By Robert Sanders, Public Affairs
Posted February 16, 2000

An advance by Berkeley physicists could help unlock the secrets of high-temperature superconductors.

Using a scanning tunneling microscope they built specifically to study these materials, Berkeley scientists have obtained pictures of the electron clouds around impurity atoms in a copper oxide superconductor. Impurities play a key role in superconductors, raising or lowering the temperature at which they become superconducting.

"We now have the technology to look at individual impurity atoms in these very complicated materials, opening a new door to research on high-temperature superconductors," said lead investigator J.C. SÈamus Davis, associate professor of physics.

The feat is reported in the Feb. 17 issue of the British journal Nature by Davis, former postdoctoral associates Shuheng Pan, now an assistant professor of physics at Boston University, and Eric Hudson, now a National Research Council fellow at the National Institute of Standards and Technology in Gaithersburg, Md.; UC Berkeley graduate student Kristine Lang; and professor Shin-ichi Uchida and Dr. Hiroshi Eisaki of the University of Tokyo's Department of Superconductivity.

Ever since IBM scientists in 1993 used a scanning tunneling microscope to image the electron clouds around copper atoms in a metal, scientists have tried to extend this feat to other, more complex materials.

"Scanning tunneling microscopy is really the first technique that can look at quantum mechanical wave functions in a material, at how the electrons do their quantum mechanical dance on the surface," Davis said.

In particular, scientists have been yearning to look at copper oxide superconductors, in the hope that understanding how the electrons move around the atoms will give hints as to how to build better high-temperature superconductors.

High-temperature superconductors are materials that conduct electricity perfectly at temperatures substantially above absolute zero, that is, at 87 degrees above absolute zero (-300öF) instead of 4 degrees above zero (-452öF), the temperature at which normal superconductors operate. Already copper oxide superconductors are used in electric power transformers, mobile phone base stations and some experimental biomedical devices, such as magnetic resonance imaging machines.

The dream is to find substances that are superconducting at room temperature, or about 280 degrees above absolute zero (80öF).

"Nobody knows exactly why, when you put all these chemicals together in the right amounts, you get high temperature superconductivity. No one knows the recipe to make new higher temperature superconductors," Davis said. "To find that recipe you have to understand how the system works at the atomic level, which is where we are attacking the problem."

Davis and his colleagues at Berkeley built a one-of-a-kind, high resolution scanning tunneling microscope that works at low temperatures -- around 4 degrees above absolute zero -- and thus can look at materials like high-temperature superconductors.

"The idea is that impurities interfere with the mechanism creating the superconductivity, it in the vicinity of the impurity and creating a localized state, which scanning tunneling microscopes can probe on an atomic scale," wrote theoretician Alexander V. Balatsky of Los Alamos National Laboratory in a commentary in the same issue of Nature. "By learning how the superconductivity is destroyed, we hope to better understand the inner workings of the high-critical-temperature mechanism. The approach is similar to a child disassembling a toy to see how it works."

February 16 - 22, 2000 (Volume 28, Number 22)
Copyright 2000, The Regents of the University of California.
Produced and maintained by the Office of Public Affairs at UC Berkeley.
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