An artist’s illustration of particles and antiparticles on graphene. Image: Matteo Ceccanti and Simone Cassandra.
An artist’s illustration of particles and antiparticles on graphene. Image: Matteo Ceccanti and Simone Cassandra.

Researchers at the University of Manchester in the UK have succeeded in observing the so-called Schwinger effect, an elusive process that normally only occurs in cosmic events. By applying high currents through specially designed graphene-based devices, the team – based at the National Graphene Institute – succeeded in producing particle-antiparticle pairs from a vacuum.

A vacuum is assumed to be completely empty space, without any matter or elementary particles. However, 70 years ago, Nobel laureate Julian Schwinger predicted that intense electric or magnetic fields can break down the vacuum and spontaneously create elementary particles.

This requires truly cosmic-strength fields such as those around magnetars or created briefly during high-energy collisions of charged nuclei. It has been a long-standing goal of particle physics to probe these theoretical predictions experimentally and some are currently planned for high-energy colliders around the world.

Now, though, the researchers – led by another Nobel laureate Sir Andre Geim, in collaboration with colleagues from the UK, Spain, US and Japan – have used graphene to mimic the Schwinger production of electron and positron pairs.

In a paper in Science, they report specially designed devices made from graphene, including narrow constrictions and superlattices, which allowed them to achieve exceptionally strong electric fields in a simple, table-top setup. Spontaneous production of electron and hole pairs was clearly observed (holes are a solid-state analogue of positrons), and the details of the process agreed well with theoretical predictions.

The researchers also observed another unusual high-energy process that so far has no analogies in particle physics or astrophysics. They filled their simulated vacuum with electrons and accelerated them to the maximum velocity allowed by graphene’s vacuum, which is 1/300 of the speed of light. At this point, something seemingly impossible happened: the electrons appeared to become superluminous, providing an electric current higher than allowed by general rules of quantum condensed matter physics.

The origin of this effect was explained as spontaneous generation of additional charge carriers (holes). The theoretical description of this process provided by the research team is rather different from the Schwinger one for empty space.

“People usually study the electronic properties using tiny electric fields that allows easier analysis and theoretical description. We decided to push the strength of electric fields as much as possible using different experimental tricks not to burn our devices,” explained the paper’s first author Alexey Berduygin from the University of Manchester.

“We just wondered what could happen at this extreme,” added Na Xin, also from the University of Manchester. “To our surprise, it was the Schwinger effect rather than smoke coming out of our set-up.”

“When we first saw the spectacular characteristics of our superlattice devices, we thought ‘wow … it could be some sort of new superconductivity’,” said Roshan Krishna Kumar, another leading contributor from the University of Manchester. “Although the response closely resembles that routinely observed in superconductors, we soon found that the puzzling behaviour was not superconductivity but rather something in the domain of astrophysics and particle physics. It is curious to see such parallels between distant disciplines.”

This research is also important for the development of future electronic devices based on two-dimensional quantum materials. It establishes limits on wiring made from graphene, which was already known for its remarkable ability to sustain ultra-high electric currents.

This story is adapted from material from the University of Manchester, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.