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One of the most exciting materials developments of the last few years is totally twisted.
A closer look at graphene has revealed even more of its magical glory,
and taken it a step further…using SQUIDs.
And no, probably not the squids you're thinking of, I'll explain later.
We're talking, of course, about “magic angle" graphene.
First discovered back in 2018, it involves taking graphene, which is just a single sheet of carbon atoms,
and stacking two of those sheets on top of one another.
The twist?
The sheets are ever so slightly offset from each other, about 1.1 degrees.
And this “magic angle" gives the twisted graphene some really exciting properties.
The twist makes the graphene bilayer able to become either an insulator, blocking electricity flow,
or a superconductor, letting electricity flow with minimal loss of energy to resistance.
The discovery was huge news when it broke—I actually covered it a while back.
It started a whole new field called 'twistronics' (which is really fun to say),
that explores how the structure of 2D materials influences their electronic behavior.
And now, that same research team who twisted that "magic angle" into a graphene bilayer in the first place
has mapped the entire structure of "magic angle" graphene for the first time.
Some research has previously tried to get an up close look at "magic angle "structures like this
using a scanning tunneling microscope, but could only look at little chunks at a time—
like a few hundred square nanometers at most.
The scanning tunneling microscope approach just takes way too long—
it's kinda like trying to use a flashlight to count each of the seats in a really huge, dark sports arena.
See, in a magic angle graphene structure, there will be minute differences in how twisted it actually is
at any point in the structure—it won't always be exactly 1.1 degrees at every single point.
So, here's where the squids come in.
This team used a technique called a “scanning nano-SQUID.”
SQUID stands for Superconducting Quantum Interference Device,
which definitely takes the cake for most science-fiction-like name with the coolest acronym.
It's kinda complicated, but essentially, it works like this:
the nano-SQUID is made of two half circles of superconducting material
attached to the tip of scanning electron microscope.
When the "magic angle" graphene structure is exposed to a small magnetic field, currents ripple across it.
The structure itself, including the degree of its twist, determines how those currents behave.
The SQUID can measure those subtle changes in the currents and in this way,
map the exact angle of the graphene structure at any given point...
down to variations of .0002 degrees.
That's uhhhh...pretty precise!
So, the team used the nano-SQUID technique to map two different twisted graphene bilayers—
one with a smaller range of variation in its twists, and one with a larger range of variation.
The structure with the smaller range of variation showed more intense exotic physics properties
than the one with the broader range.
This observation is a key step to understanding how different twist structures
may change graphene bilayer behavior.
And if that weren't cool enough, the team also added another bilayer,
for a total of four "magic angle" layers.
Twists on twists!
This totally novel four-layer structure exhibited the same awesome versatility as just two layers—
the ability to alternate between insulating and superconducting.
But this time, the researchers were able to tune those capabilities—
by turning an electric field up or down, they could control what state the structure was in,
making it what we call 'highly tunable'.
Y'know, kinda like tuning a radio between stations,
but instead with "magic angle "superconducting graphene that exhibits exotic physics. Casual.
While twistronics is still a very new field, pretty far from being implemented in any real-world applications,
it's cool to fantasize about the devices that it might enable,
like superconductors that could function at high temperatures.
And lots of research is going into twisted graphene's "magic" properties,
and I'm pretty excited to see what comes next.
If you have no idea what I've been talking about for this whole video,
then you should check out the video we first made on this discovery when it came out.
If you have more materials science breakthroughs you want us to cover,
let us know down in the comments below, and subscribe to Seeker for all your exotic physics news.
As always, thank you so much for watching, and I'll see ya next time.
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The Secrets of ‘Magic’ Angle Graphene Are Now Fully Revealed

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Summer published on July 30, 2020
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