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Manchester group discover new family of quasiparticles in graphene-based materials: Findings to help achieve Holy Grail of 2D materials – superfast electronic devices

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Home > Press > Manchester group discover new family of quasiparticles in graphene-based materials: Findings to help achieve Holy Grail of 2D materials – superfast electronic devices

Abstract:
A group of researchers led by Sir Andre Geim and Dr Alexey Berdyugin at The University of Manchester have discovered and characterised a new family of quasiparticles named ‘Brown-Zak fermions’ in graphene-based superlattices.

Manchester group discover new family of quasiparticles in graphene-based materials: Findings to help achieve Holy Grail of 2D materials – superfast electronic devices


Manchester, UK | Posted on November 13th, 2020

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

The study follows years of successive advances in graphene-boron nitride superlattices which allowed the observation of a fractal pattern known as the Hofstadter’s butterfly – and today (Friday, November 13) the researchers report another highly surprising behaviour of particles in such structures under applied magnetic field.

“It is well known, that in zero magnetic field, electrons move in straight trajectories and if 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 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!”

“Such behaviour is radically different from textbook physics.” adds Dr Piranavan Kumaravadivel.

“We attribute this fascinating behaviour to the formation of novel quasiparticles at high magnetic field,” says Dr Alexey Berdyugin. “Those quasiparticles have their own unique properties and exceptionally high mobility despite the extremely high magnetic field.”

As published in Nature Communications (doi: 10.1038/s41467-020-19604-0), the work describes how electrons behave in an ultra-high-quality superlattice of graphene with a revised framework for the fractal features of the Hofstadter’s butterfly. Fundamental improvements in graphene device fabrication and measurement techniques in the past decade have made this work possible.

“The concept of quasiparticles is arguably one of the most important in condensed matter physics and quantum many-body systems. It was introduced by the theoretical physicist Lev Landau in the 1940s to depict collective effects as a ‘one particle excitation’,” explains Julien Barrier “They are used in a number of complex systems to account for many-body effects.”

Until now, the behaviour of collective electrons in graphene superlattices were thought in terms of the Dirac fermion, a quasiparticle that has unique properties resembling photons (particles with no mass), that replicate at high magnetic fields. However, this did not account for some experimental features, like the additional degeneracy of the states, nor did it match the finite mass of the quasiparticle in this state.

The authors propose ‘Brown-Zak fermions’ to be the family of quasiparticles existing in superlattices under high magnetic field. This is characterised by a new quantum number that can directly be measured. Interestingly, working at lower temperatures allowed them to lift the degeneracy with exchange interactions at ultra-low temperatures.

“Under the presence of a magnetic field, electrons in graphene start rotating with quantised orbits. For Brown-Zak fermions, we managed to restore a straight trajectory of tens of micrometres under high magnetic fields up to 16T (500,000 times earth’s magnetic field). Under specific conditions, the ballistic quasiparticles feel no effective magnetic field,” explain Dr Kumaravadivel and Dr Berdyugin.

In an electronic system, the mobility is defined as the capacity for a particle to travel upon the application of an electrical current. High mobilities have long been the Holy Grail when fabricating 2D systems such as graphene because such materials would present additional properties (integer and fractional quantum hall effects), and potentially allow the creation of ultra-high frequency transistors, the components at the heart of 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 millions of cm²/Vs, which means particles would travel straight across the entire device without scattering. Importantly, this was not only the case for classical Dirac fermions in graphene, but also realised for the Brown-Zak fermions reported in the work.

These Brown-Zak fermions define new metallic states, that are generic to any superlattice system, not just graphene and offers a playground for new condensed matter physics problems in other 2D material based superlattices.

Julien Barrier added “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.”

The high mobility means that a transistor made from such a device could operate at higher frequencies, allowing a processor made out of this material to perform more calculations per unit of time, resulting in a faster computer. Applying a magnetic field would usually scale down the mobility and make 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.

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Contacts:
James Tallentire
44-075-322-82824

@UoMNews

Copyright © University of Manchester

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Scientists synthetize new material for high-performance supercapacitors

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Home > Press > Scientists synthetize new material for high-performance supercapacitors

Photo: modified rGO supercapacitor electrodes
Photo: modified rGO supercapacitor electrodes

Abstract:
Scientists of Tomsk Polytechnic University jointly with colleagues from the University of Lille (Lille, France) synthetized a new material based on reduced graphene oxide (rGO) for supercapacitors, energy storage devices. The rGO modification method with the use of organic molecules, derivatives of hypervalent iodine, allowed obtaining a material that stores 1.7 times more electrical energy. The research findings are published in Electrochimica Acta academic journal (IF: 6,215; Q1).

