Seminars

A “magic angle “gives graphene new extraordinary properties

Scientists at the Massachusetts Institute of Technology (MIT) reported a surprising finding with graphene last year: simply by turning two layers of this material on top of each other at an angle of 1.1 degrees (the so-called magic angle) behaved as a superconductor in which electric currents flow without resistance. In addition, this configuration was always accompanied by enigmatic correlated insulating phases, such as what is observed in the also mysterious cuprate superconductors (a ceramic material) of high temperature. Now, researchers from the Institute of Photonic Sciences (ICFO) in Barcelona have managed to greatly improve the quality of the device that graphene carries with this configuration, and in doing so, they have encountered something even bigger and totally unexpected. The authors, who publish their study in Nature, were able to observe a large number of unknown superconducting and correlated states, in addition to an unprecedented set of magnetic and topological states, opening a path to a completely new and richer physics.

Superconductivity at room temperature is the key to many technological objectives, such as efficient energy transmission, frictionless trains or even quantum computers, among others. When discovered more than 100 years ago, superconductivity was only plausible in materials cooled to temperatures close to absolute zero. Later, in the late 1980s, scientists discovered high-temperature superconductors using cuprates. Despite the difficulty of building superconductors and the need to apply extreme conditions (very strong magnetic fields) to study the material, this field took off as a holy grail among scientists. Since last year, the excitement of research has increased. The double carbon monolayers have captivated scientists because, unlike cuprates, their structural simplicity has become an excellent platform to explore the complex physics of superconductivity. In their experiment, ICFO scientists used the so-called “peel and stack” van der Waals assembly technique to make the two graphene monolayers stacked and rotated with the magic angle. Then they used a mechanical cleaning process to remove impurities and release local tension. Thus they were able to obtain extremely clean and extremely disordered rotated graphene bilayers, solving a multitude of fragile interaction effects.
By changing the density of the electric charge carrier inside the device with a nearby capacitor they saw that the material could be adjusted to behave as an insulator, such as a superconductor, or even an exotic orbital magnet with topological texture in a phase never observed before. The device entered a superconducting state for lower densities, a completely new advance What is even more surprising is the fact that the device entered a superconducting state for lower densities, an advance never published for any superconductor and completely new in the field. Finally, the researchers were also able to increase the superconducting transition temperature to more than 3 kelvin, reaching record highs twice as high as in previously published studies for graphene devices with a magic angle.
According to the authors, the exceptional thing about this approach is that graphene – a material that is generally poor in electron phenomena that interact strongly – has now been the tool that allows access to this complex and exceptionally rich physics. So far, there is no theory that can explain the superconductivity in the magical angle of graphene at the microscopic level, however, with this new discovery, a new opportunity has emerged to reveal its origin.

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