Reported by Alfie Mussett (13Y) and Pameer Saeed (13O)
Edited by Mrs Hare
Before we grapple with superconductivity, we need to know what ordinary conductivity is.
The basic facts about electricity
An atom is made up of a nucleus containing protons and neutrons with electrons orbiting the outside. As the electrons get further from the nucleus they are held more weakly and in some elements such as metal copper they can become free.
If a power supply is connected across the metal then the electrons flow and form an electric current.
Electrical conductivity measures a material’s ability to conduct an electric current. It is commonly represented by the Greek letter σ (sigma).
Electron flow is often pictured as water flowing through a pipe but this is a poor analogy. A better one is to imagine the electrons as a swarm of angry bees. They actually travel very slowly with a drift velocity of −0.00029 m/s.
Electrons are always colliding with their surroundings resulting in a great deal of heat being produced and this why your computer and other electrical equipment get very hot. Ideally we want to stop this heat being produced. The heat can have a detrimental effect on the equipment and causes a decrease in the current flow (Copper’s resistance increases with increasing temperature).
Joule heating, also known as ohmic heating and resistive heating, is the process by which the passage of an electric current through a conductor releases heat.
The above picture shows Dr Steele demonstrating ohmic heating in iron. Lots of bright sparks.
Tungsten in old fashioned light bulbs relies on ohmic heating to produce the light. Ohmic heating is also used to heat water in kettles and cook food in electric ovens.
Unfortunately most ohmic heating is wasted heat.
To reduce this problem and increase conductivity we need lower temperatures.
But what are lower temperatures?
Decreasing the temperature of things can have unusual effects.
A balloon deflates when it is put in a Dewar of liquid nitrogen and Dr Steele could fit in two balloons when previously only one would fit. If there had been enough time he could have fitted one hundred balloons in the Dewar.
Rubber loses its flexibility when it gets very cold. The vapour seen in the above right picture is caused by water vapour condensing to water because of the decreasing temperature due to the presence of the liquid nitrogen. This effect stops when everything reaches the same temperature.
Very cold rubber also becomes very brittle and breaks into pieces when hit with a hammer.
Liquid nitrogen is quite easy to liquefy but the lowest temperature that can be obtained using it is 77K. Helium is a better choice for cooling but it is much rarer (It was first discovered in the Sun) and more expensive and difficult to liquefy. Getting the temperature down to 4.2K is an incredible feat.
Resistance of mercury
You would think that there would be a linear relationship between resistance and temperature.
In fact a Dutch physicist Heike Kamerlingh Onnes discovered that at the temperature of 4.2 K the resistance of mercury abruptly disappeared. In the same experiment, he also observed the superfluid transition of helium at 2.2 K, without recognizing its significance.
21 September 1853 – 21 February 1926
He was awarded the Nobel Prize in 1913 for his discovery.
In superconducting materials, the characteristics of superconductivity appear when the temperature T is lowered below a critical temperature Tc. For mercury this is 4.2K.
Highest elemental Tc is niobium 9.25 K. Lowest lithium, 0.4 mK!!
Many of the elements in the periodic table superconduct if you cool them down enough …and if you apply pressure or look at thin films, even more stuff does! Ironically good metals don’t superconduct: copper wiring, gold jewellery, silver pins…nothing.
A magnet levitating above a high-temperature superconductor, cooled with liquid nitrogen. Persistent electric current flows on the surface of the superconductor, acting to exclude the magnetic field of the magnet (Faraday’s law of induction). This current effectively forms an electromagnet that repels the magnet.
The Meissner effect is the name given to the expulsion of the magnetic field from the superconductor during its transition to the superconducting state. The German physicists Walther Meissner and Robert Ochsenfeld discovered the phenomenon in 1933 by measuring the magnetic field distribution outside superconducting tin and lead samples.
Normally electrons, being negatively charged, should repel each other. In condensed matter physics, a Cooper pair or BCS pair is two electrons (or other fermions) that are bound together at low temperatures in a certain manner first described in 1956 by American physicist Leon Cooper. Cooper showed that an arbitrarily small attraction between electrons in a metal can cause a paired state of electrons to have a lower energy than the Fermi energy, which implies that the pair is bound. In conventional superconductors, this attraction is due to the electron–phonon interaction. The Cooper pair state is responsible for superconductivity, as described in the BCS theory developed by John Bardeen, Leon Cooper, and John Schrieffer for which they shared the 1972 Nobel Prize.
Leon Cooper born February 28, 1930 John Bardeen May 23, 1908 – January 30, and John Schrieffer 1991 born May 31, 1931
Electrons distort the lattice, leaving behind a trail of positive charges they’ve attracted towards themselves. ‘Cooper pairs’ of electrons, bound by the mutual attraction to the positive trails, are immune to scattering. Distorting the lattice makes moving through a metal something like wading through treacle: this is why good metals, whose electrons are not wading through treacle, are bad superconductors, because the treacle is the essential glue which binds the electron pairs together.