Scientists synthetize new material for high-performance supercapacitors


Tomsk, Russia | Posted on January 19th, 2021

A supercapacitor is an electrochemical device for storage and release of electric charge. Unlike batteries, they store and release energy several times faster and do not contain lithium.

A supercapacitor is an element with two electrodes separated by an organic or inorganic electrolyte. The electrodes are coated with an electric charge accumulating material. The modern trend in science is to use various materials based on graphene, one of the thinnest and most durable materials known to man. The researchers of TPU and the University of Lille used reduced graphene oxide (rGO), a cheap and available material.

“Despite their potential, supercapacitors are not wide-spread yet. For further development of the technology, it is required to enhance the efficiency of supercapacitors. One of the key challenges here is to increase the energy capacity.

It can be achieved by expanding the surface area of an energy storage material, rGO in this particular case. We found a simple and quite fast method. We used exceptionally organic molecules under mild conditions and did not use expensive and toxic metals,” Pavel Postnikov, Associate Professor of TPU Research School of Chemistry and Applied Biomedical Science and the research supervisor says.

Reduced graphene oxide in a powder form is deposited on electrodes. As a result, the electrode becomes coated with hundreds of nanoscale layers of the substance. The layers tend to agglomerate, in other words, to sinter. To expand the surface area of a material, the interlayer spacing should be increased.

“For this purpose, we modified rGO with organic molecules, which resulted in the interlayer spacing increase. Insignificant differences in interlayer spacing allowed increasing energy capacity of the material by 1.7 times. That is, 1 g of the new material can store 1.7 times more energy in comparison with a pristine reduced graphene oxide,” Elizaveta Sviridova, Junior Research Fellow of TPU Research School of Chemistry and Applied Biomedical Sciences and one of the authors of the article explains.

The reaction proceeded through the formation of active arynes from iodonium salts. They kindle scientists` interest due to their property to form a single layer of new organic groups on material surfaces. The TPU researchers have been developing the chemistry of iodonium salts for many years.

“The modification reaction proceeds under mild conditions by simply mixing the solution of iodonium salt with reduced graphene oxide. If we compare it with other methods of reduced graphene oxide functionalization, we have achieved the highest indicators of material energy capacity increase,” Elizaveta Sviridova says.

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The research work was conducted with the support of the Russian Science Foundation.

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Contacts:
Alina Borovskaia
7-923-419-5528

@TPUnews_en

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Researchers guide a single ion through a Bose Einstein condensate

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Jan 20, 2021 (Nanowerk News) Transport processes are ubiquitous in nature but still raise many questions. The research team around Florian Meinert from the 5th Institute of Physics at the University of Stuttgart has now developed a new method that allows them to observe a single charged particle on its path through a dense cloud of ultracold atoms. The results were published in Physical Review Letters (“Transport of a single cold ion immersed in a Bose-Einstein condensate”) and are subject in a Viewpoint of the accompanying popular science journal Physics (“Tracking a Single Ion in an Ultracold Gas”). Artistical visualization of the trajectory of a positively charged ion (yellow) through the BEC (green) Artistical visualization of the trajectory of a positively charged ion (yellow) through the BEC (green). (Image: Celina Brandes, University of Stuttgart) Meinert‘s team uses a so called Bose Einstein condensate (BEC) for their experiments. This exotic state of matter consists of a dense cloud of ultracold atoms. By means of sophisticated laser excitation, the researchers create a single Rydberg atom within the gas. In this giant atom the electron is a thousand times further away from the nucleus than in the ground state and thus only very weakly bound to the core. With a specially designed sequence of electric field pulses, the researchers snatch the electron away from the atom. The formerly neutral atom turns into a positively charged ion that remains nearly at rest despite the process of detaching the electron. In a next step, the researchers use precise electric fields to pull the ion in a controlled way through the dense cloud of atoms in the BEC. The ion picks up speed in the electric field, collides on its way with other atoms, slows down and is accelerated again by the electric field. The interplay between acceleration and deceleration by collisions leads to a constant motion of the ion through the BEC. “This new approach allows us to measure the mobility of a single ion in a Bose Einstein condensate for the very first time,” Thomas Dieterle, PhD student on the experiment, is pleased. The researchers’ next goal is to observe collisions between a single ion and atoms at even lower temperatures, where quantum mechanics instead of classical mechanics dictates the processes. “In future, our newly created model system – the transport of a single ion – will allow for a better understanding of more complex transport processes that are relevant in many-body systems, e.g. in certain solids or in superconductors,” Meinert is sure. These measurements are also an important step on the way to investigate exotic quasi-particles, so-called polarons, which can arise through interaction between atoms and ions. The neighboring lab at the institute already works on an ion microscope that will allow to directly observe collisions between atoms and ions. While an electron microscope uses negatively charged particles to create an image, this is what happens in an ion microscope with positively charged ions. Electrostatic lenses deflect ions similar to light rays in a classical optical microscope. The work was created in the Center for Integrated Quantum Science and Technology IQST, a consortium of the universities of Stuttgart and Ulm and the Max Planck Institute for Solid State Research in Stuttgart. The aim of the center is to promote synergies between physics and related natural and engineering sciences and to represent quantum science from the basics to technological applications. Researchers and practitioners from the fields of physics, chemistry, biology, mathematics, and engineering science at IQST investigate the world of quanta in its entirety and in some cases cooperate directly with the industry.

