Superconductor?

Imagine life in a superconducting state. Electrical charges experience no resistance, so nothing heats up. No parts wear down or decay because there is no burnout. The applications are endless and revolutionary, and include inexpensive, sustainable power delivery, mega-memory quantum computers, magnetic high-speed trains, and powerful magnetic resonance imaging capable of scanning individual cells.

Superconductors have the potential to make life so much easier – except for one big snag. There is no theory to explain why electrons behave as they do in a high-temperature, super-conducting state, and the best minds in physics have been at a loss to explain the phenomenon.

Superconductivity, discovered in 1911, occurs in many metals at extremely low temperatures, near absolute zero (-273.15°C). The need to use cumbersome, costly cooling devices based on liquid helium put nearly all practical applications for superconductors on hold for years. Then, in 1986, came a groundbreaking discovery. Two scientists working for IBM in Switzerland, Georg Bednorz and Alex Müller, found that copper oxides became superconductors at unprecedented high temperatures. Liquid nitrogen, which is cheap and plentiful, can readily cool the material to the required level, although the highest temperature at which scientists have produced superconductivity is still a frigid 16O°C above absolute zero. There is still a long way to go to reach the ultimate goal – a room-temperature superconductor (about 300°C above absolute zero). More significantly, 14 years after the discovery of high-temperature superconductors, the microscopic mechanism that causes this phenomenon remains unknown.

Theoretical physicists around the world have grappled with this problem and have come up with conflicting theories. The path to a solution seems likely to lie in experiments designed to study and characterize the odd behaviour of electrons in the superconducting state. Bob Birgeneau, the new president of the University of Toronto, is one of a handful of key experimenters working to solve the mystery. “Quantum physics is 75 years old,” says Birgeneau. “Yet there is still a deep hole in our understanding of matter, solids and liquids at the quantum level. We hope to fill that hole.”

This task is difficult because the behaviour of electrons in interacting quantum states is hard to predict. First of all, electrons are impossibly small (like a football in a subatomic stadium). Microscopic phenomena in physics simply don’t have parallels in our everyday macroscopic world. Secondly, the laws of quantum physics are consistently right, even when they don’t appear to make sense. They are counterintuitive.

It was fundamental physics that laid the foundation for the wonders of electronic engineering – the transistors, integrated circuits, lasers and fibre optics that drive our high-tech world. Scientists have since moved into the study of more complex states of matter. Birgeneau offers an analogy: “Picture the electron as an individual in a crowd. To predict the behaviour of that electron, it is often enough to know the average weight and height of the surrounding electrons. Now, in quantum many-body physics, my territory, knowing the average behaviour in the neighbourhood is not good enough. To predict the behaviour of an individual electron, we must know the precise weight and height of all of the surrounding electrons.”

Birgeneau experiments with exquisite, single crystals of high-temperature superconducting materials, which he probes with neutrons. These crystals are composed of isolated sheets of copper oxide; his experiments suggest that there are magnetic “rivers” of superconducting charge in the copper oxide layers, and this appears to underlie the wondrous properties of high-temperature superconductors. His experiments are designed to find out what is fundamental and what is mere detail because, as he reminds us, “we still do not have the mathematical tools, the theory, to describe this exotic behaviour.”

Current research at U of T complements Birgeneau’s own work, and the president will form part of a core group determined to unravel the paradoxical nature of high-temperature superconductivity. Physicists Louis Taillefer and John Wei are each looking at the same class of problem, although with a different focus. Taillefer is an expert at making high-purity materials and measuring their properties at very low temperatures. Wei is setting up a new tunnelling spectroscopy lab along with facilities for making very thin samples, in an effort to study the problems microscopically. Two theorists, Michael Walker also a professor of physics, and WaIker’s former graduate student Bob Gooding (PhD 1987), now at Queen’s University in Kingston, Ont., will be part of the group, and Birgeneau’s senior researcher from MIT, Shuichi Wakimoto, will also work on campus.

When Birgeneau began his scientific career with a summer job at the Chalk River Nuclear Laboratories in Northern Ontario after graduation from U of T in 1963, his timing couldn’t have been better. Canadian scientists like Bertram Brockhouse (MA 1948, PhD 1950), awarded the Nobel Prize for physics in 1994, had recently pioneered the use of neutron beams from the new Chalk River nuclear reactor to probe the atomic vibrations of elemental metals, semi-conductors and simple compounds. A resulting paper on the interatomic properties of nickel, which Birgeneau co-authored, was his most cited paper for two decades. “It was an extraordinary privilege for me as a young U of T graduate to be able to work with this group of Canadians who were true world leaders.”

Now, those high-level experiments are carried out collaboratively in multibillion-dollar facilities in the United States, Japan and Europe. Birgeneau dreams that the Chalk River nuclear reactor, in danger of being mothballed, might be transformed into a neutron research centre, putting Canada back in a leadership position in the international field of neutron scattering.

How will Birgeneau the scientist and Birgeneau the president balance these two demanding roles? “My first responsibility is to help lead the University of Toronto into the realm of the very best universities in the world in both education and research,” he says. Then he adds, “But my soul is that of a scientist, and I need to show that it is possible both to be a high-level administrator and to carry out leading-edge research.”