Physicists discover unusual superconductivity in twisted three-layer graphene

Physicists used high-resolution scanning tunneling microscopy and spectroscopy to study

superconductivity of three-layer graphene,twisted at a magical angle. Experiments have shown that superconductivity in this material exhibits several very unusual properties that cannot be described using BCS theory.

Artist's rendering of three-layer graphene. Image: Polina Shmatkova and Margarita Davydova

Scientists measured the evolution of the superconducting gap usingas electrons are removed from the trilayer by a switch that turns the electric field on or off. The superconducting gap is a property that describes how difficult it is to add or remove individual electrons in a superconductor, the authors explain.

Because the electrons in a superconductor tend toto form pairs, it takes a certain amount of energy to break these pairs. It can be different for pairs moving in different directions relative to the crystal lattice. As a result, the "gap" has a specific shape, which is determined by the probability that the pairs will be broken at a certain amount of energy.

Even though superconductors existFor a long time, a surprisingly new feature of twisted two-layer and three-layer graphene materials is that superconductivity in these materials can be turned on by simply applying a voltage to an adjacent electrode.

Stevan Naj-Perge, physicist at Caltech, co-author of the paper

Scientists have found that in a twisted three-layergraphene has two superconductivity modes with superconducting gap profiles of different shapes. If one of the modes can be explained by a theory somewhat similar to the BCS, then the presence of two modes shows that an additional transition is likely taking place inside the superconducting phase, the researchers note. 

The authors of the work believe that an increase in the numberlayers makes superconductivity more reliable, while remaining easily tuned. This property opens up various possibilities for using twisted three-layer graphene as superconducting devices for quantum research and possibly in quantum information processing.

Cover image: A tunneling microscope image of three-layer graphene. Source: Caltech

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