines.

Assistant Professor Joseph Checkelsky of MIT, and co-author of the study, told Materials World, ‘The kagome network itself has large voids, which is not often a stable structure in nature. To overcome this, we synthesised a material that has a kagome network of iron atoms, but fills the voids with another atom – tin.’ The elements were reacted at around 800oC and then quenched in ice water to room temperature in order to freeze the metastable kagome structure.  

Different behaviours

The kagome layers produce strange, quantum-like behaviours when passed over by a current – electrons veer, bending back with the lattice, rather than flowing straight through. ‘The electrons bend in the crystal because of a quantum mechanical effect connected to the kagome structure itself. This veering is evidence that the physics of the material indeed arise from the structure of the lattice,’ said Checkelsky. This behaviour persists at room temperature, or higher, thanks to the iron layer. ‘Iron allows the system to have a robust magnetic order (above room temperature), while tin is a heavy atom that imports a relativistic aspect to electronic motion known as spin-orbit coupling. This could lead to perfect conduction, akin to superconductivity, in future generations of materials.’

Researchers measured the energy spectrum of the crystal using the photoelectric effect – the ejection of electrons from the surface of a metal in response to incident light. The ejected electrons are detected as a function of take-off angle and kinetic energy, and the images are a snapshot of the electronic levels occupied by electrons. It was revealed that electrons flow through the crystal in a manner that would suggest that they have gained a relativistic mass, which can theoretically be explained by the presence of the iron and tin atoms. 

Iron is magnetic, and therefore gives rise to chirality – a molecule is chiral if it is not superimposable on its mirror image – whereas the heavier tin atom possesses a nuclear charge, which produces a large local electric field. When a current passes, it recognises the field as magnetic, not electric, and bends away from it. 

‘Work on future materials to remove other conventional electronic states in similar materials could lead to dissipationless conductors... These kagome metals offer a new materials design pathway to realising new platforms for quantum circuitry,’ said Checkelsky. The team feel that the production process could be scalable due to the commercial availability of iron and tin, but this process would require different synthesis methods, for example in the form of thin films. 

‘Finding other examples of kagome metals is an important direction in establishing that these effects are not simply isolated to one compound, but are potentially part of larger family of materials,’ said Checkelsky. Researchers are now pursuing this to find the varieties of kagome metals that might exist with different properties.