Discovery of ‘magic-angle graphene’ that behaves like a high-temperature superconductor is Physics World 2018 Breakthrough of the Year

13 Dec 2018 Hamish Johnston

The Physics World 2018 Breakthrough of the Year goes to Pablo Jarillo-Herrero of the Massachusetts Institute of Technology (MIT) in the US and colleagues for making the discovery that led to the development of “twistronics”, which is a new and very promising technique for adjusting the electronic properties of graphene by rotating adjacent layers of the material. The technique was first used by the team to create “magic-angle graphene”, which behaves like a high-temperature superconductor. Nine other achievements are highly commended in the Top Ten Breakthroughs of 2018 and cover topics ranging from the first full body PET/CT scan to a zero-carbon aeroplane propelled by a jet of ions.

New twist: magic-angle bilayer graphene is the 2018 Breakthrough of the Year. (Courtesy: Pablo Jarillo-Herrero)

Graphene is a layer of carbon just one atom thick that has a honeycomb lattice. Bilayer graphene is a stack of two layers in which the two lattices are usually oriented in a specific way. Twistronics began when Jarillo-Herrero and colleagues discovered Mott insulator behaviour in pristine bilayer graphene when the orientation of the two layers were twisted by a magic angle.

The team, a collection of researchers from MIT, Harvard University and the National Institute of Materials Science (NIMS) in Japan, then showed that by adding electrons to the twisted bilayer using an applied electric field, they could make it superconducting.

The development of twistronics has already triggered several important follow-up discoveries in graphene research. Scientists at Columbia University devised a way to finely tune the angle between adjacent layers of 2D materials and thereby control the electronic properties. This highlights the potential for twistronics as an alternative paradigm for device engineering.

Further theoretical investigations have provided insights into the electronic transitions in bilayer and multilayer graphene systems. Theorists have highlighted the potential for unconventional superconductivity, including topological superconductivity and the existence of topological “Majorana states” at the edge of the material. These states could be particularly useful for creating quantum bits in quantum computers because they are more robust to environmental perturbations than many of the alternatives.

More recently, adding a twist between layers of 2D materials has also helped prevent Umklapp scattering, which degrades carrier mobility at high temperatures.

The Physics World Top 10 Breakthroughs of 2018 are awarded to research reported in 2018 in physicworld.com. The winners are chosen by Physics World editors and the criteria for judging included:

Significant advance in knowledge or understanding

Importance of work for scientific progress and/or development of real-world applications

Of general interest to Physics World readers

Now for our nine highly commended breakthroughs, which are listed below in no particular order.Ready to roll: Leif Asp with a bobbin of carbon fibre yarn. (Courtesy: Chalmers University of Technology)

Multifunctional carbon fibres enable “massless” energy storage

To Leif Asp at Chalmers University of Technology and his collaborators in Sweden, Italy and France for demonstrating the potential for effectively massless energy storage using multifunctional carbon fibres. Despite progress in energy storage technology, batteries still make up a significant part of the weight for devices such as laptops and even cars. Rather than focussing solely on optimized battery materials to tackle lightweight demands, Leif Asp and co-authors demonstrated that exploiting the electrochemical properties of carbon fibres used for structural support could drop device masses by as much as 50%.

Compensator expands global access to advanced radiotherapy

To Eric Ford from University of Washington Medical Center and his colleagues for creating a low-cost method for implementing intensity-modulated radiotherapy (IMRT). IMRT is a precision treatment technique that uses complex multileaf collimators (MLCs) to shape the photon beam and spare more healthy tissue. But while IMRT is available in essentially all radiotherapy clinics in high-income countries, it is largely absent in vast regions of low- and middle-income countries. To address this shortfall, Ford and a multi-institutional team developed a cost-effective alternative to the MLC, replacing it with a ring of compensatorsmade from lightweight plastic moulds filled with attenuating beads such as tungsten beads. The proposed device can be retrofitted to existing linac and cobalt teletherapy units – allowing clinics to add IMRT without having to purchase a new treatment system.

IPCC Special Report on 1.5 °C climate change

To the Intergovernmental Panel on Climate Change (IPCC) for its Special Report on 1.5 °C climate change, which was released in October. The work of 91 authors and review editors from 40 countries, the report resulted from the Paris climate talks in 2015 and highlights the climate-change impacts that could be avoided if the world gets its act together and limits global warming to 1.5 °C. “Every extra bit of warming matters, especially since warming of 1.5 °C or higher increases the risk associated with long-lasting or irreversible changes, such as the loss of some ecosystems,” said Hans-Otto Pörtner, co-chair of IPCC Working Group II. Read more in our special collection on climate change at 1.5 °C.

