Science at the Frontier (1992) / Chapter Skim
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10 Physics: Quasicrystals and Superconductors: Advances in Condensed Matter Physics
10 Physics: Quasicrystals and Superconductors: Advances in Condensed Matter Physics
Pages 233-254
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From page 233... ...
Both topics high-temperature superconductivity and quasicrystals concerned fields that "seemed to be, if not dead, at least not so active in the 1970s and the early 1980s" but suddenly came to life as the result of major, surprising discoveries, Fisher said. High-temperature superconductivity, which has received wide coverage in the popular press, has become the focus of intensive effort among applied and theoretical physicists and materials scientists.
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From page 234... ...
In the same way, a three-dimensional space can be filled periodically with crystal units that have fourfold or sixfold symmetry but not, according to theory, by crystal units with fivefold symmetry. That theory now has been upset by the discovery of crystal units that have fivefold symmetry and fill space completely.
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From page 235... ...
In the 1930s, another characteristic of superconducting materials was described. If an ordinary metal is placed in a magnetic field, the magnetic field permeates the material.
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From page 236... ...
Anything that destroys the Cooper pairs for example, heat that increases the lattice vibrations above a certain limit destroys superconductivity. Among other things, the BCS theory explains why superconductivity occurs in metals only at very low temperatures.
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From page 237... ...
The success of the BCS theory came against a background of slow, dogged advances in superconducting materials research. Starting from the 4.15-K transition temperature of mercury described by Onnes in 1911, physicists discovered a series of materials with progressively higher transition temperatures.
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From page 238... ...
This time the claim was verified, and the world of superconductivity entered a new era. Within a matter of months, Paul Chu at the University of Houston and Maw-Kuen Wu at the University of Alabama reported that a copper oxide ceramic containing yttrium and barium had a transition temperature well above 90 K, easily in liquid nitrogen territory.
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From page 239... ...
2CuO _ ·~ _ Nb3Sn _ - ~ NbN '- Nb3AI0.75Geo.25 -- -- V Si Hg _ -- NbC O 1 1 1 1 1910 1930 ~ 1950 1 1970 1990 Year Z39 - Liquid nitrogen FIGURE 10.1 History of progress in the quest for an ever-higher transition temperature.
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From page 240... ...
"It is pretty clear that we have a pairing mechanism, and the question then becomes what mechanism is holding these pairs together," Zettl said. The BCS theory makes some specific predictions about the properties of electron pairs.
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From page 241... ...
But it is almost impossible to accommodate both the observed high transition temperatures and the small isotope effect within BCS theory, Zettl saidone of the many unresolved issues about the new materials. The issue of dimensionality has become very important.
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From page 242... ...
The smaller value is consistent with BCS theory, while the larger value is not." The issue of strong binding versus weak binding of the electron pairs thus remains confusing, said Duncan Haldane of Princeton University, another speaker at the symposium, who focused on the theoretical issues. One critical factor in trying to develop a theory to explain the new materials, Haldane said, is their odd behavior in terms of electrical resistance above the transition temperature.
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From page 243... ...
The charge-polarization and spin-polarization theories are based on two sharply different pictures of the electron structure of the material. BCS theory assumes that the conduction electrons that form a Cooper pair are delocalized and move relatively independently.
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From page 244... ...
Cooper pairing occurs as a result of interactions between the spins of the holes and neighboring electrons. One theory that aroused enthusiasm for a while was based on exotic particles called anyone or semions, which are neither bosons nor fermions, Haldane said.
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From page 245... ...
The copper oxide superconductors, like the low-temperature superconductors now used in large magnets, do not expel magnetic fields completely. Instead, the magnetic field penetrates the material in some areas, forming vortices that are not superconducting; the rest of the material remains superconducting.
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From page 246... ...
Verification of the vortex glass theory has important implications for physicists working on industrial uses of the new superconductors. For example, the theory might help researchers introduce defects into superconducting materials in a way designed to make the transition to the vortex glass state, and thus to superconductivity, occur at higher temperatures.
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From page 247... ...
, who spoke at the symposium, described quasicrystals as "ordered atomic solids that possess longrange quasiperiodic positional order and a long-range non-crystallographic orientational order." In other words, they fill space completely without having the symmetry and order of classic crystals. Their atomic arrangements exhibit overall symmetries, such as fivefold, eightfold and tenfold symmetries, that have never before been observed.
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From page 248... ...
(An icosahedron is a regular polyhedron with 20 identical triangular faces and six fivefold symmetry axes.) The observation of this icosahedrally symmetric pattern was startling precisely because an icosahedron, rotated about one of its axes, has the fivefold rotational symmetry that conventional crystallography held to be impossible for a crystal.
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From page 249... ...
One explanation immediately invoked by doubters was that the material was not a truly quasiperiodic crystal, but merely a set of crystallites arranged in an icosahedrally symmetric cluster, a phenomenon called twinning. A growing crystal sometimes starts reproducing itself in one direction, producing a twin of itself.
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From page 250... ...
Only a year earlier, there appeared to have been convincing arguments that it was impossible to grow such a perfect quasicrystal: to cover a surface with Penrose tiles, the tiles must be placed carefully in the precise order needed to ensure complete coverage; a slight error produces an incomplete configuration, with gaps in the coverage. Because of the long-range nature of the quasiperiodic order, critics expressed doubts that atoms could come together to grow perfect quasicrystals.
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From page 251... ...
In computer simulations, the new rules produced infinitely large Penrose tilings, removing most of the theoretical objections. "The results provide new insights as to how materials with only shortrange atomic interactions can grow large, nearly perfect quasicrystal grains," reported Onoda et al.
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From page 252... ...
There is an important exception to that observation, however. One of the computer simulations run by Steinhardt started with a seed that had a specific defect, one that was discovered in the course of investigation.
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From page 253... ...
If the Penrose tiles are marked with specially chosen line segments, the segments join across tile edges to form five sets of infinite, parallel lines that are pentagonally oriented (Figure 10.3~.
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From page 254... ...
1988. Growing perfect quasicrystals.
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