Scientists Discover New Family of Quasiparticles in Graphene-Based Materials

A group of researchers led by Sir Andre Geim and Dr Alexey Berdyugin of the University of Manchester have discovered and characterized a new family of quasiparticles called “Brown-Zak fermions” in graphene-based superlattices.

The team achieved this breakthrough by aligning the atomic lattice of a graphene layer with that of a boron nitride insulating sheet, radically changing the properties of the graphene sheet.

The study follows years of successive advancements in graphene-boron nitride superlattices that allowed the observation of a fractal pattern known as the Hofstadter butterfly – and today (Friday the 13th November) researchers report another very surprising behavior of particles in such structures under applied magnetic. field.

“It is well known that in a zero magnetic field, the electrons move in rectilinear paths and if you apply a magnetic field they start to bend and go round in circles”, explain Julien Barrier and Dr Piranavan Kumaravadivel, who have carried out the experimental work.

“In a layer of graphene that has been aligned with the boron nitride, the electrons also begin to bend – but if you set the magnetic field to specific values, the electrons again move in a straight line, as if it there was no longer a magnetic field! “

“Such behavior is radically different from the physics of textbooks.” adds Dr Piranavan Kumaravadivel.

“We attribute this fascinating behavior to the formation of new quasiparticles with a high magnetic field,” says Dr Alexey Berdyugin. “These quasiparticles have their own unique properties and exceptionally high mobility despite the extremely high magnetic field.”

As published in Nature communications, the work describes how electrons behave in a very high-quality graphene superlattice with a revised framework for the fractal characteristics of the Hofstadter butterfly. Fundamental improvements in graphene device fabrication and measurement techniques over the past decade have made this work possible.

“The concept of quasiparticles is arguably one of the most important in condensed matter physics and multibody quantum systems. It was introduced by theoretical physicist Lev Landau in the 1940s to describe collective effects as an “excitation of a particle” “, explains Julien Barrière” They are used in a number of complex systems to take into account the effects at several bodies. “

Until now, the behavior of collective electrons in graphene superlattices has been thought of in terms of Dirac’s fermion, a quasi-particle that has unique properties resembling photons (massless particles), which replicate to magnetic fields. high. However, this did not take into account some experimental characteristics, such as the further degeneration of states, nor did it correspond to the finite mass of the quasi-particle in that state.

The authors propose that “Brown-Zak fermions” are the family of quasiparticles existing in superlattices under high magnetic fields. This is characterized by a new quantum number which can be measured directly. Interestingly, working at lower temperatures allowed them to reverse degeneration with exchange interactions at ultra-low temperatures.

“Under the presence of a magnetic field, the electrons in graphene begin to rotate with quantized orbits. For Brown-Zak fermions, we succeeded in restoring a rectilinear trajectory of tens of micrometers under high magnetic fields up to 16T (500,000 times the Earth’s magnetic field). Under specific conditions, ballistic quasiparticles do not feel any effective magnetic field ”, explain Dr Kumaravadivel and Dr Berdyugin.

In an electronic system, mobility is defined as the ability of a particle to move when an electric current is applied. High mobilities have long been the holy grail when manufacturing 2D systems such as graphene because such materials would have additional properties (whole and fractional quantum hall effects), and potentially allow the creation of ultra-high frequency transistors, the components at the heart. a computer processor.

“For this study, we prepared graphene devices that are extra large with a very high level of purity.” says Dr Kumaravadivel. This allowed us to achieve mobilities of several million cm² / Vs, which means that the particles would move directly through the whole apparatus without diffusion. Importantly, this was not only the case for the classical Dirac fermions in graphene, but also realized for the Brown-Zak fermions reported in the work.

These Brown-Zak fermions define new metallic states, which are generic for any superlattice system, not just graphene, and provide a playing field for new problems in condensed matter physics in other superlattices. based on 2D materials.

Julien Barrier added: “The results are important, of course for fundamental studies on electron transport, but we believe that understanding quasiparticles in new superlattice devices under high magnetic fields can lead to the development of new ones. electronic devices. “

The high mobility means that a transistor made from such a device could operate at higher frequencies, allowing a processor made from this material to perform more calculations per unit of time, resulting in a faster computer. The application of a magnetic field would generally reduce mobility and render such a device unusable for certain applications. The high mobilities of Brown-Zak fermions at high magnetic fields open a new perspective for electronic devices operating under extreme conditions.