‘Magic’ angle graphene and the creation of unexpected topological quantum states | Hacker News Reader

‘Magic’ angle graphene and the creation of unexpected topological quantum states

princeton.edu

Graphene continues to impress condensed matter physics researchers. Just by taking this material made out of carbon atoms you can get superconducting states, correlated insulating states, ferromagnetism, and some others. By using field-effect-transistor-style device geometries, you can tune between these states by changing a voltage on a gate. This is a big deal, because it's much easier to change a gate voltage than it is to grow an entirely new material with a different doping level, which is usually what you need to do with other systems. The paper that this article is about is a nice measurement of the topological nature of the moire bands. It's cool stuff and could potentially be important for quantum computing, but don't forget that these devices are still extremely difficult to make.

Regardless of whether graphene has left the lab and made it into consumer electronics or not, it's still super interesting for academics.

Source: I work on this stuff.

Are there any upcoming advancements on scale production methods?

I think CVD graphene (grown, production-scale stuff) has gotten to decent quality these days. There's much more to these devices than graphene, however.

You may have noticed that literally every experimental paper on this stuff has Kenji Watanabe and Takashi Taniguchi as authors. This is because they grow the best hBN crystals in the world and they give them for free to pretty much any researcher who asks nicely. This is amazing and without them the whole field would be way worse off. Their crystals are the bulk sort that needs scotch tape to use, much like graphite needs scotch tape to isolate graphene. I don't think CVD hBN is nearly as good as their stuff yet. The hBN provides an encapsulating dielectric for the graphene, and is critical to getting high-quality devices.

That's just one example of a problem with scaling this stuff, and there are others as well. The field is working to improve these things though, since right now it's super difficult to reproduce any given piece of physics. I think at this point there are exactly 3 ferromagnetic TBG devices in the world, for instance.

Any graphene research in particular that you're personally excited about? Past/ongoing/future, theoretical/practical, anything!

The graphene news that I as a non-academic get exposed to, and that is in a form that I can understand (i.e. not highly technical papers), is mostly limited to sensationalist battery improvement articles. A real shame, as so much seems to be going on behind the scenes!

I personally think that the Hofstadter butterfly stuff is absolutely gorgeous, and it was a large part of why I started working on 2D materials in the first place.

https://en.wikipedia.org/wiki/Hofstadter%27s_butterfly

In the '70s, Hofstadter wrote about this neat fractal pattern that would show up in the band structure of a material in a sufficiently large magnetic field. Specifically, the magnetic field strength times the crystal unit cell area needed to be big. Magnetic field strength is limited by how much current you can put around a superconducting solenoid. Unit cell area is generally something that you can't change for ordinary crystals: it's just set by the chemistry of the material.

Back in 2012-ish, a few groups managed to artificially increase the area of the unit cell by multiple orders of magnitude by aligning the graphene with hexagonal boron nitride, which has the same crystal structure and a very similar size. When aligned, the moire pattern itself has a large size, and that was enough to see the butterfly.

That is indeed very cool and inspirational! I'll be checking out out some of the other concepts that wiki page links to later.

Thanks for sharing, I would have otherwise never come across it.

I remember where I was when I read about the magic angle discovery.

It was exciting to sense almost immediately that the work would likely earn a Noble Prize one day.

Is this more of a "cool story bro" or is there something meaty to the press statement? I don't know much about topological properties in condensed matter that seem to be in vogue right now.

The press statement has all the "graphene is a miracle" parts that clash with the reality that graphene doesn't leave the lab much (has it left it even once?).

Superconducting, for instance: https://phys.org/news/2020-02-superconductivity-graphene.htm...

I do agree that the submitted link is light on proposed applications for this phenomena

This is like a standard university press statement for a standard high quality experimental article. It seems like the researchers have more experimental evidence of topological phases in Magic angle graphene and found some results which cannot be explained by existing theory. This paper has little impact for practical applications, but could lead to more discoveries in the interesting field of topological insulators, i.e., churn more papers.

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