New highly adjustable composite with a twist

Fractal arrangement of the periodic table. Dots identify moiré parameter values ​​that correspond to systems with periodic microgeometry. Here, the short period and the large period are identified by large dots and small dots, respectively, and show a self-similar fractal arrangement of the periodic system. Credits: Ken Golden / University of Utah.

Notice the pattern that is created when the circles move from each other. These patterns, created by two sets of lines offset from each other, are called: Moire (Pronounced mwar-AY) Effect. As an optical illusion, the moiré pattern creates a neat simulation of movement. However, on the atomic scale, if one sheet of latticed atoms is slightly offset from another, these moiré patterns provide exciting and important physics with interesting and unusual electronic properties. You can create it.

Mathematicians at the University of Utah have discovered that various composite materials can be designed from moiré. pattern It is created by rotating and stretching one grid with respect to another.Their electricity and others Physical characteristics The resulting moire pattern can change very rapidly in some cases, depending on whether it repeats regularly or not.Their findings are published in Communication physics..

The mathematics and physics of these twisted grids apply to a variety of material properties, says Kenneth Golden, a prominent professor of mathematics. “The underlying theory also applies to materials on a wide range of length scales from nanometers to kilometers, showing how wide the range of potential technical applications of our discoveries is.”

Two concentric circles that move parallel to each other create a moiré pattern. Credits: Jacopo Bertolotti, CC0

With a twist

Before we reach these new discoveries, we need to graph the history of two important concepts: aperiodic geometry and twisttronics.

Aperiodic geometry means a pattern that does not repeat. An example is the diamond-shaped Penrose tiling pattern. If you draw a box around a part of the pattern and start sliding it in any direction without rotating it, you won’t be able to find the part of the pattern that matches it.

Aperiodic patterns designed over 1000 years ago appeared in the giriff tiles used in Islamic architecture. More recently, in the early 1980s, materials scientist Dan Shechtman discovered crystals with an aperiodic atomic structure. This revolutionary crystallography won the 2011 Nobel Prize in Chemistry to Shectmann because the classical definition of crystals contains only regularly repeating atomic patterns.

Now let’s move on to Twistronics. It has also won the Nobel Prize in this field. In 2010, Andre Geim and Konstantin Novoselov were awarded the Nobel Prize in Physics for discovering graphene, a material made up of a single layer of grid-like carbon atoms that looks like a wire mesh. Graphene itself has its own interesting properties, but in recent years physicists have found that stacking two layers of graphene and slightly rotating one results in a superconductor, which is very powerful. I found that it would be. This field of study of the electronic properties of twisted double-layer graphene is called “twistronics”.

Two-phase composite material

In a new study, Golden and his colleagues imagined something different.It’s like twisttronics, but instead of two layers of atoms, a moiré pattern formed from interfering lattices geometrically places two different material components, such as good and bad conductors. Composite material.. They call the new material a “twisted double layer composite” because one of the grids is twisted and / or stretched relative to the other grid. Examining the mathematics of such materials, they found that the moiré pattern produced some amazing properties.

“As the twist angle and scale parameters change, these patterns produce a myriad of microgeometry, with very small changes in parameters causing very large changes. Material property“Ben Murphy, co-author of the paper and assistant professor of mathematics, said.

For example, twisting a grid only twice will change the moiré pattern from a regularly repeating state to a non-repeating state, and all patterns are not random, but may even appear to be randomly disturbed.If the pattern is ordered and periodic, the material can conduct Current very well Displays on / off operations similar to the semiconductors used in computer chips. However, for patterns that look aperiodic and chaotic, the material can be an insulator that crushes the current, “similar to the rubber on the handle of a tool that helps eliminate electric shock,” a recent study. Principal author David Morrison states that he holds a PhD in Physics from the University of Utah under the supervision of Golden, who has completed his PhD.

This kind of sudden transition from an electric conductor to an insulator reminded researchers of yet another Nobel Prize-winning discovery, the Anderson localization transition of quantum conductors. This 1977 Nobel Prize in Physics discovery uses the mathematics of wave scattering and interference to allow electrons to move freely, trap or localize (insulator) materials (conductors). Explains how. But Golden says that the quantum wave equation used by Anderson does not work on the scale of these twisted double-layer composites, so something else is happening to create this conductor / insulator effect. I say it must be. “We observe geometry-driven localization transitions that have nothing to do with wave scattering or interference effects. This is a surprising and unexpected discovery,” says Golden.

The electromagnetic properties of these new materials change significantly with small changes in the helix angle, and one day engineers will use those changes to accurately adjust the properties of the material, for example, the visible frequency of light that the material receives (also known as). Color) may be selected. The frequency that allows passage and blocks it.

“In addition, our mathematical framework applies to the adjustment of other properties of these materials, such as magnetism, diffusion, heat, optics, and electricity,” said Professor Elena Cerkaev, co-author of mathematics and research. I am saying. Similar behavior in acoustic and other mechanical analogs. ”

Researchers enhance charge density waves with twisted heterostructure moiré engineering

For more information:
Order of quasi-periodic composite disordering, Communication physics (2022). DOI: 10.1038 / s42005-022-00898-z

Provided by
University of Utah

Quote: New, highly adjustable composites — with a twist (June 14, 2022) from https: // Acquired on June 14, 2022.

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New highly adjustable composite with a twist

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