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. Massive shout out to anyone who has bought
a poster and to my wonderful patreon supporters. You are all helping me keep making educational
content which is free for anyone to watch, and also helping me to decouple myself from
the fickle youtube algorithm so that I can concentrate on making killer content, that’s
also bloat free. So thank you so much and I’ll see you on
the next video.
