The Meissner effect is responsible for the incredible ability of superconductors to levitate above magnets – or indeed for magnets to levitate above superconductors.
Read more: “Instant Expert: Superconductors”
Exactly 100 years ago, a Dutch physicist called Heike Kamerlingh Onnes and his team made a remarkable and completely unexpected discovery. They found that certain metals completely lose their electrical resistance when cooled to within a few degrees of absolute zero. A coil of wire made from such a metal could carry an electrical current round and round forever, without needing a power source to drive the current. No one had predicted this phenomenon and at the time no one could explain it. The effect was named superconductivity – the prefix “super” implying that it was in a completely different league from anything seen before. It took more than half a century to figure out how superconductivity might work and how to make it useful. More recently, though, we have come to realise that we understand the phenomenon far less well than we thought.
Advertisement
The big discovery
spent most of his scientific career in the quest to achieve the lowest temperatures possible. His big breakthrough came in 1908 when he managed to make liquid helium, the last of the gases to be liquefied because it requires temperatures as low as 4 degrees above absolute zero – the lowest possible temperature, equivalent to 0 kelvin or -273.15 °C.
Kamerlingh Onnes was one of the first people to really understand that advances in low-temperature physics critically depended on having first-rate technicians, expert glassblowers and skilled craftspeople to build and maintain the delicate equipment. It was not enough to potter around alone in a ramshackle laboratory, as many of his contemporaries did. For this reason Kamerlingh Onnes’s team at the University of Leiden in the Netherlands beat physicist James Dewar at the Royal Institution in the race to make liquid helium.
Once he had the technology to achieve the lowest temperatures available anywhere, Kamerlingh Onnes started exploring how matter behaves at low temperature. One of the questions he wanted to answer was how the electrical resistance of a metal changes as it approaches absolute zero. Experiments had shown that resistance falls as you cool from room temperature, but what happens when you get really cold? Some had speculated that resistance would fall steadily to zero, others that it would rise sharply as the electrons froze, or even simply flatten off at a constant value.
The first studies indicated that it flattened off, but Kamerlingh Onnes realised that this was actually down to the presence of impurities in the metal causing electrons to scatter. He needed a metal that was really pure and so he chose to focus on mercury, a liquid at room temperature that could be repeatedly distilled to make it as pure as possible. To make wires of mercury, his technician filled very narrow U-shaped glass capillaries and then carefully froze them. The capillaries had electrodes at either end so it was possible to pass a current through them and measure the resistance at various temperatures.
On 8 April 1911, Kamerlingh Onnes and his team observed that the resistance of the mercury wire suddenly disappeared below around 4 kelvin (see graph). They had discovered superconductivity. Subsequent experiments showed that mercury would behave this way even at lower purity, and that some other metals exhibited superconductivity, including tin and lead.
Levitation
Superconductivity has another trick up its sleeve besides the loss of electrical resistance. In 1933, German physicists Walther Meissner and Robert Ochsenfeld performed an experiment at Germany’s national measurement laboratory in Berlin to study what happens to magnetic fields near a superconducting material as it is cooled.
Magnetic fields are normally quite content to pass through any material, but as soon as the temperature fell low enough for superconductivity to occur, Meissner and Ochsenfeld found that the magnetic field was expelled from their material. It couldn’t pass through, but instead was forced to pass around the superconductor, doing a circuitous detour as if sensing some kind of invisible “NO ENTRY” sign.
“Magnetic fields pass around superconductors as if they sense an invisible ‘no entry’ sign”
The Meissner effect, as it is now known, is responsible for the incredible ability of superconductors to above magnets – or indeed for magnets to levitate above superconductors.
Magnetic fields are unable to go through the superconductor due to currents that run across its surface. These are known as screening currents, because they act to screen the interior of the superconductor from the externally applied magnetic field. The currents themselves also produce their own magnetic field around the superconductor that repels an external one. This repulsive force balances the force of gravity and allows the superconductor to hover eerily in mid-air.
Later in the 1930s, Fritz and Heinz London, two brothers working at the University of Oxford, developed a theoretical framework for understanding the Meissner effect. Their results hinted that the response of the superconductor to a magnetic field is due to electrons in the superconductor being locked into a peculiar configuration that Fritz London christened a “macroscopic quantum state”.
The quantum connection
Theoretical physicist Fritz London was the first to show that superconductivity is a quantum phenomenon. We can understand what led to his idea with a thought experiment involving two black boxes. One box contains a permanent magnet and the other a coil of wire connected to a battery. At first the two boxes are indistinguishable – the current produced by the battery creates the same pattern of magnetic field lines as the permanent magnet. The similarity doesn’t end there because the permanent magnet’s field comes from microscopic electric currents within it.
Yet there is a difference between the boxes. Leave them for a while and the magnetic field around the box containing the circuit will disappear when the battery goes flat. In contrast, the permanent magnet retains its magnetism and its field pattern is unchanged. Why is it that the currents driving the permanent magnet are not subject to wear and tear, while those driven by batteries are?
This puzzle is related to the behaviour of an atom’s electrons. Electrons orbit around the nucleus, creating microscopic currents, and because they normally stay in their orbits their energy levels do not wear out or run down. In the early 20th century, quantum mechanics began to explain this in terms of electrons residing in what are called stationary states and this is the origin of permanent magnetism.
London grasped that similar reasoning could apply to the behaviour of a superconductor. The current in a superconducting coil behaves much more like the currents created by electrons around atoms in a magnet than those in an ordinary coil of wire. Indeed, if a third black box contained a coil of superconducting wire, a current started around it, and its resulting magnetic field, would last indefinitely.
This is why London thought of superconductivity as a macroscopic quantum state. He realised that a superconducting wire was just like a big atom, with the current going round and round the wire just like the electrons orbiting round and round the atom. Both are immune from wear, tear and decay.
Read next article: “Superconductors: Getting warmer“