Coherent state, like a ballroom dance rather than a disco. A quantum waltz. The electrons never collide with anything.
BCS theory—discovered 1957, Nobel prize 1972.
The greater the number of electron cooper pairs the smoother the current flow.
Quantum field theory can explain cooper pairing and the Higgs field.
In theoretical physics, quantum field theory (QFT) is a theoretical framework for constructing quantum mechanical models of subatomic particles in particle physics and quasiparticles in condensed matter physics, by treating a particle as an excited state of an underlying physical field. These excited states are called field quanta. For example, quantum electrodynamics (QED) has one electron field and one photon field, quantum chromodynamics (QCD) has one field for each type of quark, and in condensed matter there is an atomic displacement field that gives rise to phonon particles. Ed Witten describes QFT as “by far” the most difficult theory in modern physics.
BCS theory seems to explain why good metals are poor conductors. Electrons have to be able to distort the surrounding lattice and copper, silver, and gold can’t do this.If you put in typical values, you get no Tcs > 30 K or so. Niobium (Nb) has the highest elemental Tc = 9 K (−264°C) at ambient pressure. Lithium is rubbish.
Luckily, there are more than the elements…
Cuprates: first, BaxLa5-x which was 30 K, got its two discoverers at IBM a Nobel prize within a year. Then, things went crazy with Paul Chu’s discovery of YBCO: both lN2 temperature and BCS shattered!
Copper-oxide superconductors were discovered in the late 1980s and have a relatively high transition temperature compared with past alloys.
On submission to PRL, Paul Chu substituted Yb for Y. (Very similar names because named after the Swiss town of Ytterby, which is near a large quarry where many of the rare earths were first discovered…yttrium, ytterbium, erbium and terbium are all named after it.) He maintains genuine error. Several Yb papers did come out, and it is a superconductor, but not as good as YBCO!
In 2000 Iron-arsenide superconductors were discovered.
We still don’t really how cooper pairs stick together.
Why do we want room temperature superconductors?
This would decrease energy losses from 10% to 3%, and underground cables can be much smaller, even requiring less insulation! Therefore they are more efficient and cheaper to make.
Power cables at Long Island NY
Large Hadron Collider
The blue cylinders are the bending magnets. There are super conducting cables producing the magnets.
These superconducting cables are less expensive than copper wires despite the need for liquid helium and hydrogen.
Until recently have wires and cables been possible as ceramic magnets can’t be drawn into wires.
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI), nuclear magnetic resonance imaging (NMRI), or magnetic resonance tomography (MRT) is a medical imaging technique used in radiology to visualize internal structures of the body in detail. MRI makes use of the property of nuclear magnetic resonance (NMR) to image nuclei of atoms inside the body. MRI can create more detailed images of the human body than are possible with X-rays.
MRI scanners. There is a coherent quantum field but the process requires superconductors. 3MW of power required.
If anyone’s ever had an MRI scan, you’ve been inside a coherent quantum state four degrees above absolute zero.
Nuclear fusion powers the Sun.
On Earth fusion could be used to produce several years of electricity. The raw materials are deuterium and lithium which form tritium.
The Joint European Torus (JET) investigates the potential of fusion power as a safe, clean, and virtually limitless energy source for future generations. The largest tokamak in the world, it is the only operational fusion experiment capable of producing fusion energy. As a joint venture, JET is collectively used by more than 40 laboratories of EURATOM Associations. The European Fusion Development Agreement, EFDA for short, provides the work platform to exploit JET in an efficient and focused way. As a consequence more than 350 scientists and engineers from all over Europe currently contribute to the JET programme.
The operating temperature is 150,000,000°C. Plasma can’t be stored by conventional methods so superconducting magnets need to be used.
In the UK we spend £2200 per person per year on energy and only £1.20 is spent on fusion research per person per year.
The common comment about nuclear fusion:
Hasn’t that been 30 years away for the last 30 years?
Hasn’t that been 50 years away for the last 50 years?
Hasn’t that been 30 years away for the last 50 years?
Well we might solve this if every person in the developed world gave £50
Science is woefully underfunded compared to the scale of the problems it’s trying to solve.
SCMaglev at a test track in Yamanashi Prefecture, Japan, in November 2005
Maglev (derived from magnetic levitation) is a method of propulsion that uses magnetic levitation to propel vehicles with superconducting magnets rather than with wheels, axles and bearings. With maglev, a vehicle is levitated a short distance away from a guide way using magnets to create both lift and thrust. High-speed maglev trains promise dramatic improvements for human travel if widespread adoption occurs.
The above pictures show Dr Steele demonstrating the Maglev effect.