Manchester scientists caught Hofstadter鈥檚 butterfly in one of the most ancient materials on Earth
Researchers in the at The University of Manchester have revisited one of the most ancient materials on Earth 鈥� graphite, and discovered new physics that has eluded the field for decades.
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, characterised by different stacking order of consecutive atomic planes. The majority of naturally appearing graphite has hexagonal stacking, making it one of the most 鈥渙rdinary鈥� materials on Earth. The structure of graphite crystal is a repetitive pattern. This pattern gets disrupted at the surface of the crystal and leads to what's called 'surface states', which are like waves that slowly fade away as you go deeper into the crystal. But how surface states can be tuned in graphite, was not well understood yet.
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 moir茅 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, employs moir茅 pattern to tune the surface states of graphite, reminiscent of a kaleidoscope with everchanging pictures as one rotates the lens, revealing the extraordinary new physics behind graphite.
In particular, Prof. Mishchenko expanded twistronics technique to three-dimensional graphite and found that moir茅 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 moir茅 potential at an aligned interface could penetrate through more than 40 atomic graphitic layers.
This research, published in the latest issue of , studied the effects of moir茅 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 鈥� a 2.5D Hofstadter鈥檚 butterfly.
Prof. Artem Mishchenko at 糖心Vlog官方, who has already discovered the said: 鈥淕raphite gave rise to the celebrated graphene, but people normally are not interested in this 鈥榦ld鈥� 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 鈥� so much yet to be explored鈥�. Ciaran Mullan, one of the leading authors of the paper, added: 鈥淥ur work opens up new possibilities for controlling electronic properties by twistronics not only in 2D but also in 3D materials鈥�.
Prof. Vladimir Fal鈥檏o, Director of the National Graphene Institute and theoretical physicist at the Department of Physics and Astronomy, added: 鈥淭he unusual 2.5D quantum Hall effect in graphite arises as the interplay between two quantum physics textbook phenomena 鈥� Landau quantisation in strong magnetic fields and quantum confinement, leading to yet another new type of quantum effect鈥�.
The same team is now carrying on with the graphite research to gain a better understanding of this surprisingly interesting material.
Image credit: Prof. Jun Yin (co-author of the paper)
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