Unusual quasiparticles discovered in graphene-based materials

Scientists have discovered a new family of quasiparticles that defy textbook physics. Researchers found the particles, called Brown-Zak fermions, in graphene-based superlattices.

Physicists spotted the particles and their odd behavior -- described Friday in the journal Nature Communications -- after aligning a single layer of graphene with an insulating boron nitride sheet.

Typically, in the absence of a magnetic field, electrons travel in straight lines. When a magnetic field is applied, the paths of electrons start to bend and the particles begin to move in circles.

"In a graphene layer which has been aligned with the boron nitride, electrons also start to bend -- but if you set the magnetic field at specific values, the electrons move in straight line trajectories again, as if there is no magnetic field anymore," study co-author Piranavan Kumaravadivel said in a news release.

"Such behavior is radically different from textbook physics," said Kumaravadivel, a physicist at the University of Manchester in Britain.

Researchers hypothesized that the novel electron behavior is triggered by novel quasiparticles, which form at especially high magnetic field values.

"Those quasiparticles have their own unique properties and exceptionally high mobility despite the extremely high magnetic field," said Alexey Berdyugin, study co-author and Manchester physicist.

For years, scientists have been studying graphene-boron nitride superlattices to better understand -- and take advantage of -- a fractal pattern known as the Hofstadter's butterfly.

The latest discovery has forced physicists to reconsider everything they thought they knew about the fractal phenomenon.

"The concept of quasiparticles is arguably one of the most important in condensed matter physics and quantum many-body systems," said lead author Julien Barrier.

"It was introduced by the theoretical physicist Lev Landau in the 1940s to depict collective effects as a 'one particle excitation.' They are used in a number of complex systems to account for many-body effects," said Barrier, also a physicist at the University of Manchester.

Until now, scientists thought the behavior of electrons inside graphene-boron nitride superlattices were dictated by Dirac fermions, photon-like quasiparticles that proliferate at high magnetic field values.

However, Dirac fermions failed to account for some of the unusual material properties of graphene-boron nitride superlattices.

Researchers suggest Brown-Zak fermions is part of a family of quasiparticles that explain some of these mysterious properties.

In the lab, researchers fabricated extremely large graphene-boron nitride superlattices, offering material's electrons greater mobility.

When scientists supplied the superlattice with extremely high magnetic fields, 500,000 times Earth's magnetic field, they circular-moving electrons began traveling in straight lines. The electrons were able to maintain their straight-line paths for the entire length of the superlattice sheets without scattering.

Researchers suggest such tremendous electron mobility makes graphene-boron nitride superlattices an ideal material for the creation of ultra-high frequency transistors, a key component in computer processors.

Greater electron mobility allows transistors to operate at higher frequencies, boosting the speed and efficiency at which they can power computations.

"The findings are important, of course for fundamental studies in electron transport, but we believe that understanding quasiparticles in novel superlattice devices under high magnetic fields can lead to the development of new electronic devices," said Barrier.

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