EXPLORER PET/CT produces first total-body scans

To the EXPLORER consortium for producing the first human images using their total-body PET scanner. The EXPLORER PET/CT is the world’s first medical imaging system that can capture a 3D image of the entire human body simultaneously. Developed by UC Davis scientists and a multi-institutional consortium, EXPLORER can scan up to 40 times faster, or use up to 40 times less radiation dose, than current PET systems, making it possible to conduct repeated studies in an individual, or dramatically reduce dose in paediatric studies. The high-sensitivity scanner can also create movies that track radiolabelled drugs as they move around the body.

Combustion-free, propeller-free plane takes flight

To Steve Barrett and colleagues at MIT for demonstrating the first flight of a propeller-free plane that is not powered by a combustion engine. The plane, which has a five-metre wingspan, is instead propelled by an “ionic wind” of charged ions generated by wire electrodes running off a battery. Announcing the flight, Barrett said he was inspired by a childhood love of Star Trek: “The future looked like it should be planes moving silently with no moving parts – maybe a blue glow but certainly no propellers or turbines or anything like that. So I started looking for what physics would make flight with no moving parts possible.”Cruising altitude: Time-lapse image of the ion-driven aeroplane in flight. (Courtesy: Nature)

Quantum mechanics defies causal order, experiment confirms

To Jacqui Romero, Fabio Costa, Kaumudibikash Goswami, Christina Giarmatzi, Michael Kewming and Andrew White of the University of Queensland and Cyril Branciard of the University of Grenoble Alpes for their experimental demonstration that quantum mechanics can allow events to occur with no definite causal order. This is unlike classical physics – and everyday life – in which there is a strict causal relationship between consecutive events. To observe indefinite causality the team created a “quantum switch”, in which a photon can take two paths. One path involves the photon being subjected to operation A before operation B, while in the other path B occurs before A. If the operations are performed close together in time, then it becomes impossible to tell which was done first. Indefinite causal order – along with team’s quantum switch – could prove useful for processing quantum information.

Activating retinal stem cells restores vision in mice

To Bo Chen from Icahn School of Medicine at Mount Sinai and an international research team for successfully restoring vision in mice by activating retinal stem cells. In cold-blooded vertebrates, Müller glia cells (MGs) act as retinal stem cells that can replenish damaged retinal neurons and restore vision. In mammals, however, MGs do not have regenerative capability. In this study, Chen and colleagues aimed to reactivate the MGs by performing a two-step gene transfer process to reprogram MGs in blind mice. Between four and six weeks after the reprogramming, the mice could sense light and regained their vision. While further tests are needed to determine the degree of sight improvement, the approach could one day transform treatment for patients with retinal degenerative diseases, which currently have no cure.

Ancient hydrogen reveals clues to dark matter’s identity

To Judd Bowman, Raul Monsalve, Thomas Mozdzen and Nivedita Mahesh of Arizona State University Arizona State University and Alan Rogers of the Massachusetts Institute of Technology for using the EDGES radio telescope to observe colder-than-expected hydrogengas that existed just 180 million years after the Big Bang; and Rennan Barkana, of Tel Aviv University for calculating that this could be the first direct observation of a non-gravitational interaction between dark matter and conventional matter. While further observations are needed to back-up this hypothesis, the research could help resolve one of the most important unsolved mysteries of physics: what is the nature of dark matter?Light and dark: did dark matter cool ancient hydrogen?

Superconductivity spotted in a quasicrystal

To Keiichiro Imura, Kazuhiko Deguchi, Tsutomu Ishimasa, Keisuke Kamiya, Nobuo Wada and Noriaki Sato of Nagoya University, Tsunehiro Takeuchi of the Toyota Technological Institute, Tsutomu Ishimasa of the Toyota Physical and Chemical Research Institute and Noriyuki Kabeya of Tohoku University for discovering the first superconducting quasicrystal – a metal-alloy that is a superconductor a temperatures lower than 0.05 K. Conventional superconductivity arises when pairs of electrons form via an interaction with phonons, which are particle-like deformations that propagate through crystalline lattices. Quasicrystals do not have translational symmetry and therefore do not have crystalline lattices – and should therefore not be conventional superconductors. Since the first quasicrystals were discovered in 1984, some physicists have suggested that superconductivity could occur in quasicrystals and now this discovery could lead to the creation of new materials that display fractal superconductivity.

15/12/2018 FROM PHYSICSWORLD.COM