Secrets of Superconductors
They say “nothing is perfect” but, as with any rule, there are always exceptions!
Superconductors are the world’s only perfect conductors of electricity, a phenomenon which means they have great potential for use in a wide variety of applications. But despite being discovered nearly 100 years ago, a number of mysteries still surround them.
Now Dr Grant Williams, a researcher at the MacDiarmid Institute for Advanced Materials and Nanotechnology and at Industrial Research Limited (IRL), is conducting research that may at last force superconductors to reveal their innermost secrets.
Superconductivity is a phenomenon that occurs in a number of different metals. When a superconducting substance is cooled to a certain temperature, all its electrical resistance disappears, and the material becomes a perfect conductor of electricity. This means that it can carry a very large current without any significant decrease, for a very long time. Superconductors also have unusual magnetic properties, meaning they also have great potential for magnetic devices.
The phenomenon of superconductivity was discovered in 1911 by Dutch physicist Heike Kamerlingh-Onnnes. Since this first discovery, many other superconducting materials have been found, including elemental metals, alloys, and other materials. So far, the main use of superconductors has been for making superconducting electromagnets for use in research laboratories and magnetic resonance imaging in hospitals. Another exciting potential use for superconductors is in magnetic levitation, and trains that levitate on their tracks have been used on a trial basis in Japan and Germany for a number of years. In the future, superconductors may be used for long distance transmission of electricity, for generators, motors and transformers, and in better medical imaging devices.
Each superconductor has a ‘critical temperature’, below which it changes from its ‘normal’ state to become a superconductor. For an elemental metal such as tin or lead, this is an extremely low temperature, of up to -266ºC. For a metallic alloy, it is a touch higher (up to -250ºC). These temperatures are so low, that it makes these superconductors difficult and expensive to work with. In 1987, ‘high temperature’ superconductors were discovered. These are materials which are made of copper oxides mixed with other elements, such as calcium, yttrium and lanthanum. The critical temperature of high temperature superconductors is much higher (up to -120ºC). This is a great advantage, because these materials can be cooled to their critical temperature relatively easily, with readily-available liquid nitrogen, making them much cheaper and easier to work with.
Dr Grant Williams is investigating a set of these high temperature superconductors, which form part of a group of materials called strongly- correlated oxides. “Strongly-correlated oxides have great potential for technological applications that include superconductivity, magnetic sensors, and spin polarized electronics”, he said. “They are also nice because the structure appears to be simple. One of the superconductors that is important for us is called BiSCO,” he explained.
BiSCO is made of three layers of copper oxide mixed with calcium, an insulating layer of strontium oxide, two bismuth oxide layers and another strontium oxide layer. This basic layered structure is then repeated many times. The bonding across the bismuth oxide layers is weak, which has the distinct advantage that it makes the material very easy to mould into wires. However, Dr Williams said, “there’s still some real technological challenges, and that’s why it’s taken a good 12-15 years to actually start to make wires.”
Specifically, Dr Williams’ research involves trying to understand more about the underlying physics behind the way in which strongly-correlated oxides function. This is part of a theme at the MacDiarmid Institute on novel electronic, electronic-optic and superconducting materials.
One major mystery is the mechanism by which high temperature superconductors actually work. “In high temperature superconductors, nobody really understands what is going on,” Dr Williams said. It is believed, however, that a series of complex interactions that occur within the material are important. These include interactions between certain electrons, how the electrons are ordered, and vibrations that occur within the material. Although these interactions happen all the time, in most areas of physics, they are ignored because they don’t significantly affect how a material functions. “Physicists always try to take the easiest approach,” said Dr Williams. “When you have a metal, the assumption is that you can just treat each electron independently. You can get away with it in a simple metal,” he said, “but in reality, for all the things that are interesting, like superconductivity, you have to take [them] into account.”
As a result of his research so far, Dr Williams thinks that the key to the function of high temperature superconductors is a phenomenon called dynamic phase separation. This is an electronic state where, at very small timescales, the material can form into ‘stripes’, with tiny ‘conducting channels’, separated by insulating regions. Just as quickly, these then disappear.
Dynamic phase separations can be hard to detect, because they happen so quickly that when most measurements are made, the material looks homogeneous. But, by using a method called Nuclear Magnetic Resonance (NMR), Dr Williams has been able to actually ‘see’ these dynamic phase separations occurring. “It’s really a new direction in trying to understand these things,” he said. “If we understand this, it could give a greater understanding of what the actual mechanism is.”
As well as his work with superconductors, Dr Williams is also looking at other materials in the strongly-correlated oxides group. A simple example, he explained, is a substance called strontium ruthenate, which is part of a group of compounds known as half-metals. “It’s been proposed as a new way of making magnetic RAM,” he said. “It will make it more efficient, it can be set up so it’s a permanent memory, and it has the advantage that its also radiation hard. So you can have a nuclear strike and your radio will still go,” he joked.
Dr Williams is investigating other strongly correlated oxides including, double-perovskites, ruthenates, manganites and ruthenate-cuprate compounds. “They’re interesting because some of them have a large magnetoresistance – and magnetoresistance is what is used in your hard disc. That’s how they’ve been able to increase the capacity of hard drives,” he explained. Today, hard drives are made from conventional ferromagnetic metals, but in future, half-metals will be used instead. There are also interesting physics questions related to magnetic, electronic and structural instabilities as well as electronic phase separation.
Dr Williams collaborates in his research with two other MacDiarmid Institute researchers, Professor Joe Trodahl, and Professor Jeff Tallon. “It’s nice, because we’ve got complementary resources,” said Dr Williams. “Then there’s all our international collaborations, which also make things a lot easier.”