Source: https://www.nanowerk.com/nanotechnology-news2/newsid=57061.php

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Storing information with light: photo-ferroelectric materials

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Jan 20, 2021 (Nanowerk News) Can you imagine controlling the properties of a material by just shining light on it? We are used to see that the temperature of materials increases when exposed to the sun. But light may also have subtler effects. Indeed, light photons can create pairs of free charge carriers in otherwise insulating materials. This is the basic principle of the photovoltaic panels we use to harvest electrical energy from sun. In a new twist, a light-induced change of materials’ properties could be used in memory devices, allowing more efficient storage of information and faster access and computing. This, in fact, is one of our society’s current challenge: being able to develop high-performance commercially available electronic devices which are, at the same time, energy efficient. Smaller electronic devices having lower energy consumption and high performance and versatility. Now, researchers from the Institute of Materials Science of Barcelona (ICMAB, CSIC) have studied photoresponsive ferroelectric materials integrated in devices exploiting nanotechnologies and quantum effects. Memory elements have been engineered to store non-volatile information in distinct resistance states (ON/OFF). It has been discovered that, when properly designed, their electrical resistance can be modulated by pulsed light. This means that they can switch from a low-resistance to a high-resistance state just by the application of light pulses. “Materials that show changes of resistance under illumination are abundant, although the effect is typically volatile and the material recovers its initial state after some dwell time” says ICMAB researcher Ignasi Fina, co-author of the study (Nature Communications, “Non-volatile optical switch of resistance in photoferroelectric tunnel junctions”). “For devices to be used in computing and data storage, non-volatile optical control of electrical resistance is of potential interest” and adds “for non-volatile, we mean that the information can be retained stored in the device, even when the power is off”. A photon reverses the binary 0/1 state of a memory device A photon reverses the binary 0/1 state of a memory device. (Image: ICMAB) Currently two different devices are required to use optical signals for non-volatile data storage: optoelectronic sensors and memory devices. The ICMAB study features these properties in one single material able to modulate its resistance by pulsed light: a photo-ferroelectric material. Ferroelectric materials have electrically switchable spontaneous non-volatile electric polarization. In ferroelectric ultrathin films of such material sandwiched between appropriate metals, a quantum mechanical phenomenon effect appears called the tunneling current. This effect allows a charge current flow across the ferroelectric layer, which is genuinely insulating, in an amount that depends on the direction of its polarization. In the explored devices, first an electric field is used once to write the ON/OFF states, and it is combined with the optical stimulus to promote the ON/OFF change of states, and reversibly modulate the resistance (from high to low, and vice versa). These devices are energy efficient for two main reasons: firstly, the energy consumption is reduced when the memory state is written, as it does not need any charge current flow. Secondly, as the information is stored in a non-volatile manner, the state is preserved and there is no need to refresh the information (re-writing) as continuously done in current RAMs memories of all computers, for example. The observed optical switch is not restricted to the studied materials and, thus, opens a path towards further investigations on this phenomenon. As for future applications, Ignasi Fina envisions the following: “The studied devices combine light sensor and memory functions. In addition, as shown in the study, the device behaves like a memristor. A memristor is a device that can display multiple resistance states according to the stimulus it has received, and is one of the basic devices for the development of neuromorphic computing systems. Therefore, the developed device opens a path to be explored in relation to its integration into neuromorphic vision systems, where the system learns to recognize images.”

