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University of Manchester researchers, led by Sir Andre Geim and Dr Alexey Berdyugin, discovered and characterized a new family of quasiparticles called “Brown-Zak fermions” in graphene-based super lattices. This was achieved by aligning the atomic lattice of a graphene layer to that of a boron nitride insulating sheet, drastically changing the properties of the graphene sheet.
The study follows years of subsequent advances in graphene-boron nitride superlattices that previously allowed for the observation of a fractal pattern known as the Hofstadter butterfly – and now, with this current work, the researchers report another very surprising behavior of the particles. in such structures under applied magnetic field.
“It is known that in a zero magnetic field, electrons move in straight paths and if a magnetic field is applied they begin to bend and move in circles,” explain Julien Barrier and Dr Piranavan Kumaravadivel, who conducted the experimental work.
“In a layer of graphene that has been aligned with boron nitride, the electrons also start to bend – but if you set the magnetic field to specific values, the electrons again move in straight lines, as if there were none. more magnetic field! “
“This behavior is radically different from textbook physics.” adds Dr. Piranavan Kumaravadivel.
“We attribute this fascinating behavior to the formation of new high magnetic field quasiparticles,” says Dr. Alexey Berdyugin. “Those quasiparticles have their own unique properties and exceptionally high mobility despite the extremely high magnetic field.”
The work describes how electrons behave in a very high quality graphene super lattice with a revised structure 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 undoubtedly one of the most important in condensed matter physics and quantum many-body systems. It was introduced by theoretical physicist Lev Landau in the 1940s to describe collective effects as an ‘excitation of a particle’ “, explains Julien Barriera” They are used in a number of complex systems to account for multi-body effects. “
Until now, the behavior of collective electrons in graphene superlattices was thought of in terms of Dirac fermion, a quasiparticle that has unique properties similar to photons (massless particles), which replicate at high magnetic fields. However, this did not take into account some experimental features, such as further degeneration of states, nor did it correspond to the finite mass of the quasiparticle in this state.
The authors propose that “Brown-Zak fermions” are the family of quasiparticles existing in high magnetic field super lattices. This is characterized by a new quantum number that can be directly measured. Interestingly, working at lower temperatures allowed them to lift degeneration with exchange interactions at ultra-low temperatures.
“In the presence of a magnetic field, electrons in graphene start spinning with quantized orbits. For Brown-Zak fermions, we were able to restore a straight trajectory of tens of micrometers under magnetic fields as high as 16T (500,000 times the magnetic field). under specific conditions, ballistic quasiparticles do not perceive any effective magnetic fields, “explain Dr. Kumaravadivel and Dr. Berdyugin.
In an electronic system, mobility is defined as the ability of a particle to travel upon application of an electric current. High mobilities have long been the holy grail when fabricating 2D systems such as graphene because such materials would exhibit additional properties (whole and fractional quantum hall effects) and potentially allow the creation of ultra-high frequency transistors, the core components of a computer processor.
“For this study we have prepared large graphene devices with a very high level of purity.” says Dr. Kumaravadivel. This allowed us to reach a mobility of several million cm² / Vs, which means that the particles would travel straight through the entire device without dispersing. Importantly, this was not only the case with the classical Dirac fermions in graphene, but also made for the Brown-Zak fermions reported in the work.
These Brown-Zak fermions define new metal states, which are generic to any super lattice system, not just graphene, and offers a playground for new condensed matter physics problems into other material-based 2D super lattices.
Julien Barrier added: “The results are important, obviously for fundamental studies on electron transport, but we believe that understanding quasiparticles in new super lattice devices under high magnetic fields can lead to the development of new electronic devices.”
High mobility means that a transistor made with such a device could operate at higher frequencies, allowing a processor made with this material to perform more calculations per unit of time, resulting in a faster computer. Applying a magnetic field would usually reduce mobility and render this device unusable for certain applications. The high mobility of Brown-Zak fermions at high magnetic fields opens a new perspective for electronic devices operating in extreme conditions.
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