Be Smart

- It may look like there's just one pattern here, but there's actually two patterns just perfectly lined up.

When I give 'em a little twist though- - Wow!

- We see a completely new and amazing pattern emerge.

That looks so cool, I mean, I could do this all day.

But this same simple trick is now letting scientists do something that sounds impossible.

You take one material, you don't add anything to it, you don't mix it with anything, you can change what kind of material it is, and what it can do.

From something that conducts electricity like a metal to something that doesn't, like a perfect insulator, even something that behaves like a magnet, or even this revolutionary type of material that lets electricity flow with no resistance at all.

And thanks to that cool trick, we can do all of this in one material without changing a single atom.

The material that can do this isn't some exotic new chemical that costs like a million dollars a gram or something.

You can actually find it inside of a pencil.

And here's how simple it is.

You strip off two sheets of carbon, each just a single atom thick, you stack them, and then you simply give one of them a tiny twist.

And that carbon sandwich is capable of becoming all of these different types of materials.

And all of that happens thanks to that simple twist.

So how big is the twist?

45 degrees, 10?

Just 1.1 degrees is all it takes, not 1, not 1.2.

This magic angle is the secret that makes everything possible.

- We reapply quantum mechanics, and we're like, you know, sort of trying to be modern-day wizards.

- These twisted materials are challenging almost everything scientists assumed about why certain materials act the way that they do.

A whole new field of physics, all based on a simple twist.

So how can such a simple change lead to such mysterious effects and potentially world-changing applications all from a material that's so ordinary that you can find it in any pencil on your desk?

That's what we're gonna figure out today.

(intriguing orchestral music) Hey smart people, Joe here.

You may not know the name of what you're seeing here, but you're probably familiar with it.

It's called the moire effect.

It's why they tell you not to wear tiny little patterns when you're gonna be on camera, and we even find it in the world around us.

So what's going on here?

♪ When a grid's misaligned ♪ ♪ With another behind ♪ ♪ That's a moire ♪ You see what I did there?

It's a joke.

Well, the moire effect describes what happens whenever we have two repeated patterns that are just slightly offset.

The patterns interfere basically.

Some points on the pattern double up, and some of the gaps double up.

And what you're left with is this brand new repeated pattern.

It even happens in sound, too.

(constant beeping) If I play two notes that are exactly tuned together, well, I hear one sound.

But if I throw one of them slightly out of tune, (distorted beeping) we get this weird wobbly thing going on.

Those sound waves are interfering just like our patterns, sometimes adding up to make a louder sound, and sometimes canceling out to quieter moments.

Now that's a moire!

(hand whooshing) (glass shattering) (cat growling) But there's one important thing that makes these patterns useful and not just pretty.

Did you notice it?

Look how much bigger the new pattern is than that pattern that made it.

For these honeycomb layers, the new superpattern that we create, it's also a hexagon.

The bigger the twist, the smaller the superpattern.

A tiny twist, that makes a bigger superpattern.

And that is something you need to remember because it's one of the keys that unlocks this material's magic.

We live in a world full of 3D stuff.

This pencil's 3D, I'm 3D, probably.

But there's a whole field of physicists who play with what they call 2D materials, sheets that are so thin that they're only one-atom thick.

- So I think the thing that's most special about 2D is that we live in 3D.

You can move, charge in and out of a 2D system by moving it into the third-dimension.

Hi, I'm Allan MacDonald, I'm a Physics Professor at the University of Texas in Austin.

I came to understand that I liked science because the answers weren't known and were ambiguous.

And, you know, for a while, I went to the dark side and was interested in economics, but that was too vague for me.

(chuckles) And so, I came back to physics.

- Allan's favorite flat playground to mess around with is a material called graphene.

"Well, what's graphene?"

I'm glad you asked.

- [DJ] Hit it.

(groovy hip-hop upbeat music) - On the tip of any pencil, now that's graphite, that's basically a big chunk made up of these different sheets of carbon, all mish-mashed, pointing in different directions.

Of course, the graphite in this pencil isn't the only form that carbon can take, right?

So what makes this different from a diamond?

Well, graphite is soft, gray, and is an excellent conductor of electricity.

A diamond is the hardest material you can buy, it's transparent, and it blocks electricity completely.

Both are made of 100% carbon, and those carbons have the same number of electrons.

Basically every single atom in them is identical, except for one key difference.

The difference between a pencil and a diamond is how the carbons are stacked and arranged.

In a diamond, the layers of the carbon crystal are bonded to each other; but in graphite, it's layers of carbon sheets, just one atom thick, kind of just sit next to each other.

Graphene is carbon arranged in these honeycomb sheets.