Source: https://www.nanowerk.com/nanotechnology-news2/newsid=57060.php

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Do simulations represent the real world at the atomic scale?

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Jan 20, 2021 (Nanowerk News) Computer simulations hold tremendous promise to accelerate the molecular engineering of green energy technologies, such as new systems for electrical energy storage and solar energy usage, as well as carbon dioxide capture from the environment. However, the predictive power of these simulations depends on having a means to confirm that they do indeed describe the real world. Such confirmation is no simple task. Many assumptions enter the setup of these simulations. As a result, the simulations must be carefully checked by using an appropriate “validation protocol” involving experimental measurements. To address this challenge, a team of scientists at the U.S. Department of Energy’s (DOE) Argonne National Laboratory, the University of Chicago and the University of California, Davis, developed a groundbreaking validation protocol for simulations of the atomic structure of the interface between a solid (a metal oxide) and liquid water (Physical Review Materials, “Validating first-principles molecular dynamics calculations of oxide/water interfaces with x-ray reflectivity data”). The team was led by Giulia Galli, a theorist with a joint appointment at Argonne and the University of Chicago, and Paul Fenter, an Argonne experimentalist. Computer simulation at the atomic scale Pictorial representation of joint experimental and computational study of materials. The study utilized the Advanced Photon Source (upper panel) and Argonne Leadership Computing Facility (lower panel). The team addressed the atomistic structure of interfaces, which are ubiquitous in materials. (Image: Emmanuel Gygi, University of California, San Diego) “We focused on a solid/liquid interface because interfaces are ubiquitous in materials, and those between oxides and water are key in many energy applications,” said Galli. “To date, most validation protocols have been designed for bulk materials, ignoring interfaces,” added Fenter. “We felt that the atomic-scale structure of surfaces and interfaces in realistic environments would present a particularly sensitive, and therefore challenging, validation approach.” The validation procedure they designed uses high-resolution X-ray reflectivity (XR) measurements as the experimental pillar of the protocol. The team compared XR measurements for an aluminum oxide/water interface, conducted at beamline 33-ID-D at Argonne’s Advanced Photon Source (APS), with results obtained by running high-performance computer simulations at the Argonne Leadership Computing Facility (ALCF). Both the APS and ALCF are DOE Office of Science User Facilities. “These measurements detect the reflection of very high energy X-ray beams from an oxide/water interface,” said Zhan Zhang, a physicist in Argonne’s X-ray Science division. At the beam energies generated at the APS, the X-ray wavelengths are similar to interatomic distances. This allows the researchers to directly probe the molecular-scale structure of the interface. “This makes XR an ideal probe to obtain experimental results directly comparable to simulations,” added Katherine Harmon, a graduate student at Northwestern University, an Argonne visiting student and the first author of the paper. The team ran the simulations at the ALCF using the Qbox code, which is designed to study finite temperature properties of materials and molecules using simulations based on quantum mechanics. “We were able to test several approximations of the theory,” said Francois Gygi from the University of California, Davis, part of the team and lead developer of the Qbox code. The team compared measured XR intensities with those calculated from several simulated structures. They also investigated how X-rays scattered from the electrons in different parts of the sample would interfere to produce the experimentally observed signal. The endeavor of the team turned out to be more challenging than anticipated. “Admittedly, it was a bit of a trial and error at the beginning when we were trying to understand the right geometry to adopt and the right theory that would give us accurate results,” said Maria Chan, a co-author of the study and scientist at Argonne’s Center for Nanoscale Materials, a DOE Office of Science User Facility. “However, our back and forth between theory and experiment paid off, and we were able to set up a robust validation protocol that can now be deployed for other interfaces as well.” “The validation protocol helped quantify the strengths and weaknesses of the simulations, providing a pathway toward building more accurate models of solid/liquid interfaces in the future,” said Kendra Letchworth-Weaver. An assistant professor at James Madison University, she developed software to predict XR signals from simulations during a postdoctoral fellowship at Argonne. The simulations also shed new insight on the XR measurements themselves. In particular, they showed that the data are sensitive not only to the atomic positions, but also to the electron distribution surrounding each atom in subtle and complex ways. These insights will prove beneficial to future experiments on oxide/liquid interfaces.

Source: https://www.nanowerk.com/nanotechnology-news2/newsid=57058.php

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