Results to help fulfill the holy grail of 2D materials – ultra-fast electronics

Researchers are discovering a new family of quasiparticles in GrapheneAb based materials.

A group of researchers led by Sir Andre Jim and Dr. Alexei Berdyogene at the University of Manchester have discovered and distinguished a new family of quasiparticles called “brown-Zach fermions” in supernets based on graphene.

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

The study comes after years of successive advances in supernets of graphene and boron nitride that allowed the observation of a fractal pattern known as the Hofstadter butterfly – and today (Friday, November 13, 2020) researchers reported another surprising behavior of particles in such structures under the applied magnetic field.

“It is known that in a zero magnetic field, electrons move in straight paths and if you apply a magnetic field they start to bend and move in circles,” explain Julian Barrier and Dr. Piranavan Kumaravadevil, who conducted the experimental work.

“In a graphene layer that has been aligned with boron nitride, the electrons also start to bend – but if you set the magnetic field at specific values, the electrons move in straight line paths again, as if there is no magnetic field anymore!”

“Such behavior is fundamentally different from textbook physics,” adds Dr. Piranavan Kumaravadville.

“We attribute this remarkable behavior to the formation of new quasiparticles in a high magnetic field,” says Dr. Alexei Berdyogene. “These quasi-particles have their unique properties and high mobility despite an extremely high magnetic field.”

Also posted on Nature Communications, The work describes how electrons behave in a super-fine mesh of graphene with a refined framework for the fractal features of a Hofstadter butterfly. Fundamental improvements in graphene device fabrication and measurement technologies in the past decade have made this work possible.

It can be said that the concept of quasiparticles is one of the most important concepts in condensed matter physics and multi-body quantum systems. It was introduced by theoretical physicist Lev Landau in the 1940s to depict collective effects as “the excitation of a single particle,” as Julian Barrier explains, “They are used in a number of complex systems to account for multiple body effects.”

Until now, the behavior of collective electrons in supernumerary graphene networks has been viewed from the perspective of the Dirac fermion, a quasi-particle with unique photon-like properties (particles without mass), which multiply in high magnetic fields. However, this did not take into account some experimental features, such as additional decay of the states, and did not match the finite mass of the quasiparticle in this case.

The authors suggested that Brown Sach fermions are a family of quasiparticles found in supernets under a high magnetic field. This features a new quantum number that can be directly measured. Interestingly, working at lower temperatures allowed them to raise decay by exchange reactions at extremely low temperatures.

“In the presence of a magnetic field, electrons in graphene begin to spin in quantum orbits. For Brown-Zak fermions, we were able to restore a straight path of tens of micrometers under magnetic fields as high as 16 terra (500,000 times the Earth’s magnetic field). Under certain conditions, Ballistic quasiparticles do not feel any effective magnetic field, ”Dr. Comaravadevil and Dr. Perdyogen explain.

In an electronic system, mobility is defined as the ability of a particle to travel when an electric current is applied. High mobility has always been the holy grail when fabricating 2D systems such as graphene because such materials would introduce additional properties (integer and fractional quantum hall effects), and would potentially allow the creation of high-frequency transistors, which are the components at the core of a computer processor.

“For this study, we prepared very large graphene devices with very high purity,” says Dr. Kumaravadevil. This allowed us to achieve mobility capabilities of several million cm² / V, which means that the particles will travel directly through the entire device without scattering. Importantly, this was not only the case for the classic Dirac fermions in graphene, but also realized the Brown-Zach fermions mentioned in the work.

Brown Zak fermions define new metallic states, which are generic to any super lattice system, not just graphene, and provide a playground for new condensed matter physics problems in other supernets based on 2D materials.

“The results are important, of course, for basic studies in electron transport, but we believe that understanding quasiparticles in new devices with a supernet under high magnetic fields could lead to the development of new electronic devices,” Julian Barrier added.

High portability means that a transistor made with such a device can operate at higher frequencies, which allows a processor made from this material to perform more calculations per unit time, resulting in a faster computer. Typically, applying a magnetic field reduces movement and makes this device unusable in certain applications. The high mobility of Brown-Zak fermions in high magnetic fields opens a new perspective for electronic devices operating under extreme conditions.

Reference: 13 November 2020, Nature Communications.
DOI: 10.1038 / s41467-020-19604-0