Some people argue that the biggest challenge facing the human race right now is the question of how to generate electric power in a sustainable and economic friendly way. A more pressing problem, however, may be to avoid the large amounts of electricity lost to transmission everyday. Imagine a loop of wire that can have an electric current flowing through it indefinitely with no power source. Forever. No recharge. No top up. Sounds impossible? Not if that wire is a superconductor. Don’t be fooled by the name, this isn’t sci-fi, it’s not some sort of magic, it’s a very real counter-intuitive physical phenomenon provided by quantum mechanics: superconductivity.
Everyday, we are used to being able to switch on the light whenever we want to. Where does this energy actually come from? It’s sent to our homes from power plants far away using electrical transmission lines, usually made from copper. During this transmission however, around 10% of the energy is lost in the form of heat. Yes, that’s right. 10% of energy we generate is only good for heating cows, sheep and any other living organism unfortunate enough to be near the transmission line. Thus, power plants amp up current to high voltages when transmitting it across the country to overcome energy lost.
In the heat of the moment
So why does all this wastage happen? It’s due to the effects of electrical resistance. Electrical current consists of electrons flowing through the wire. These electrons interact with the atoms in the wire, leading to a dissipation of the electrical energy to heat. Unfortunately, the electrons that carry electricity through the material can’t simply sail along. They are scattered about by all kinds of obstacles – impurities, defects, and vibrations of the material’s crystal lattice. This leads to the concept of electrical resistance.
A great beneficial example of this are old-fashioned light bulbs, where so much current is pumped through a little wire that it starts to glow and radiate heat – producing light, whenever you switch the button.
So what is a superconductor?
A superconductor has zero resistance. No, not very little, or almost zero – but exactly, mathematically and absolutely none: zero. This means that theoretically, electricity can flow through a loop of superconducting material forever with no power source and no energy loss.
The first person to observe superconductivity was the Dutch physicist Kamerlingh Onnes in 1911, when he was studying the electrical behaviour of liquid mercury at very low temperatures. Temperatures so low, in fact, that they make freezing seem boiling hot. Everything was as expected, until he cooled the material below 4.2 K, where he observed that the resistance suddenly dropped to zero.
Hold up. Sounds too good to be true. What’s the role of temperature?
Just when you thought superconductors were having their cake and eating it too, as usual, there’s a catch. Temperature is crucial in superconductivity: it only works when it’s really cold. In superconductivity, instead of using Celsius or Fahrenheit, we measure temperatures in degrees Kelvin. 0 K is the point of absolute zero: it is physically impossible to attain a lower temperature. Measured in every day units, this is -273 °C or -460 °F.
As you can see, 4.2K, corresponding to -269 °C, is really damn cold. Each superconducting material has a critical temperature, below which it is able to act as a superconductor. This is because superconductivity requires the existence of a long-range order at a quantum mechanical level, and to achieve this, we need to freeze out all motion of the atoms in the material.
But how does the resistance go away?
Right, time for a bit of physics. This is where things might get a bit tricky. A basic rule of Physics is that opposites attract and like charges repel: electrons repel electrons, but attract protons. In superconducting materials, a special process allows attraction between electrons, due to their interaction with the atomic crystal lattice of the material.
In metals, periodic distortions of the lattice form excitations called phonons. These phonons essentially behave like quantum mechanical particles. In superconductors, two electrons can attract each other via the exchange of a phonon, which acts as an attractive force between them.
The attraction between the electrons allows the formation of tightly bound electron pairs, so-called Cooper pairs, named after Leon Cooper, who first came up with this baffling idea. The carriers of charge and energy in a superconductor are therefore not electrons like in normal conductors, but rather these Cooper pairs.
What makes these Cooper pairs so special?
These Cooper pairs again act as a single particle and have very different quantum-mechanical properties compared to single electrons: they are classified as bosons. A special property of bosons compared to single electrons is that they can collectively occupy a ground state, which is very low in energy. If an impurity or an obstacle wants to get in the way, it would have to scatter the Cooper pair into a different energy state.
This is prevented by an energy gap between the ground state and all other states, meaning that impurities are unable to scatter the Cooper pairs and they can move freely at zero resistance. Think of them as forming a team cooperating to avoid obstacles – two is always better than one…
All this can only happen at low temperatures, because the contribution of phonon interactions only becomes relevant when the thermal vibrations of the crystal structure are very small.
Can we make superconductors at room temperature? Unconventional superconductors
The theory of Cooper pairs explained above is the Bardeen-Cooper-Schrieffer (BCS) theory of conventional superconductivity. The connection between Cooper pairs and superconductivity was first understood by these three physicists in 1957, almost half a century after the first observation by Onnes. For a long time, the microscopic theory of superconductivity was one of the greatest unsolved problems in physics.
After the publication of the BCS theory, many assumed that superconductivity was solved – and also predicted that the theory implied that superconductivity was impossible above 30K, ending any fantasies of high temperature superconductors.
But in 1986, a German-Swiss collaboration observed superconducting behaviour in an exotic lanthanum-based alloy at 35K. This opened up a new field: unconventional superconductivity. However, whilst providing no resistance to electrons, these superconductors have provided an astonishing amount of resistance to being explained by scientists. The dynamics of these conductors is not explained by BCS theory and in fact there is no general theory to this day.
SO COOL, in all senses of the word. But what can you actually do with them?
We’ve established that superconductors are exactly that: super efficient conductors. And in the real world, there are plenty of useful applications for materials with zero resistance.
The most obvious use of superconductors is to replace the lossy national grid with slick, lossless superconducting wires, resulting in enormous energy and cost savings. And it’s not just the transportation they’re useful for, but also generating the electricity itself: replacing the usual copper wire generators with superconducting magnets could increase efficiency from 50% to a whopping 99%.
But of course, this is all somewhat limited by the temperature aspect. Cooling miles and miles of superconducting grid is quite the impracticality, so in the meantime, what else can be done with superconductors?
Superconductor electromagnets give us the ability to magnetically levitate big objects such as trains. That’s right, actual levitation. We kid you not! This super futuristic idea, seemingly ripped straight out of a scene of Star Wars, could eliminate the losses due to friction between the train tracks and wheels.
This technology actually already exists in our lives, and goes by the name of MAGLEV. It is the science behind Japan’s incredible Yamanashi Maglev test line that achieved speeds of over 600 km/h – that’s nearly twice as fast as Harmony, the previous world leading speed train.
What about superconductors for better MRI scanners? Korean superconductor group KRISS have developed a technology using superconductor derived magnetic fields that is able to provide a better resolution image from inside a body than an MRI scan and without the need for strong magnetic fields.
And finally, nothing gets a particle physicist as excited as a technological advancement in the field of superconductors because their favorite toys, particle colliders like CERN’s Large Hadron Collider, are able to accelerate particles to speeds closer to the speed of light before having them smash each other into smithereens.
In a world where efficiency is everything, superconductors offer one of the few 100% efficient solutions.