It's essentially a two-dimensional crystal.

This is the repeating unit of that crystal.

And if we tie all those units together, we can create an infinite honeycomb.

On a microscopic scale, graphene is the strongest material we've ever measured, and it conducts electricity even better than copper, and you can make it at home with some simple ingredients.

Let me start by just rubbing some pencil on this paper here, make a nice dark stripe.

Now, in this graphite streak is a bunch of different single atom layers of carbon, but they're all stacked up in some mish-mash way.

So to find them, all I need is some Scotch tape.

I'm gonna stick this down on my graphite, rapidly peel up, and I'm gonna rip up a bunch of the paper, so I'm gonna have to do this again.

(screen beeping) I'm gonna start by putting it down on my stripe here, getting some of that on there, and rapidly lifting it up.

See, I've got some graphite on there, but it's still too many layers.

Stick my tape together, pull it apart.

It's gotten a little thinner.

Now I pull that one apart again.

And I just repeat this process over and over again, making ever increasingly thin layers of graphite until eventually... - [Narrator] Later... - I can't really see it anymore with my eye, but there are sheets of single-atom-thick graphene on this tape now.

(scoffs) I'm basically a wizard.

I'm not kidding, by the way, this isn't some cute analogy.

They literally do use Scotch tape to make graphene.

These guys even won a Nobel Prize for it.

Now, graphene is really special because of where its electrons are.

This is a model of a single-atom-thick layer of graphene.

These blue balls are all carbon atoms, and they're bonded to their neighbors here with these paired electrons.

But remember, carbon has four electrons in its outer layer.

So where's the other one?

Well, in a graphene layer, it's actually kind of floating around above or below this single atom layer in an electron sea.

Now where this is different from a diamond is that that would be bonded to the layer next to it.

Now this sea of electrons above the layer is the reason that electricity can flow through a single layer of graphene so well.

Electromagnetic energy can sort of surf along that electron sea, and it can do that at a significant fraction of the speed of light.

But those really fast electrons are a problem if you want graphene to do more interesting things.

And it turns out interesting things happen when we start stacking two layers of graphene together.

Oh, with a twist.

In 2011, Allan and his colleagues did some calculations and they made a bold prediction.

They proposed the existence of this magic angle at which the moire pattern will be perfectly suited to make that material act in a really weird way.

- We discovered that at certain twist angles, the electron waves would interfere as they move through, and that meant that the wave basically could not propagate.

- And that angle turned out to be 1.1 degrees.

- For us, it was a surprise.

- So, what's so special about 1.1 degrees?

Well, at this magic angle, notice that we've created a larger moire pattern.

In some places, the carbons sit on top of one another, and in others, carbons sit on top of holes in the honeycomb, and we have just about every arrangement in between.

Now, this larger moire pattern is an even bigger hexagon, a new repeating crystal unit, a superpattern.

And here is why.

When they stack the two sheets of graphene, they sit less than a billionth of a meter apart.

- Electrons can, you know, quantum-mechanically tunnel from one layer to the other.

- The sheets are stacked so close that electrons can hop between the layers millions of billions of times every second.

But if we rotate one sheet, those electrons become misaligned with the electrons in the other layer.

And at that magic twist angle, those hopping electrons are misaligned in just the right way, that they slow down almost to a stop.

- When they hop from layer to layer, and then it can hop back, and that's what happens at the magic angle, that the electron hopping back interferes with the part of the wave that didn't hop, and so, that other two waves cancel each other and the electron can't move.

In a single graphene sheet, the electron velocity is 300 times smaller than the speed of light, so 10 to the 6 meters per second.

But if you twist it to 1 degree, the electrons slow down so that they're only going, you know, a thousand meters a second.

You could almost see them move.

The way in which electrons nearly stop in graphene is really unusual.

- When electrons slow down, they can interact in these really strange ways, they can even cooperate with each other.

- One way to think of it is, if electrons are moving fast, then, you know, they're sort of by each other before they notice that they feel each other's propulsive Coulomb force.

But when they're moving slowly, then they're close together for a long time.

And in quantum mechanics, it turns out that when that happens, that means the interaction becomes totally dominant.

- [Joe] And that's when you can start doing interesting things- - Exactly.

(loud techno upbeat music) - It's like being at a loud party.

There are conversations happening all around you, but you can't hear them over the noise.

But then, turn the music down, (soft techno upbeat music) and all of a sudden, you can interact with people all over the room, and that's when a party gets interesting.

Drawing it up on a blackboard is one thing though, how do you actually twist two sheets of graphene when each is just an atom thick and basically invisible to that precise magic angle?

