Scientists at The University of Manchester’s National Graphene Institute have discovered new physics in graphite through the application of twistronics, revealing a 2.5-dimensional mixing of surface and bulk states. The research opens new possibilities in controlling electronic properties in both 2D and 3D materials.
Researchers in the National Graphene Institute (NGI) at The University of Manchester have revisited graphite, one of the most ancient materials on Earth, and discovered new physics that has eluded the field for decades.
The Complexity of Graphite
Despite being made entirely of layers of carbon atoms arranged in a honeycomb pattern, natural graphite is not as simple as one may think. The manner in which these atomic layers stack on top of one another can result in different types of graphite. These are characterized by different stacking orders of consecutive atomic planes. The majority of naturally appearing graphite has hexagonal stacking, making it one of the most “ordinary” materials on Earth. The structure of a graphite crystal is a repetitive pattern. This pattern gets disrupted at the surface of the crystal, leading to what’s called ‘surface states’, which are like waves that slowly fade away as you go deeper into the crystal. However, the way that surface states can be tuned in graphite was not well understood.
New Insights Through Twistronics
Van der Waals technology and twistronics (stacking two 2D crystals at a twist angle to tune the properties of the resulting structure to a great extent, because of the moiré pattern formed at their interface) are the two leading fields in 2D materials research. Now, the team of NGI researchers, led by Prof. Artem Mishchenko, is employing the moiré pattern to tune the surface states of graphite, reminiscent of a kaleidoscope with ever-changing pictures as one rotates the lens, revealing the extraordinary new physics behind graphite.
In particular, Prof. Mishchenko expanded the twistronics technique to three-dimensional graphite and found that moiré potential does not just modify the surface states of graphite, but also affects the electronic spectrum of the entire bulk of graphite crystal. Much like the well-known story of The Princess and The Pea, the princess felt the pea right through the twenty mattresses and the twenty eider-down beds. In the case of graphite, the moiré potential at an aligned interface could penetrate through more than 40 atomic graphitic layers.
Observations and Implications
This research, published in a recent issue of the journal Nature, studied the effects of moiré patterns in bulk hexagonal graphite generated by crystallographic alignment with hexagonal boron nitride. The most fascinating result is the observation of a 2.5-dimensional mixing of the surface and bulk states in graphite, which manifests itself in a new type of fractal quantum Hall effect – a 2.5D Hofstadter’s butterfly.
Prof. Artem Mishchenko at The University of Manchester, who has already discovered the 2.5-dimensional quantum Hall effect in graphite said: “Graphite gave rise to the celebrated graphene, but people normally are not interested in this ‘old’ material. And now, even with our accumulated knowledge on graphite of different stacking and alignment orders in the past years, we still found graphite a very attractive system – so much yet to be explored”. Ciaran Mullan, one of the leading authors of the paper, added: “Our work opens up new possibilities for controlling electronic properties by twistronics not only in 2D but also in 3D materials.”
Prof. Vladimir Fal’ko, Director of the National Graphene Institute and theoretical physicist at the Department of Physics and Astronomy, added: “The unusual 2.5D quantum Hall effect in graphite arises as the interplay between two quantum physics textbook phenomena – Landau quantization in strong magnetic fields and quantum confinement, leading to yet another new type of quantum effect.”
The same team is now continuing with the graphite research to gain a better understanding of this surprisingly interesting material.
Reference: “Mixing of moiré-surface and bulk states in graphite” by Ciaran Mullan, Sergey Slizovskiy, Jun Yin, Ziwei Wang, Qian Yang, Shuigang Xu, Yaping Yang, Benjamin A. Piot, Sheng Hu, Takashi Taniguchi, Kenji Watanabe, Kostya S. Novoselov, A. K. Geim, Vladimir I. Fal’ko and Artem Mishchenko, 19 July 2023, Nature.