Allan H. MacDonald
Wolf Prize Laureate in Physics 2020
The 2020 wolf prize in Physics is awarded to: Pablo Jarillo-Herrero, Allan H. MacDonald and Rafi Bistritzer.
Prof. Allan H. MacDonald
The University of Texas at Austin – USA
“For pioneering theoretical and experimental work on twisted bilayer graphene.”
Since the 2004 groundbreaking experiments regarding the two-dimensional material -graphene, several research groups were soon studying the properties of twisted bilayer graphene. Graphene is a significant foundation for an entirely new generation of technologies. The hope is that graphene-based applications will benefit the environment and reduce costs. Electronic and computer industry requires materials whose conductance can be controlled.
The work of Jarillo-Herrero, MacDonald and Bistrizer has shown that the conductance properties of graphene interfaces can be controlled via the spatial misfit angle between the layers and then at certain angles the electrons exhibit surprising physical behavior. This physical discovery has the potential of leading to an energy revolution.
In 2011, a group led by Allan Macdonald, a theoretical physicist from the University of Texas, researched an intriguing behavior of twisted bilayer graphene, where the atomic lattices of two stacked graphene layers are laterally rotated with respect to each other by a small misfit angle. According to the calculations of MacDonald and Bistrizer (who did his post-doctoral thesis under the supervision of MacDonald at that time), the tunneling velocity of electrons between the layers depends on the misfit angle and completely vanishes at the “magic angle” of 1.1 degrees. It was hoped that this discovery would lead to the creation of a new type of super-conductor, namely a material that allows electrical current to pass with no impedance and with no energy loss.
The original paper by MacDonald and Bistrizer, which describes their discovery, was not received with enthusiasm by the scientific community and was even forgotten for several years.
At the same time, Jarillo-Herrero was working on twisted bilayer graphene in his lab at MIT. He became convinced that the ideas expressed by Macdonald and Bistrizer had substance.
His research team therefore invested considerable efforts in creating and measuring twisted bilayer graphene of various twist angles. The experiments proved successful in 2017 when it was found that positioning the layers at an angle of 1.1 degrees relative to one another (“the magic angle”) resulted in unusual electrical properties, precisely as MacDonald and Bistrizer have suggested. In this position, at sufficiently low temperatures, the electrons move from one layer to the other, creating a lattice with unusual qualities. The paper that described the phenomenon, which was published in Nature in 2018, revolutionized physics and triggered a flood of additional papers.
The discovery opens the door to building a super-conductor from bilayer graphene, in which electron movement is completely controlled by external electrical current. This electrical behavior resembles the behavior of copper-based superconductors called Cuprates. Cuprates demonstrate electrical conductivity with no resistance in relatively high temperatures compared with other super-conductors. For this reason, Cuprates now form a source of hope for realizing the dream of electrical conductivity with no energy loss at temperatures close to room temperature. If this mission is achieved, it would lead to a far-reaching energy revolution. However, one obstacle that prevents this revolution is that we do not yet have a theory that explains the behavior of superconductors at high temperatures. In the absence of a solid theoretical foundation, it is difficult to develop new, better materials. This is one of the reasons for the excitement around the discovery of bilayer graphene and the magic angle, which allows us to understand better what happens on the microscopic level when transitioning from a conductor to a superconductor state.
Allan H. MacDonald (1951, Canada) received the B.Sc. degree from St. Francis Xavier University, Antigonish, Nova Scotia, Canada in 1973 and the M.Sc. and Ph.D. degrees in physics from the University of Toronto in 1974 and 1978, respectively. He was a member of the research staff of the National Research Council of Canada from 1978 to 1987 and has taught at Indiana University (1987-2000) and the University of Texas at Austin (2000-present) where he now holds the Sid W. Richardson Chair in Physics. He has contributed to research on the quantum Hall effect, electronic band structure theory, magnetism, and superconductivity among a variety of other topics. Prof. MacDonald is a fellow of the American Physical Society, a member of the American Academy of Arts and Sciences and the US National Academy of Sciences, and a recipient of the Herzberg Medal, the Ernst Mach Honorary Medal, and the Buckley Prize.