It's extremely difficult to deposit a two layers of graphene on a surface with an angle between them, because the atoms in the second layer naturally want to align with the layer below them. The possibility of superconductivity at the magic angle was discussed theoretically, but a lot of clever engineering work had to go into realizing these experimentally. They also depend on the coupling between layers, which can be tuned by pressure: one of the talks in the March Meeting session discussed stacks in which the angle was deliberately set to something slightly off from the "magic" value of around 1.1 degrees where superconductivity occurs, but tuned into the superconducting condition by compressing the stack to push the layers slightly closer together, effectively increasing the strength of the interaction between layers. The period of the moiré pattern depends on the angle between the two lattices, so the properies of the two-layer stack can be changed by changing the angle. The really beautiful thing about these, though, is that they provide the opportunity to tune the properties of the artificial material. And that, in turn, determines the conditions under which the material becomes an insulator, a conductor, or even a superconductor.Īs with the high-pressure materials discussed in last week's post, these graphene bilayers represent a way of artificially creating structures with patterns and periods not found in naturally occurring samples. That coupling between layers turns into a periodic variation in the energy of an electron moving through graphene, which in turn changes the band structure that the electrons see. This might not seem like a thing that would matter- after all, what does the graphene care about patterns seen by humans observing from above? The layers in the stack are close enough together, though, that electrons in one layer can also feel the presence of the atoms in the other. The period of this pattern depends on the angle between the sheets- the larger the angle, the closer together the moiré hexagons are. If you look at this figure showing two hexagonal lattices rotated by 14 degrees relative to one another, you can see a larger pattern, also hexagonal, formed by the moiré effect. Two hexagonal arrays overlaid with an angle of 14 degrees between them, showing the moiré effect. What does this have to do with superconductors? The same basic physics that produces the moiré effect in overlaid groups of straight lines can also work with overlays of more complicated patterns like the hexagonal mesh found in graphene. Thus, the standard advice to avoid small stripes when appearing on television. The separation between the stripes depends on the angle between the patterns- the bigger the angle, the closer together the stripes are, as seen in the figure above.Ī mismatch caused by pixel lines on the television not aligning with stripes on the clothing of a person speaking into the camera will produce a moiré pattern on the screen, The moiré stripes can shift dramatically with the small changes in orientation that naturally occur as someone moves and speaks, and the resulting "swimming" effect is distracting. The angle means that the lines will necessarily cross, and their intersections will form a repeating pattern of dark "stripes" when seen from a distance. The classic example of a moiré pattern comes from overlaying two sets of straight lines with an angle between them.
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