Researchers discover new family of quasiparticles in Graph-based materials.
A group of researchers led by Sir Andre Geim and Dr. Alexey Berdyugin from the University of Manchester 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 an insulating boron nitride layer and dramatically changing the properties of the graphene layer.
The study follows years of successive advances in graphene-boron nitride superlattices that enabled the observation of a fractal pattern known as the Hofstadter butterfly – and today (Friday, November 13, 2020) researchers report another highly surprising behavior of particles in such structures under applied magnetic field.
“It is known that electrons move in straight paths in the zero magnetic field and when you apply a magnetic field, they start to bend and move in circles,” explain Julien Barrier and Dr. Piranavan Kumaravadivel who carried out the experimental work.
“In a graphene layer that is aligned with the boron nitride, electrons also begin to bend. However, if you set the magnetic field to certain values, the electrons move in straight paths again as if there were no more magnetic fields!”
“Such behavior is radically different from textbook physics,” adds Dr. Piranavan Kumaravadivel added.
“We attribute this fascinating behavior to the formation of novel quasiparticles in a high magnetic field,” says Dr. Alexey Berdyugin. “These quasiparticles have their own unique properties and an exceptionally high mobility despite the extremely high magnetic field.”
As published in Communication with natureThe thesis describes how electrons behave in a high quality superlattice of graphene with a revised frame for the fractal features of the Hofstadter butterfly. Fundamental improvements in graphene device manufacture 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 in quantum many-body systems. It was introduced by the theoretical physicist Lev Landau in the 1940s to represent collective effects as “single-particle excitation”, “explains Julien Barrier.” They are used in a number of complex systems to account for multi-body effects. ”
So far, the behavior of collective electrons in graphene superlattices has been studied using the Dirac fermion, a quasiparticle with unique properties that resemble photons (particles without mass) that replicate at high magnetic fields. However, this did not take into account some experimental features such as the additional degeneracy of the states, nor did it agree with the finite mass of the quasiparticle in this state.
The authors suggest that ‘Brown-Zak fermions’ are the family of quasiparticles that exist in superlattices under a high magnetic field. This is characterized by a new quantum number that can be measured directly. Interestingly, at lower temperatures, they were able to reverse the degeneracy through exchange interactions at extremely low temperatures.
“When a magnetic field is present, electrons in graphs begin to rotate with quantized orbits. For Brown-Zak fermions, we managed to restore a straight trajectory of tens of micrometers under high magnetic fields of up to 16 T (500,000 times the Earth’s magnetic field). Under certain conditions, the ballistic quasiparticles do not feel an 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 electrical current is applied. High mobilities have long been the holy grail of making 2D systems like graphene, as such materials have additional properties (integer and fractional quantum hall effects) and would potentially allow the creation of ultra-high frequency transistors, the components at the heart of a computer processor.
“For this study, we made particularly large graphene devices with a very high degree of purity,” says Dr. Kumaravadivel. This enabled us to achieve mobilities of several million cm² / Vs, which means that particles would travel directly across the device without scattering. It is important that this was not only the case with classical Dirac fermions in graphs, but also with the Brown-Zak fermions described in the work.
These Brown-Zak fermions define new metal states that are generic to any superlattice system, not just graphene, and provide a playground for new condensed matter physics problems in other 2D material-based superlattices.
Julien Barrier added: “The results are of course important for fundamental studies of 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.”
The high mobility means that a transistor made from such a device can operate at higher frequencies, allowing a processor made from this material to do more calculations per unit of time, resulting in a faster computer. The application of a magnetic field would normally reduce mobility and render such a device unusable for certain applications. The high mobility of Brown Zak fermions at high magnetic fields opens up a new perspective for electronic devices that work under extreme conditions.
Reference: November 13, 2020, Communication with nature.
DOI: 10.1038 / s41467-020-19604-0
