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Resonant Inelastic X-ray Scattering (RIXS) experiments carried out on Diamond's I21 beamline were crucial in the discovery of a new exciton in the 2D van der Waals antiferromagnetic material NiPS3. This exciting fundamental research, recently published in Nature1, could ultimately lead to new developments in optical data transmission and quantum computing.
Excitons transport energy and emit light
First proposed by Soviet physicist Yakov Frenkel in 1931, an exciton is a combination of an electron and an electron-hole (an empty electron state in an atom's valence band). An exciton is free to move through a non-metallic crystal, and transports energy, but carries no electrical charge. When the electron recombines with the hole, the energy of the exciton is released as a photon of light. Excitons allow conversions between heat, light and work, and find applications in devices such as photovoltaics and laser diodes.
Shining light onto a semiconductor crystal can produce excitons, which could - theoretically - be used to speed-up data transmission. At the moment, transistors are needed to convert data signals into light for transmission down optical fibres. Data transmission would be faster without that conversion step. But if we want to use excitons for data transmission, we need to understand them a lot better.
We're one step closer to that goal, thanks to work undertaken by a team led by Professor Je-Geun Park of Seoul National University, and in which Diamond's Resonant Inelastic X-ray Scattering (RIXS) beamline (I21) played a crucial role. The researchers found a new type of exciton in NiPS₃, one of a recently-discovered class of antiferromagnetic van der Waals materials. They demonstrated that NiPS₃ hosts an entirely different magnetic exciton state from the conventional excitons known to date.
On the trail of a new kind of exciton
The first hints of the new exciton arose in 2016, in photoluminescence experiments carried out by Professor Hyeonsik Cheong of Sogang University. More data was collected soon afterwards by Prof. Jae Hoon Kim of Yonsei University, in optical absorption experiments. Prof. Park's group reported the first realisation of exact 2D magnetic van der Waals materials using NiPS₃ in 2016. When Prof. Park discussed this work at a scientific meeting, Diamond's Physical Sciences Director, Dr Laurent Chapon, suggested that he consider running an experiment on the new I21 beamline.
I21 is a Resonant Inelastic X-ray Scattering (RIXS) beamline, and at that time was approaching completion. Prof. Park was allocated two days of beamtime during the beamline's commissioning phase. If promising data could be collected during those two days, then more beamtime could be made available to the project.
Prof. Park, said:
Collecting RIXS data in a short period of time, and when we were working on an entirely new beamline, there's no question of arriving fully prepared. During our experiments, I21 required minor modifications, and new code had to be written.
The experiment could not have been a success without a very dedicated local contact, and principal beamline scientist Dr Kejin Zhou was indispensable during that first session. It was a pleasure working with him.
That short session did produce promising data, and the team carried out further experiments during two more sessions. The results they collected at Diamond were critical to the success of the project, not only confirming the existence of the exciton but also providing a wealth of information over a wide momentum-energy space. This was is extremely useful to build up a theoretical model for the experimental observations.
The next stage of the research was for Dr Beom Kim and Professor Young-Woo Son of the Korea Institute for Advanced Study to create a theoretical model of the exciton. They explored 1,500,000 quantum states - a Herculean computational effort. Combining their theoretical results with the RIXS data gives a full understanding of the new exciton. The team could be confident that what they had discovered was a genuine quantum exciton state.
Prof. Parks, said:
For a material to host such a novel state of an exciton physics, it requires a direct bandgap and most importantly, magnetic order with strong quantum correlation. These conditions are satisfied in the magnetic van der Waals material NiPS3, an intrinsically correlated system.
Prof. Parks is continuing to collaborate with Kejin Zhou, although plans to carry out further experiments at Diamond have been put on hold due to the COVID-19 pandemic. Having examined a bulk sample of NiPS3, he would like to carry out RIXS experiments on a very thin layer and other related materials.
While it is too early to predict future applications for this novel exciton, there is a tantalising possibility that it could be used for quantum computing. We see the benefits of standard computing technology in our everyday lives, but there are some problems it will never be able to tackle. These are of a size and complexity that would require more computational power than currently exists on Earth. Quantum computing works differently, leveraging quantum mechanical phenomena to manipulate information in quantum bits (qubits). However, there are few quantum sources. If this exciton can be conclusively proven to be one (and there are still some hurdles to be overcome), then it could open a door to more widespread quantum computing.
To find out more about the I21 beamline, or to discuss potential applications, please contact Principal Beamline Scientist, Kejin Zhou.
1 Kang S et al. Coherent many-body exciton in van der Waals antiferromagnet NiPS3. Nature 583, 785-789 (2020). DOI: 10.1038/s41586-020-2520-5
Read the full Nature publication here.
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