- So the key problem is this, that you could easily pick up a crystal and put it down on another crystal, but there was no way to know exactly what the orientation of either crystal was; you wouldn't know what you'd end up with.

- [Joe] So how do you actually take one single atom thick layer of carbon, pick it up, twist it by a precise angle, and lay it down on another?

Well, I visited a lab to watch them do exactly that.

- [Kenneth] This is not graphene, but this is a monolayer tungsten diselenide.

What I'll do is, I'll try to pick up this half- - Okay.

- And then I can rotate the stage and then pick up this half, and like, in-turn, stack.

If I focus onto the substrate, you'll see this thing is basically this thing.

And what I'll do, I'll try to pick up half of this.

And what you'll hopefully see is that the adhesive may contact with just part of the flake.

- [Joe] We're seeing the press, right?

- Yes.

- [Joe] Okay.

- [Kenneth] I'm only pressed into this half.

What I can do is I could very quickly peel off- - Okay.

- This half and tear it off while keeping this- - [Joe] You're gonna rip off the Scotch tape here, basically.

- [Kenneth] So what you see here, I've picked up half of this flake here, and the other half, down here, you'll see that- - Oh, so you ripped off the top, that's amazing.

- The other half is still here.

I can basically rotate the whole thing.

And now that it's rotated, I could basically place the one that I picked up onto here.

So right now I am moving the stage up so that you can see it come in focus so that hopefully- - [Joe] It's gonna splat it right on top, huh?

- [Kenneth] Exactly.

- [Joe] How do you unpeel it from your sticky finger?

- [Kenneth] Yes, so what I can do is I can use the temperature.

It gets less sticky as I increase the temperature.

At around 90 to 100 degrees in Celsius, of course, the flakes just don't stick anymore.

As I slowly take this temp off, now these flakes remain here.

And- - The moment of truth.

- [Kenneth] Yeah.

Let me focus the microscope a little bit.

And as you can see- - It's there.

- Just by the color of it- - Look at that.

- [Kenneth] It is staying there.

And so, now I have here a twisted bilayer that I've just stacked.

- Can we get some confetti for Kenneth, please, just the editing note?

(Kenneth chuckles) That was amazing.

Now, MacDonald's team predicted that this material would be special, but they had no idea just how powerful it would be.

- What I remember is Emmanuel showed me his, you know, PowerPoint.

animation of this trick, and I told him immediately, "Send me those slides."

(crew member and Allan chuckle) - Ah, our old friend, the periodic table!

What is it really that distinguishes two different atoms?

Well, technically, it's the number of protons in the nucleus, that's why we put them in that order.

But that's also usually matched by the number of electrons that are buzzing around the nucleus.

But when it comes to whatever properties something has, what physicists and chemists mostly care about is the number of electrons per atom.

That's what determines whether the material's a conductor that lets electricity flow, an insulator that blocks electricity, or a semiconductor can kind of do both.

Now to understand this better, let's look at a very simplified analogy.

Imagine materials are like parking garages.

Each car represents an electron, and each parking spot is one way that an electron is allowed to exist in this material.

Now each spot can only fit one car, but each spot comes as a pair, one for cars that are parked front ways, and one for those cool dudes who insist on backing in everywhere.

"Oh, well, why is that?"

Well, because that's the rules of this parking garage, and the rules of quantum physics.

Well, parking spots on the same floor, they cost the same to park in.

This whole garage together, how tall it is, how many spots it has on each floor, that represents every way that an electron can be inside a given material.

If we leave some spaces open and empty, the cars can easily move to another spot, right?

That's basically conducting electricity.

If all of the parking spots are filled and electron cars can't move in the material anymore, well, that's an electrical insulator.

But let's say we wanna take a conductor and turn it into an insulator.

Well, all we need to do is drive some more cars into the garage to fill up all those empty spots, right?

Thing is, in a normal crystal, there's a limit to how many new cars you can drive into the old atomic parking garage.

You know, it's something around one car per 1,000 parking spots, and that's not enough for us to fill all the gaps, and it means we can't change the atomic properties very much just by adding new electrons to our crystal.

But everything changes in moire materials.

Electrons, they stop paying attention to individual carbon atoms, and instead they treat the larger moire hexagon as a single giant atom.

- When you find two graphene sheets in their lowest energy state, the normal state is this alignment, where the layer behind is one of the atoms is in the middle of the hexagons of the layer above.

So, you know, what happens in a moire material, as you move on this longer length scale, the alignment up between the layers gradually changes from this one to this one and back to this one and so on.

