 
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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