Fields of Dreams
Cathy Foley
On
18 March 1987, the New York Hilton Hotel was the venue for what The New
York Times referred to as the ‘Woodstock’ of physics. The world’s top
physicists were gathered at the annual meeting of the American Physical
Society to hear about the breakthrough of high-temperature superconductors – a
revolution in materials science.
Never before had the sleepy world of materials science been so shaken up by an announcement: high-temperature superconductors, the newspapers trumpeted, were going to change the world. Now, as I look back, I wonder how many people even knew what we were talking about.
High-temperature superconductors are materials that can carry electricity but have no electrical resistance, as well as a range of other amazing properties, when they are cooled to –200oC. That may sound pretty chilly, but it’s more than 70oC warmer than normal superconductors. And the warmer they are, the easier and cheaper they are to create – making superconductors a more feasible proposition.
This really was a revolution. If these amazing new materials could be incorporated into existing devices, they promised an incredible range of new technology. Affordable magnetically levitated trains, power lines that lose no current over distance, super-efficient electronics, ultra high-density storage devices, and unbelievably sensitive magnetic field sensors able to map heart and brain function, detect customs contraband or find new mineral deposits – all of this could now be possible. And that was just the beginning.
But if the announcement set the world of physics on fire, it also caught us by surprise, because superconductivity wasn’t supposed to happen when things heated up. Suddenly, here was an amazing class of materials with enormous potential, but the science and technology able to exploit this discovery still had to be developed.
Scientists all over the world were scrambling to work out how to make and use these wonderful new superconductors, and I was part of the scramble.
I’m a solid-state physicist – I study the properties of solid materials. I’m based at the National Measurement Laboratory in Sydney, part of CSIRO Telecommunication and Industrial Physics. My work is all about understanding how different materials function, how to test them, how to use them for specific purposes. Before the discovery of high-temperature superconductors, I had worked on a wide range of materials of varying conductivity, from semiconductors to high-conductivity metals. It’s always challenging because I’m usually working with materials about which little is known, and devising tests and techniques that have often never been tried before. That’s the stuff of science really, extending our knowledge into new areas.
And, back in 1987, high-temperature superconductors (HTSs) were about as new and as unknown as any area of science could be. Even so, the scientific world quickly appreciated the importance of this discovery, and within eight months, Alex Mueller and Georg Bednorz were awarded a Nobel Prize for synthesising the first HTSs – the shortest delay between original research and an award in the history of physics.
Before I get into what we did with HTSs, let’s look at superconductors in general. Many materials have superconductive properties. If you cool some metals such as lead, aluminium and niobium, they suddenly lose all resistance to the flow of electricity when a critical temperature is reached. Zero resistance means no loss of electrical energy as it passes through the conductor.
Superconductors have two other important properties. One is that magnetic fields cannot penetrate into the material, so you can use it to levitate a magnet or build a maglev train, as is being done in Japan. The other is that you can create tiny structures called Josephson junctions. These are so sensitive to magnetic fields, they can be used in devices called Superconducting Quantum Interference Devices (or SQUIDs) that can detect magnetic fields so small, they can map the magnetic fields of the human brain and heart.
It sounds almost like magic – materials with no electrical resistance that repel magnetic fields and can be used to build supersensitive magnetic-field detectors. Of course, in physics, as in life, there’s no such thing as a free lunch. The disadvantage of these marvellous materials is that the critical temperature is unbelievably low: about –270oC. To freeze something so deeply, it’s necessary to use liquid helium as a coolant, which is very expensive – about $35 per litre – and difficult to work with. Indeed, this has severely limited our ability to use superconductors.
Enter high-temperature superconductors – they become superconducting when they’re cooled to a mere –200oC. That’s still pretty cold, but much easier to reach than –270oC. Indeed, it’s possible to reach –200oC simply by using liquid nitrogen, which is cheap, easy to use and readily available even in remote areas. Most high-school science classes use it at some time. Also, there are now super-refrigerators that can easily reach –213oC.
All in all, high-temperature superconductors (HTSs) promised a revolution in the application of superconductors – if we could work around their shortcomings. Unlike the low-temperature materials such as niobium, which are metals, these new HTS materials are ceramics. The compound I have worked on mostly is yttrium barium copper oxide (commonly labelled YBCO). In bulk form it looks like a black bathroom tile or brick, and trying to make wires out of a brick is no small feat.