And the electrons don't see the individual atoms, they only see the change in this pattern as they move through the system, and that's why it looks like the size of an atom is the size of the moire period.

- They're called superatoms, and they're made of 10,000 or so individual carbon atoms.

- So when you make the moire materials, it's like having a parking garage where the size of the floors has been reduced.

So the number of states available on each floor is limited.

So you can easily have the cars move from spot to spot and move around that floor, if you like.

- [Joe] Suddenly we don't need to push many electrons around to change how the material behaves.

Just connect that twisted graphene to just a few volts, bam, our parking spots are full; it's suddenly an insulator.

Turn the dial back, bam, it's a conductor again.

Turn it to a different setting, oh, snap now it's a magnet?

- So you can do experiments, which with real crystals would require you to grow a whole bunch of different crystals out of different elements from the periodic table.

Instead, you could do the same thing by taking one material and just changing the voltage.

- This is pretty spooky.

The material didn't change, it's all just still carbon, but just a few volts and a twist, and we made it behave like totally different stuff.

All of this fell in line with what MacDonald had predicted in 2011, but then something happened that he hadn't dared to imagine.

The twisted graphene turned into a superconductor.

Superconductivity was first discovered in 1911.

A Dutch physicist was cooling down a chunk of mercury and watching its electrical resistance, basically how hard it is for electrons to go through it.

The resistance dropped and dropped.

And then around minus 269 degrees Celsius, the resistance hit zero.

The electrons could suddenly glide freely around.

Electrons don't bounce around like pinballs; we're in the quantum age, folks, you gotta start thinking of these things as waves.

Electricity flowing through a wire is like a wave of electron energy propagating from one end to the other.

How difficult it is for that wave to flow is what we call resistance.

- You know, the rules for friction, which is electrical resistance, are different than in a classical fluid, they're determined by quantum mechanics.

Quantum mechanics is very favorable because this has many types of disorder, in particular, the crystal itself doesn't slow the electrons down.

- [Joe] What slows electrons down is atoms vibrating, and the electrons sort of trip over those vibrations.

When the resistance is high, more of the energy of their movement gets radiated out as heat.

When you render a big YouTube video on your laptop, it gets hotter, right?

When you run electricity through a space heater full of wires that have high resistance or the "electrical friction," that generates enough heat to even keep you warm in winter.

But superconductors somehow can transmit electricity without losing any of that heat.

We have some ideas for how this happens at these very, very low temperatures, and that involves electrons pairing up and forming a sort of atomic force field, which allows them to skate through the material without any of that friction.

But the thing is, we also know that that can't be the whole story when it comes to superconductors, 'cause scientists have also seen superconductors that work at higher temperatures, up to minus 140 degrees Celsius and at room pressure.

Yes, that is still insanely cold, but it's considerably warmer than a few degrees above absolute zero, which is where the first superconductors operated.

So those warmer superconductors can't be operating on that same force field principle, there must be something else responsible for high-temperature superconductivity, and, frankly, we don't know what that is yet.

But even though we don't know how these warmer, but still very cold superconductors work yet, some scientists think that twisted graphene sandwiches could be the perfect place to finally crack the mystery.

Guess if you're looking to win a Nobel Prize, it sounds like a decent place to start.

Imagine a world with room temperature superconductors and what that could do; power lines lose nothing on the way to your PlayStation, MRIs that don't need to be bathed in liquid helium, floating trains, flying cars.

This is the future that our comic books promised us.

Well, maybe not those last two things, but how about quantum computers?

- For building superconducting quantum computers, it's certainly my belief that ultimately we'll use moire materials to do that 'cause it's just too natural and too perfect as a natural way of making arrays of superconductors where you can apply a little gate voltage and turn some regions of a device into insulators and some region into superconductors.

- And it all starts here.

Sure, there's a few steps in between and some big machines and some extremely precise physics in there, but this is absolutely mind-blowing to me; to get these futuristic new materials, we're not digging them out of the ground, we're not melting other materials and mixing them together.

- Yes, we are alchemists, that's what we are in these so-called moire materials, as I like to call 'em.

It really is a kind of alchemy.

We can do alchemy even though all of the atoms really underneath it are not changing, but it's as if the properties... It's if the atoms are changing.

- [Joe] You worried the inquisition is gonna show up at your door.

(both chuckling) - Well, nowadays, one does- - You never know.

- (chuckles) Never know.

- The thing is, the moire isn't just a pretty pattern, it isn't an illusion, it's a real physical alignment that lets these sheets of carbon really truly act like a new material.

It makes you think twice about what really makes something something.

It's more than just what it's made of.

At the end of the day, taking a new angle on things really can make all the difference.

Stay curious.