A less complicated way to make quantum materials

A depiction of a carefully-designed substrate that causes a deposited sheet of graphene to ripple. This distortion generates currents that reside on only one side of the nanoribbon structure. Credit: Võ Tiến Phong

Scientists have been trying to figure out how to use graphene because of its unique properties, like high tensile strength and conductivity, since the early 2000s.

A type of graphene called “twisted bilayer graphene” has been shown to be superconducting, which means that it can move electricity very easily. This type of graphene is made of two sheets of graphene twisted together in a certain “magic” angle. However, making devices with this method is still difficult because of the low yield of making twisted bilayer graphene.

Graphene is now a material with the same electronic properties as twisted graphene bilayers. A new study has shown how patterned, periodic deformation of one layer of graphene turns it into this material. This system also has a lot of different and interesting conductors at the boundary. This work has the potential to make quantum devices like orbital magnets and superconductors in the future. By better understanding how unique properties happen when single sheets of graphene are subjected to periodic strain, this work could lead to these devices in the future. The study, which was published in Physical Review Letters, was done by graduate student V Tin Phong and professor Eugene Mele in Penn’s Department of Physics & Astronomy in the School of Arts & Sciences. They worked together on it.

Single layers of graphene can be used instead of the more complicated twisted bilayer method. This method is called “bed of nails,” and it applies an external force or strain to the graphene in a regular pattern. To better understand the quantum geometrical properties of this system, Mele and Phong set out to learn more about the theory behind how electrons move in this single-layer system, which is why they did this study.

Following computer simulations of single-layer tests in which only one side of a material was used, the researchers were surprised to find new evidence of unusual things that happened on the surface of the material. “Generally, bulk topology is linked to surface properties, and when that’s the case, all surfaces get the same thing,” says Mele. Having edge modes on one side but not the other made me think this was a very strange thing to see.

This was surprising because in this system, the average pseudo-magnetic field, which is caused when the system is strained, was zero. It was positive in one area and negative in the other, which the researchers thought would cancel out any unique effects. It’s likely that there won’t be any interesting physics if there is no magnetic field. Instead, we found that even though the average magnetic field is zero, it still has some interesting physics at the edge of what you see.

To figure out why this happened, Phong took a closer look at a similar experiment in which single sheets of graphene were bent to make a constant strain field instead of a periodic one. He found that this system had the same topological index, which meant that edge states that only happen on one side of the material would also happen in this system, as well. Phong: “The physics here was similar and seemed to be the right answer for the phenomenology we were working on,” he tells the story.

There are flat bands in twisted bilayer graphene that are made by depositing an atomically thin layer onto a bed of nails. This causes the graphene sheet to be bent at a certain frequency.

The researchers are already working on getting even more information about these single-layer systems, and they’re not done yet. One way to do more research is to work with assistant professor Bo Zhen to look at the same thing with light waves. The researchers also want to see if other unique properties that are found in twisted bilayer graphene might also be found in single-layer systems.

“Physics is simple, but the phenomenology that comes out of it is not. You can get the system to behave the way you want in a more controlled way, but the phenomenology is not. People are still finding new things as they talk. It’s very interesting, and we’re still finding new things “In Phong’s words:

In addition, because these single-layer systems are easier to work with, this better theoretical understanding could help make future discoveries in edge state physics, like new devices that could be very small and quick.

A lot of people are trying to figure out how these twisted graphene bilayers work, says Mele. “We’re trying to figure out what the physical system needs to do that,” he says. Using this technology, we’re making things that you can’t make from the top down at a length that’s interesting. For example, we’re making things that are bigger than atoms and smaller than you can make with lithography. If you can control that, you can do a lot of different things with it.

Further information: Võ Tiến Phong et al, Boundary Modes from Periodic Magnetic and Pseudomagnetic Fields in Graphene, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.128.176406

Journal information: Physical Review Letters

Source: University of Pennsylvania