Josephson junctions, those tiny superconducting circuits that are ultra-sensitive to changes in magnetic fields, can be made from YBCO, but that’s a challenge, too. They are quite easy to build using normal metal superconductors, where they are about three thousandths of a millimetre (0.003 mm) in width. You make these using the traditional microelectronic techniques used in the mass manufacture of silicon chips.
However, Josephson junctions in YBCO have to be less than one-thousandth of that size to work, and there’s no established process of building them. So we had to invent a whole new technology to make Josephson junctions. And we did. To create them we grew an ultra-thin layer of ceramic onto a platform in such a way that the boundaries between the grains of the ceramic are aligned to produce the necessary effect. The Josephson junctions built in this way are seven millionths of a millimetre long, and we can use them to build SQUIDs that are among the most sensitive magnetic field detectors technology has ever devised.
Which is where my science jumps from the abstract to the real. It’s one thing to appreciate the beautiful subtleties of quantum theory (which is the window through which we understand superconductivity) and devise clever techniques for building structures only visible through electron microscopes. But for me this wonderful science also needs to provide real world solutions and help people. And building better magnetic field detectors does just that.
Magnetic fields are all around us. The electrical signals of our bodies, our hearts and our brains all produce a magnetic field; mineral deposits distort the Earth’s magnetic field; metallic objects change magnetic fields. But we’re talking about really, really weak fields. The Earth’s magnetic field is about a hundredth the strength a fridge magnet, and these SQUIDs can detect magnetic fields a thousand millionth the strength of that.
And they do work: we have developed a new HTS system for mineral exploration, operating our SQUID systems in the hot Australian desert and the freezing Arctic. SQUIDs have proven to be useful in finding deep deposits of highly conductive minerals that can’t be found easily by other means. We were then asked to fly a SQUID behind an aircraft so the search for minerals could be done faster and be taken further. We achieved this, but it wasn’t easy. We are the only group ever to have flown in a SQUID!
HTS technology has lots of other uses. We can spot defects in the heartbeat of a foetus (impossible to do by other means) and monitor adult hearts without electrodes. We can look for unwanted metal fragments in food, preventing large quantities of food wastage in the manufacturing process, and detect nuclear submarines from the wake they leave behind.
And this is where it gets tricky. I love science, its challenge and its potential. But there are also downsides. Scientists have to live with the implications of their discoveries, and these are not always benign. The late Professor Peter Mason awakened me to this in 1982, when I attended a lecture at Macquarie University, in Sydney, during which he talked about nuclear weapons and the arms race, and the responsibility of scientists. Even though I had studied nuclear physics as an undergraduate, I was completely unaware that nuclear fission had been used in weapons of mass destruction.
It was at this meeting that I decided to never again ignore the social responsibility of science. I joined Scientists Against Nuclear Arms, later renamed Scientists for Global Responsibility, and whenever possible now I try to encourage discussions on the implications and ethics of research, not just the potential benefits. My involvement with like-minded scientists has given me a core belief that science must be used with forethought and care.
While I am now keen to ensure my research will only be used for the common good, I have found that it is not as easy as first thought. My research could be used to create very fast, low-noise radio-amplifiers in satellite communication systems, which will improve international communications. But might not such satellites perhaps relay the order to fire nuclear weapons more effectively? These are things every scientist needs to consider: there are always unintended consequences, unintended applications.
Being a scientist today means much more than just doing excellent science. We have to seek our own funding, we have to take greater responsibility for the researchers who work for us and we’re constantly pressed to promote and communicate what we do with the wider community. Now there are growing calls that we should lead the discussion on the impacts and morality of what we make possible. Sometimes it seems there’s no time left to do the science itself!
Many scientists moan about these multiple responsibilities. They feel squeezed. I know I do sometimes. It’s not easy. Some days I just want to be left alone in my lab to get on with my research.
But is it all worth it? Would I ever change my career?
There’s nothing quite like being on the forefront of human knowledge, pushing
back the darkness and discovering new and exciting things. So the answer is
No. I have the best job in the world.
Dr Cathy Foley is the research leader of the Superconducting Devices and Applications Project with CSIRO Telecommunications and Industrial Physics, based at the National Measurement Laboratory in Sydney.
The article first appeared in the November edition of Newton, a new science journal from the publishers of Australian Geographic, and is reproduced with permission.