Feb. 20, 2004

McMaster physicist heats up physics debate

A small piece of physics puzzle could take MRIs, cell phones, to new levels

Hamilton, ON - The phenomenal force behind MRI machines and magnetically levitated trains – called superconductivity – is under investigation at McMaster, where researchers have brought the physics community one step closer to understanding how it works. In the distant future it could mean MRI machines so affordable they’re in every doctor’s office, but for now it’s another piece of a complex puzzle that physicists around the world have been contemplating for more than a decade.

Tom Timusk, a professor in physics and former colleague of Nobel Prize-winning McMaster physicist Bertram Brockhouse, has worked with post doctorate fellow J. Hwang and G.D. Gu, a collaborator at Brookhaven National Laboratory to uncover a property about high-temperature superconductivity that disproves a recently published proposal by his international colleagues. His findings are published this week in the article “High-transition-temperature superconductivity in the absence of the magnetic-resonance mode,” in the journal Nature.

Superconductivity allows for the flow of electric current without resistance, providing for powerful, uninterrupted transmission of energy. Timusk likens the electrons to an army moving in the same direction with their arms linked, so that if one encounters resistance the others carry it forward without slowing down. As a result, in a closed superconductor circuit, no energy is ever lost.

Low-temperature superconductivity is used in MRI machines, but Timusk said the technology that cools the coils to reach low temperatures is costly, necessitating the MRI’s small and tunnel-like shape. That’s something that he believes might change in the not so distant future, after researchers develop a stronger understanding of why high-temperature superconductivity works.

“There are a number of applications for high-temperature superconductivity, but right now I’m interested in finding out the basic mechanisms behind why it works and how it’s different from the low-temperature variety,” said Timusk.

Physicists know that in low-temperature superconductivity atomic vibrations – called phonons – bind electrons together, creating that “arm linking” effect. How the high temperature variety maintains its force has remained a mystery.

A research group from Berkeley recently used a sophisticated technique known as “photo emission” to show that phonons are behind the force in high-temperature superconductivity too. The group found a sharp increase or “kink” in the energy of the photo-electrons they collected, and claimed the kink was the signature of phonons binding the electrons in high-temperature superconductivity, much the same way they do in low temperature superconductivity.

Timusk used a different measurement tool known as “infra-red optics” to study the same material, and he discovered that the “kink” that appears in high-temperature superconductivity can be eliminated when oxygen is added without destroying the high temperature superconductivity. That means the infra-red instrument provides a clearer output of what’s happening to the energy of the superconducting electrons and rules out any significant role of the phonons in high temperature superconductivity.

“We’ve shown the optical method we use to study this material is sensitive and produces cleaner data than some of the more elaborate alternate instruments, such as the photo-emission spectroscopes of the Berkeley group,” he said. “This finding doesn’t solve the puzzle, but it brings us one step closer to understanding high-temperature superconductivity, and the many opportunities it has to offer.”