We’re in a heatwave here, so er I’m in the woods.
Hello everyone this is the map of superconductivity, where, as ever, I’ve broken down all the
important parts of the subject into a big picture to get you up to speed quickly and
as clearly as possible.
Superconductors are materials which, when you cool them down to a low temperature, they
lose their electrical resistance.
They also have some interesting magnetic properties which allow them to almost magically float,
but it’s not magic, it’s just plain old quantum mechanics.
We’ll look at the different kinds of superconductors, their properties, the theory behind them,
their applications in the real world, and the future avenues of research and technology.
And as we go, see if you can keep count of all the nobel prizes.
First I need to tell you about magnetic induction.
If you have a conducting material, which is a material that has electrons that can move
around freely, like a piece of metal at room temperature, and you move a permanent magnet
near to it, these electrons feel this changing magnetic field, they feel a force from it,
and start moving in a circle called an eddy current.
This is called magnetic induction.
In a normal metal this current dies away quickly because the material has electrical resistance:
the moving electrons bang into the atoms and stop moving, giving up their energy to vibrations
in the atomic lattice warming it up slightly.
But if you do the same thing to a superconductor, because they have got zero electrical resistance,
the eddy current will never stop flowing and will carry on circulating forever.
Like, age of the universe forever according to the theory, and experimentally we’ve
seen currents losing no energy over twenty five years.
Zero electrical resistance also means you can pass a direct current through a superconductor
without it losing any energy at all.
That’s cool, but they have another important property to do with magnetic fields called
the Meissner effect.
Superconductors expel any magnetic field inside them.
You know how I said that magnetic fields induce electrical currents in conductors.
Well the opposite is also true, any electrical current creates a magnetic field.
If a superconductor is in a magnetic field, it gets a load of eddy currents which create
their own magnetic fields that exactly cancel out and expel the original magnetic field.
This is a quantum effect and doesn’t happen in normal conductors.
So those are the two main features of superconductors, zero electrical resistance and the Meissner
effect.
And when they were discovered in the early nineteen hundreds physicists were like whoa!
and then since then they have investigated more and made a load of useful technology
out of them which we’ll look at in a bit.
But first we need to look at exactly what conditions are needed for a superconductor
to superconduct.
There are three conditions you need for a superconductor, low temperatures, small enough
magnetic fields and small enough electrical currents although these two are kind of the
same thing.
The specific temperature and magnetic field that breaks superconductivity depends on the
material.
The first superconductors that were studied were pure elements like mercury, aluminium
or niobium, and physicists discovered that not all of the elements superconduct, here
are the ones that do.
For each material as you cool them down they undergo a sharp transition temperature where
they suddenly start superconducting at a sharp phase transition.
Then when they are in the superconducting state if you apply a larger and larger magnetic
field or larger and larger current they have a critical field or critical current where
they suddenly stop superconducting and go back through the phase transition to a normal
conductor even if they are below the transition temperature.
Even if all of this is new you already know about phase transitions, because this is what
happens to water when it freezes or when it boils.
These are phase transitions too, but those are phase transitions in the material properties,
whereas the superconducting phase transitions are transitions in the electronic properties.
But it all comes down to what is the configuration of stuff which minimises the overall energy,
known as the gibbs gree energy.
Ifa material will be in a lower energy by freezing into a solid, or turning into a superconductor,
that’s what it will do.
Anyway, as any fan of physics knows, when we have phase transitions we’re gunna have,
say it with me, phase diagrams!
These are very handy graphs because they show us what state your thing will be in when you
change some global parameters.
This is a phase diagram for the state of water when you change temperature and pressure which
you might have seen before.
And here is the analogous example for the superconducting state and normal conducting
state for different temperatures and magnetic fields of a superconductor.
This also changes with pressure as well, but I’m not plotting that here because it’d
need a 3D plot, and superconductivity at pressure is a bit of a research niche, the vast majority
of superconductors are used at normal pressure.
