National University of Singapore

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From cars on a highway to a viscous fluid like oil, our understanding of electron behaviour is being changed by new research.

Credit: College of Design and Engineering, National University of Singapore

In high school science class, we learned that plugging a cable into an electrical circuit sets off a flow of electrons, powering everything from our lights to our phones.

Traditionally, we’ve understood how electrons behave in metals and semiconductors through a simple model: electrons are imagined as tiny, independent particles, much like cars on an open highway—each one moving freely, without interacting much with the others. It’s a straightforward perspective that has been the foundation of electronics for many years, helping us understand and design the electronic devices that underpin much of modern life.

This traditional view falls short though in the case of some emerging quantum materials such as the ultrathin, and highly conductive material graphene. In these materials, rather than behaving like individual cars on a highway, electrons instead act together in a way that resembles a viscous fluid such as oil.

This finding could be transformative for the future development of a broad range of technologies.

Assistant Professor Denis Bandurin and his team, who are from the Department of Materials Science and Engineering at the College of Design and Engineering at the National University of Singapore, are exploring how quantum materials interact with electromagnetic radiation at the nanoscale to uncover new scientific phenomena and their potential use in developing future technologies.

In a recent study, published in Nature Nanotechnology, the team reported that when graphene is exposed to electromagnetic radiation of terahertz frequencies, electron fluid heats up and its viscosity is drastically reduced, resulting in lower electrical resistance – much like how oil, honey and other viscous fluids flow more easily as they are heated on a stove.

Advancing the frontiers of THz waves detection

Terahertz (THz) waves are a special and technologically challenging part of the electromagnetic spectrum – situated between microwaves and infrared light – that have a vast range of potential applications. Being able to detect THz waves could unlock major advances in technologies.

In communications for example, current Wi-Fi technology operates at several GHz, limiting how much data can be transmitted. THz radiation, with its much higher frequency, could serve as the “carrier frequency” for ultrafast, beyond 5G networks, enabling faster data transfer for Internet of Things (IoT) connected devices, self-driving cars and countless other applications.

In medical imaging and industrial quality control, THz waves can penetrate many materials, making them useful for non-invasive scans. They are also safer than X-rays, providing a highly selective and precise imaging tool.

Going further afield, THz vision enables observational astronomy, allowing scientists to observe distant galaxies and exoplanets that cannot be seen by visible light.

THz radiation therefore offers huge potential. However, until recently, detecting it has been a significant challenge. THz waves are too fast for traditional semiconductor chips to handle and too slow for conventional optoelectronic devices.

The Viscous Electron Bolometer

The study by the NUS team showed that by harnessing the viscosity reduction effect, scientists can create innovative devices that can detect THz waves by sensing the changes in electrical resistance. Indeed, in the current study, Asst Prof Bandurin and his team has developed a new class of electronic device called a viscous electron bolometer.

Representing the first practical, real-world application of viscous electronics – a concept that was once thought to be purely theoretical – these bolometers are able to sense changes in resistance extremely accurately and quickly, operating, in principle, at the pico-second scale. In other words, trillionths of a second.

Understanding and exploiting the way electrons move together as a collective fluid opens the way for us to completely rethink the design of electronic devices. With this in mind, Asst Prof Bandurin and his team are actively working on optimising these viscous electron bolometers for practical applications.

As scientists uncover more secrets in the emerging world of quantum materials, it’s clear that traditional models of electron behaviour are no longer sufficient. By embracing this new understanding of viscous electronics, we could be on the verge of unlocking a new wave of technological possibilities.