BERKELEY, CA — A team led by investigators from Stanford University and the Department
of Energy's Lawrence Berkeley National Laboratory has gathered surprising
information about the electronic structure of the "stripe
phase," a new electronic state of solids. Their report, in the
October 8, 1999 issue of the journal Science, may help resolve an apparent
paradox between different theories of superconductivity and may explain
how copper-oxide ceramics can become superconducting at high temperatures.
Xingjiang Zhou, working with Zhi-Xun Shen of Stanford, Zahid Hussain of
the Advanced Light Source (ALS), and other colleagues, used the High
Energy Resolution Spectrometer at the ALS to uncover new clues to the
mysterious behavior of the high Tc superconductors.
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AT HIGH ENERGIES, THE FERMI SURFACES IN ND-LSCO PLOT AS STRAIGHT LINES
SET AT RIGHT ANGLES, INDICATING THAT THE CHARGE CARRIERS MOVE ALONG
ONE-DIMENSIONAL STRIPES
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"High-temperature superconductivity was discovered in 1986, but
after a dozen years we are still struggling to understand it," says
Zhou, a physicist who has a joint position at Stanford and the ALS.
"Of the 50 or so high-Tc materials discovered so far, all of them are
copper oxides."
The researchers used angle-resolved photoemission spectroscopy (ARPES)
to study the stripe phase, or charge- and spin-ordered state, in the
compound Nd-LSCO (neodymium-substituted lanthanum strontium copper oxide).
The technique employs beams of synchrotron light to knock electrons out of
a sample, probing its electronic structure by measuring both the energy
and the direction of the emitted photoelectrons.
Plots of the resulting "spectral weights" at high energies
were consistent with charges moving through the Nd-LSCO sample along
one-dimensional lines, so-called stripes, but at lower energies the
pattern was more easily explained if the charges were moving in two
dimensions -- behavior that appears to require two different theoretical
explanations.
The parent compounds of cuprates are insulators; their complex
structure, similar to that of the mineral perovskite, alternates
two-dimensional layers of oxygen and copper atoms with layers of other
atoms. Cuprates are made more metal-like, and in some cases
superconducting, by doping -- adding elements which contribute extra
electrons or create holes to carry negative or positive charges.
"With Nd-LSCO we found that at about one-eighth doping level, the
picture that best fit the data was the stripe phase -- charge carriers
segregating themselves into one-dimensional lines," says Zhou.
"In the regions between these lines, electronic spins are arranged
anti-ferromagnetically," that is, the spins are arranged so that each
spin points opposite to those around it, producing insulating regions.
Nd-LSCO has static stripes and is not superconducting, but in those
cuprates that are superconductors, dynamic stripes may come into play and
become associated with superconductivity. Whether the stripe phase is
actually responsible for high-temperature superconductivity, however,
remains the subject of vigorous debate.
The stripe phase was predicted by Jan Zaanen and Olle Gunnarsson in
1987 and discovered experimentally in cuprates by John Tranquada and
others in 1995, using neutron scattering. However, the underlying theory
which gives rise to the stripe phase, mean field theory, paradoxically
suggests that the stripe phase should always be insulating!
The traditional electronic theory of metals describes how
quasiparticles -- collective entities with particle-like properties such
as energy and momentum -- experience the field of a solid's crystal
lattice. At low energies, the charge carriers in Nd-LSCO indeed appear to
interact with variations in the field due to the crystal lattice. The
carriers move back and forth in a two-dimensional manner and exhibit
low-energy states comparable to those observed in good superconductors.
But other expected two-dimensional effects are missing from the ARPES
data.
Established theories also describe a characteristic Fermi surface that
marks where (in momentum space) a given material's uppermost energy level
is filled with electrons. At high energies, the Fermi surfaces in Nd-LSCO
plot as straight lines set at right angles, indicating that the charge
carriers move along one-dimensional bands -- features which could not
arise from the quasiparticles of traditional electronic theories.
Writing in a Science Perspective, theorist Jan Zaanen points up the
conundrum represented by these results. "How can it be that the
electrons which are 'one-dimensionalized' at high energies can rediscover
the two-dimensional world at low energies? Within established electronic
structure theory this appears as a paradox, and new physics is here at
work."
"The solution to this paradox may be a new basic starting
point," Zhou suggests. "Instead of the quasiparticles that are
responsible for superconductivity in ordinary metals, in the cuprates one
may have to start with the stripes themselves, along which charge flows
freely. The stripes appear stable at high energies, but at lower energies
they may exhibit quantum fluctuations that give rise to two-dimensional
effects."
Zhou adds that "the compound we investigated is not itself a
superconductor, but through understanding its electronic structure we can
make a major advance in understanding its high-Tc relatives."
The High Energy Resolution Spectroscopy (HERS) endstation used in
studies of highly correlated materials at the ALS was supported by the
Facilities Initiative of the Department of Energy's Office of Science, and
"is unique in the world for this kind of science," says the
ALS's Zahid Hussain.
Besides X. J. Zhou, Z.-X. Shen, and Z. Hussain, other collaborators
included P. Bogdanov and S. A. Kellar of Stanford, and T. Noda, H. Eisaki,
and S. Uchida of the University of Tokyo. The report by Zhou et al and
Zaanen's Perspective appear in the same issue of Science, October 8, 1999.
The Berkeley Lab is a U.S. Department of Energy national laboratory
located in Berkeley, California. It conducts unclassified scientific
research and is managed by the University of California.
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