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New knowledge from old method

As quantum technology makes its inroad, we need to be able to use new materials to capture and tame the promising new opportunities offered by the technology. This means we must be able to examine the interior of exotic quantum materials, and so far this has proved rather difficult. A research team with Danish participation has now found a method.

[Translate to English:] Her ses den lille nano-transistor, som forskningsgruppen har brugt som basis for deres arbejde. (Ill: Philip Hofmann)
[Translate to English:] Her ses den lille nano-transistor, som forskningsgruppen har brugt som basis for deres arbejde. (Ill: Philip Hofmann)

Modern electronics are based switching a power supply in a transistor on and off. This principle stems from the days of Ørsted, when electromagnetism laid the foundation for technological breakthroughs that pushed development forward. But what happens when technology moves into the realms of quantum physics, as it is now?

We do not know much about the materials in the quantum domain, but they may have the wondrous characteristic that they change functionality when they are exposed to different influences. For example, this means that electricity insulating materials without an electrical voltage can change and become conductors when they are exposed to a specific electrical voltage. Some of these effects may be detrimental to the uses of the materials, while others may create new opportunities that can be built on as we move towards the next generation of technology. The challenge is that, basically, we have no idea what is going on.

Researchers from all over the world are trying to look under the surface of the new quantum materials to see how they behave when exposed to different influences – just as in the world of biology when they investigate living creatures in vivo. In this context, an old-fashioned X-ray method has turned out to have new applications. In an article in the prestigious journal, Physical Review Letters, a research group with roots at AU has now demonstrated a way of exploiting these applications.

A well-used hammer
Professor Philip Hofmann from the Department of Physics and Astronomy has headed the Aarhus part of work on the new method. Together with international partners, his group has built a nano-transistor using gold and graphene. They have studied this with a new version of an X-ray method called Angle-resolved photo emission spectroscopy – or ARPES.

Roughly speaking, ARPES can be compared with taking a biopsy of tissue samples or chipping a flake off a geological sample – but here at nano scale. The method sends focused photons onto the surface of a material, thereby chipping an electron loose. The energy level and condition of the electron can then be measured using spectroscopy, which provides information about the material's electronic properties. This "nano biopsy" can demonstrate how electrons move through the graphene layer.

"The X-ray method is a very well-documented approach to examining the electronic structure of materials. You can almost call ARPES a well-used hammer; a commonplace tool for us in the world of research that we’ve been using for years to chip away electrons. The new thing here is that we’ve been able to use ARPES to create a bright spot just a few hundred nanometres in width, and by doing so demonstrate the conductivity of the graphene. This opens up for new approaches and knowledge about quantum materials," explains Philip Hofmann.

So far, ARPES has been used to examine crystal-like materials with stable substructures, but the group has shown that the method is also suitable for investigating materials with functionality, such as graphene exposed to an electrical charge that can potentially lead to changes in the electronic structure.

"It's an exciting result. Not because we’ve demonstrated the ability of graphene to conduct a current and remain stable – we already knew this – but because the new approach to an old X-ray method enables us to carry out measurements in this way. This new approach may prove very useful when we’re developing and investigating new quantum materials, where a charge can fundamentally change the properties of materials," explains Philip Hofmann.

The researchers from Aarhus University are now developing a beamline that will be affiliated with the Aarhus particle accelerator, ASTRID2. Over the next year, it will be possible to start performing measurements on the basis of the new insights about the ARPES method.

The article Accessing the Spectral Function in a Current Carrying Device was written with researchers from Carnegie Mellon University in the USA and Diamond Light Source in England.

Professor Philip Hofmann,
Department of Physics and Astronomy
Aarhus Universitet,
Email: philip@phys.au.dk,
Phone: +45 23382343