Observation of flat bands in twisted bilayer graphene

Magic-angle materials represent a surprising recent physics discovery in double layers of graphene, the two-dimensional material made of carbon atoms in a hexagonal pattern. 

When the upper layer of two stacked layers of graphene is rotated by about 1 degree, the material suddenly turns into a superconductor. At a temperature of 3 Kelvin, this so-called twisted bilayer graphene (tbg) conducts electricity without resistance.

Now, an international team of scientists from Geneva, Barcelona, and Leiden have finally confirmed the mechanism behind this new type of superconductors. In Nature Physics, they show that the slight twist causes the electrons in the material to slow down enough to sense each other. This enables them to form the electron pairs which are necessary for superconductivity.

How can such a small twist make such a big difference? This is connected with moiré patterns, a phenomenon also seen in the everyday world. When two patterned fences are in front of another, one observes additional dark and bright spots, caused by the varying overlap between the patterns. Such moiré patterns (derived from the the French name of textile patterns made in a similar way) generally appear where periodical structures overlap imperfectly.

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Image: Angle resolved photoemission spectrum revealing flat non-dispersing electronic band filled with slow electrons separated by mini gaps from the rest of electronic structure in twisted bilayer graphene device.