Full of Surprises: Graphene’s New Phases

Graphene’s isolation from its bulk sister crystal graphite blew open the field of near 2-dimensional solid-state physics, introducing us to such novel physics as massless electrons, due to the Dirac cone band structure, and technological possibilities such as spintronics, the long-range preservation an electrons spin for informatic purposes. Since then, nearly every year we add to the pallet of near 2D crystals, all with various applications and properties such as semiconductors with optically addressable spin-valley isospin freedom, magnetic insulators with single layer ferromagnetism and staked anti-ferromagnetism and even low dimensional dielectrics that enhance the optical properties of neighbouring 2D crystals. It is safe to say that the field is definitely maturing and will have far reaching effects on future research and technology.

It would have been easy to assume, that there were not many surprises left graphene to give the community, being studied so rigorously for over the past decade. However, yet again new research into novel twisted bilayer graphene has once again blown the field wide open again with access to new rich veins of new and fascinating physics. Bilayer graphene, exactly as the name suggests is simply two layers of graphene layered atop one another. Such systems have been studied for their ability to allow for the opening a tuneable band gap not allowed in single layer graphene, for applications to a number of quantum technologies such as Quantum dots of Quantum point contacts. A twister bilayer graphene system simply takes one of these sheets of graphene and rotates them slightly, off of its usual alignment found in nature.

FigTBG
Example of a twisted bilayer graphene system from J. D. Sanchez-Yamagishi et al. Nature Nanotechnology 12, pages 118–122 (2017)

It’s not immediately obvious why simply twisting the two layers of a stacked system with respect to one another would change the materials properties at all. The reason is more obvious when you consider crystal lattice structure form the top down as you change the angle between the two sheets. Long range order can be seen when you superimpose the two twisted 2D crystals on top of each other known as a Morié lattice. This long-range order can effectively be treated as the new crystal structure, with tuneable parameters given by the twist angle between the layers. So far it has been demonstrated that selecting different twist angles offers such novel physics not observed in graphene before such as non-BCS superconductivity and topological helical edge state transport.

Topological edge state transport is one of the numerous systems desired for discussed topological quantum technologies, which offer strong protection against local disorder, often the limiting factor in current quantum processors, as the disorder often limits the time window in which the processor operates in a quantum regime. Chiral edge state transport allows for electrons of specific spin to robustly conduct around the edge of a system, in a specific direction. This discovery adds a new dimension to the spintronic applications of graphene.

FigSC
Novel superconducting phases observed in twisted bilayer graphene from Y. Cao et al. Nature 556, pages 43–50 (2018)

So-called “Magic Angle” twisting of bilayer graphene also opens up a new phase of unconventional non-BCS superconductivity. Its unconventionality comes from the incredibly low carrier density at relatively high critical temperatures need to access the superconductivity regime. This suggests that the superconductivity is caused by electron correlations/ordering within the system, as opposed to electron-phonon/electron-lattice vibration interactions as is usually found.

These fascinating, easy to produce and incredibly tuneable systems not only open up new theoretical and experimental research in bilayer graphene, but also ask us what other new phases of matter we might find by applying the same method to different 2D materials, or even stacked layers of different 2D crystals. This discovery has once again excited the 2D research community, it really does seem the possibilities are endless with these almost magical materials.

By Matthew Brooks, PhD Student of the Burkard Group at Univeristät Konstanz, Germany.

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