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First-ever images of atom-scale electron clouds in high-temperature superconductors could help in design of new and better materials
16 Feb 2000

By Robert Sanders, Public Affairs

BERKELEY-- An exciting advance by physicists at the University of California, Berkeley, could help unlock the secrets of high-temperature superconductors.

Using a scanning tunneling microscope they built specifically to study these unique materials, UC Berkeley scientists for the first time 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. Séamus Davis, associate professor of physics at UC Berkeley and a researcher in the Materials Sciences Division of Lawrence Berkeley National Laboratory.

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 W. Hudson, now a National Research Council fellow at the National Institute of Standards and Technology in Gaithersburg, Md.; UC Berkeley graduate student Kristine M. 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 (-300°F) instead of 4 degrees above zero (-452°F), 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 300 degrees above absolute zero (80°F).

"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 UC 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.

Last July they reported in Science magazine their success is imaging electron clouds in a pure sample of a copper oxide high-temperature superconductor (Bi2Sr2CaCu2O8+d) - dubbed BSCCO (BIS-ko) because they are composed of bismuth, strontium, calcium, copper and oxygen. This superconductor is made up of a repeating series of layers: two bismuth oxide layers, a strontium oxide layer, and two copper oxide layers with some calcium atoms sandwiched between them.

The images are made by scanning a fine-tipped probe over the surface at a distance of a mere nanometer - a millionth of a millimeter. As the tip traverses the bismuth oxide surface layer it touches the electron clouds bulging above it and records a tiny electric current that indicates the strength of the electron cloud at that spot.

In that experiment they were able to see alterations in the electron clouds or wave functions around random, unknown impurities in the copper oxide layer.

In their new study, they enlisted Uchida's help in the precise substitution of zinc atoms for some of the copper atoms in the underlying copper oxide layers - the actual superconducting layers. This impurity disturbs and stresses the crystal structure and perturbs the electron clouds around nearby copper atoms, which can be detected at the surface.

"The idea is that impurities interfere with the mechanism creating the superconductivity, destroying 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."

The pictures obtained by the team clearly show cloverleaf-shaped electron clouds centered on each zinc atom, consistent with the d-wave orbitals these excited electrons should occupy in zinc and copper. Evidently the zinc atoms have stripped electrons from the Cooper pairs - the electron couplings that give rise to superconductivity - and concentrated them in clouds that look like a four-leaf clover. Cooper pairs in high temperature superconductors are thought to be formed from electrons on two adjacent copper atoms, whereas in other superconductors electrons forming a Cooper pair are often separated by thousands of atoms.

The clouds can be thought of as representing the probability that an electron will be found at any particular spot. This is because quantum mechanics treats electrons not as discrete particles, but as wave functions spread out around an atom or molecule, where the wave function represents the likelihood of finding the electron at a particular point.

"Scanning tunneling microscopy is one of the most direct probes of structure, allowing us to visualize the shape of the wave function," Balatsky said. "Seamus's success is a testament to the excellent quality of his experimentation."

While the pictures confirm several predictions, such as the four-leaf clover shape of the wave function, they also show new phenomena that current theory cannot explain.

Among these, Balatsky said, are the unexpected orientation of one part of the electron cloud that surrounds the zinc impurity, and the shorter-than-expected range of part of the cloud. He already has theories to explain some of these new observations.

"I am sure there will be more surprises than we can think of now," he said.

Davis's next challenge is to dope the superconductors with other impurities to see how they perturb the electron clouds in the copper oxide layer. A magnetic impurity atom, for example nickel, would allow him and his group to study the qualitatively different mechanism by which magnetic scattering influences high temperature superconductivity.

"We can now ask different questions by using different impurities," Davis said. "This potentially is a very powerful technique."

The ongoing work is supported by the David & Lucille Packard Foundation and the Department of Energy.


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