Each superconductor has got its own unique phase diagram and filling them in kept a bunch
of experimental physicists happily busy for years as they filled in all the points.
And they didn’t just stick to the elements, they started looking at compound materials,
of which there are an infinite amount, and the search for new superconductors has been
going on ever since in an attempt to find materials with higher and higher transition
temperatures with the goal of one day finding a room temperature superconductor and making
tonnes of cash.
Here’s a plot of the discovery of higher and higher temperature superconductors, and
along the way they have made some startling discoveries.
First of all, in 1935 they discovered type-II superconductors which behave differently to
the superconductors I’ve described so far, which were then called type-I superconductors.
Type-II superconductors behave differently to type-I superconductors when they are in
a magnetic field.
Here’s a phase diagram of a type-II superconductor.
Down here it still behaves just like a type-I superconductor, but then they have an intermediate
state where they do let the magnetic field penetrate them, but only in specific points
in a formation called a magnetic field vortex, or vortices for short.
Here the bulk of the material is still superconducting, but at these thread like vortices the material
is a normal conductor, the magnetic field goes all the way through in a certain small
amount called a magnetic flux quanta, and each one is surrounded by a swirling supercurrent,
which is the name for the electrical current in a superconductor.
I just threw a load of terminology at you there, so if you are confused just think of
them as a load of sausages.
And you’ll be pleased to hear that these are the only two kinds of superconductors
we’ve found so far.
There is a kind of type-1.5 superconductor, but I can’t be bothered to talk about them
here.
Wikipedia.
The next big bombshell to hit the exciting world of superconductivity research was in
1986 when researchers made a superconductor made of ceramic which was an insulator before
being cooled down.
This then led to the discovery of a bunch of high-temperature superconductors.
Although high-temperature is a little bit of a misnomer, because they still have to
be cooled way below zero with cryogenic liquids but now you can make a thing superconduct
with just liquid nitrogen at 77 kelvin, which is way cheaper than liquid helium which is
what they had to use before.
These new superconductors are known as the cuprates because they contain layers of copper
oxide, with a load of other stuff in there as well.
And there’s also many other types of superconductor iron based superconductors called the pnictides,
pure carbon based superconductors called fullerenes also known as organic superconductors and
there is others as well, but you get the picture.
People have also squeezed materials between diamond presses and discovered that many materials
start superconducting when they are at very high pressure and recently in 2020 a room
temperature superconductor was discovered called Carbonaceous sulfur hydride which superconducts
at 15 celsius, but only under a huge amount of pressure squeezed in a diamond press, so
it’s not actually practically useful, but still a landmark discovery.
Now on to the theory of superconductivity.
It was not an easy job to figure out the underlying theory of what is actually going on in superconducting
materials at the scale of the electrons.
What is the underlying process that allows for zero resistance?
The first theory, Ginzberg-Landau theory predicted properties of superconductors like, which
would be type one and type two, but this was superseded by a microscopic theory called
BCS theory, named after the initials of the creators.
They figured out that, as electrons move through the lattice of atoms, they attract the atoms
around them slightly, creating a local positive charge which creates an attractive force between
electrons.
So electrons can interact with each other through vibrations in the lattice called phonons
and this allows them to pair up into a composite entity known as a Cooper pair.
Now, if you’ve watched my map of particle physics you’ll know that electrons are spin
half particles, and obey the pauli exclusion principle so can’t exist in the same quantum
states.
But the cooper pairs behave like bosons, because when you add their spins together you get
spin zero or spin one.
Bosons can exist in the same quantum state and so all the cooper pairs form a thing called
a condensate which has special properties including not being able to easily absorb
kicks of energy, and so they flow without resistance.
This is known as an energy gap.
That’s just a very quick explanation because a full description would take a long time,
but now you’ve got the basics.
What is interesting is this theory doesn’t explain high temperature superconductivity
because the attractive force from the phonons don’t exist there.
So they need some other attractive force between electrons, and we don’t know where that
would come from in high temperature superconductors.
