Exploring nature’s dots in theoretical physics

3 May 2019

Scientists have begun investigating the value of moiré patterns in two-dimensional structures

A Hong Kong-based physicist has joined a group of US scientists to explore the inner workings of moiré-patterned structures.

Working alongside other physicists and material scientists, Professor Wang Yao (Croucher Innovation Award 2013), Department of Physics at the University of Hong Kong, is the team’s theoretician who has predicted the exotic excitons in moiré patterns, the structure created when two-dimensional materials are layered in non-perfect alignment, resulting in large-scale interference patterns.

While such materials look crystalline, they have periodic patterns on a wider scale, which scientists can control.

A moiré-patterned two-dimensional structure.

One advantage of moiré patterns is that the imprecise line-up of atoms that results in such patterns is much more commonly found in nature than perfect crystallographic ones, which makes them more readily available. Another is that moiré patterns can be stretched or shifted to change their properties.

In optics, moiré can pattern the coupling properties of photons and excitons to the nanoscale degree. Within a moiré pattern, excitons will naturally rest in the minimum energy region and once the exciton is trapped there, it begins to act like a quantum dot, or a single photon source. Previously, quantum dots could only be created chemically and were not uniform in their shape or properties. However, Yao and his co-researchers realised that a moiré-patterned quantum dot array can uniformly emit photons and be precisely changed when scientists manipulate the moiré pattern.

The team’s findings have been published in Nature. The pathway to publication, though, proved challenging. When Yao and his collaborators first drafted their findings, other scientists questioned whether the team could show that the discovery was not a fluke. Defects were known to emit a similar spectral signature. How did the researchers know that their results were due to the moiré pattern and not a defect?

After going back to the lab, the scientists were later able to prove consistent results in the polarisation of photons, revealing that emission photons were strongly polarised in the same way throughout the moiré pattern. This characteristic, unique to moiré-trapped excitons due to the correlation between light-emission polarisation and trapping location, meant uniform polarity was strong evidence for moiré trapping. Defect excitons would have had random emission polarisation.

Future work with moiré excitons includes identifying the period of a moiré pattern while measuring the photon emissions. “If you can determine the moiré period, then you can more precisely calculate the outputs for comparison with experiments,” Yao said. Experimental tools will need to be developed to understand the full spatial resolution.

Generally, theoretical physicists are not concerned with the day-to-day applications of their work. What excites Yao is the advance made in scientific approach – being at the forefront of a paradigm shift from studying either complete disorder or crystalline structures to looking at moiré patterns, which provide a middle ground. Before, scientists desperately tried to mould different materials into crystalline structures to use them in experiments. Now, thanks to the discoveries of Yao and his fellow scientists, other researchers are no longer ignoring moiré patterns but are instead realising the potential value of these natural structures.

“Engineers will come up with ideas to make use of the discoveries,” Yao said. “Physicists just want to beat the limit.”

Professor Wang Yao is a theoretical physicist working in an interdisciplinary field across condensed matter physics, quantum physics, and optical physics. He received his BSc from Peking University in 2001, and PhD in physics from the University of California, San Diego, in 2006. He joined the University of Hong Kong in 2008 and is a full professor in the Department of Physics. His research interests lie in the physics of spin and valley in solids, with a focus on two-dimensional materials and their heterostructures.

To view Professor Yao’s Croucher profile, please click here.