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New
superconductor images could help in design of new and better
materials
16
Feb 2000
IN THIS STORY:
Finding
a new recipe for higher temperature superconductors
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stories, sites, photos 
An
exciting advance by physicists at UC 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 (-300ºF) instead of 4
degrees above zero (-452ºF), 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 (80ºF).
Finding
a new recipe for higher temperature superconductors
"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."
"Scanning
tunneling microscopy is one of the most direct probes of structure,
allowing us to visualize the shape of the wave function," Balatsky
said. "Séamus's success is a testament to the excellent
quality of his experimentation."
Source:
Robert Sanders, Public Affairs
RELATED STORIES, SITES, PHOTOS:
Full
press release
Séamus
Davis home web site
Physics
of High-Temperature Superconductivity Elucidated (Berkeley
Lab)
Image
of surface of a BSCCO with caption
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