For this reason any superconductor that doesn’t follow BCS theory is called an unconventional
superconductor, and the ones that do are called conventional superconductors.
Despite a lot of work going into this high temperature superconductivity is still one
of the biggest unsolved mysteries in theoretical condensed matter physics and you’ll probably
win a Nobel prize when you figure it out.
There are a load of technologies which use superconductors.
The most widespread use of superconductors is to create large magnetic fields as you
can circulate a lot of current in a loop without burning any energy.
This is what the big tube is in MRI machines, that is basically coils and coils of superconducting
material that is cooled down with liquid helium.
Superconducting magnets are also extensively used in particle accelerators to bend and
focus the beams of particles.
They have been used in some tokamak reactors to control the plasma in the nuclear fusion
process.
And also in other areas where you want to control charged particles like mass spectrometers.
There are also many kinds of quantum devices which use superconductors.
The most useful are josephson junctions, which are small gaps between superconductors where
the cooper pairs can still flow across the gap through quantum tunneling, and so you
get a continuous flow of current even with no voltage applied.
You can use this to set up superpositions of current, and therefore superpositions of
magnetic field, where the current and magnetic field are in the superposition state of flowing
in both directions at the same time.
These josephson junctions can be combined in a specific formation called a superconducting
quantum interference device or squid for short, which is a phenomenally sensitive magnetic
field detector, the best humanity has discovered.
These are the detectors that see inside your body in MRI and fMRI machines, they are also
set the voltage standard in fundamental physics, are used as efficient radio frequency antennas
in research and in mobile phone masts, and you can use the superposition of states created
by the josephson junctions to make a tunable qubit: which are the building blocks of superconducting
quantum computers.
This stuff is my professional background back when I had a proper job, and I think quantum
technology is absolutely fascinating, especially the possible future technologies.
So what does the future hold for superconductors?
For a long time people have talked about building transmission lines to transport electricity
through superconductors to significantly reduce the amount of energy that is lost.
But to be cost effective it’d need to be a superconductor that you don’t need to
cool down very much, which can carry high currents and is strong, we don’t have this
yet, so these are the challenges, but it would be cool to be able to ship electricity around
the grid a lot more efficiently.
You can also use superconductors for levitating things like trains, but this potential has
been around for a while so I’m not sure there is a great need for it, but you know,
it’s a thing that exists.
A promising area is to make very efficient superconducting motors or generators for things
like wind turbines which could potentially decrease the cost of the electricity they
generate, so this would be really cool, to make renewable energy even cheaper.
On the quantum devices side, by far the most exciting applications are the range of quantum
computers built with superconducting qubits.
Now, superconductors aren’t the only way to build quantum computers, but google, IBM,
D-Wave and others have built very advanced superconducting quantum computers which could
potentially be used to understand and find new superconducting materials by simulating
the quantum mechanics of them, something that can’t currently be done with our most powerful
supercomputers.
So you could potentially use superconducting qubits to figure out how high temperature
superconductivity actually works and then perhaps use that to find a room temperature
superconductor.
In fact, on the research side, figuring out the mechanism of high temperature superconductors
and whether a room temperature and room pressure superconductor can exist, would be fantastic.
If we had a room temperature superconductor it could potentially revolutionise all electronics
from the power grid, to consumer electronics as you’d be able to build zero resistance
computers with way lower electricity consumption.
But this all depends on the room temperature superconductor also having a high critical
current, and critical field, as well as having material properties that make it easy to work
with to fabricate chips.
Okay so that’s the map of superconductivity.
I hope it was informative.
How many Nobel prizes did you count?
It should have been 5, which is not bad for one quantum phenomenon, which one of you is
gunna get number six?
I’ve made this map available as a digital image on flickr, and poster on my DFTBA store
and if you liked this video please remember to smash the patriarchy.
Especially in physics.
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You are all helping me keep making educational content which is free for anyone to watch,
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So thank you so much and I’ll see you on the next video.