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Superconductors Earn Their Stripes
12 March 2002 (All day)
If three sightings make a trend, then a controversial theory of high-temperature superconductivity may have become a little more chic. The "stripes" theory states that current flows without resistance along lines of electric charge inside copper-and-oxygen-based superconductors. Now, a series of new results show that, at the least, the materials actually have such stripes.
For 15 years physicists have struggled to understand the so-called cuprate superconductors, in which sheets of copper and oxygen atoms sandwich atoms of other elements. Sometimes, an atom between the sheets will pluck an electron from a copper atom, leaving behind a positively charged "hole" that can hop from one copper atom to the next. But a single hole is hindered from moving along the sheet by the magnetic fields of the copper atoms. So, according to the stripes theory (Science, 19 February 1999, p. 1106), the holes first gather into stripes, which serve as little runways with no magnetism. Physicists had, however, observed stripes of charge in only one superconductor, lanthanum strontium copper oxide (LSCO). Now, three groups are reporting similar results.
Herb Mook and colleagues at Oak Ridge National Laboratory in Tennessee shot a beam of neutrons at a large crystal of yttrium barium copper oxide (YBCO), the most widely studied of the cuprate superconductors (Science, 1 February, p. 787). By carefully analyzing the reflected beam, the researchers detected stripes of holes lying along every eighth row of copper atoms, as they describe in the 4 March issue of Physical Review Letters. Meanwhile, Aharon Kapitulnik and colleagues at Stanford University in Palo Alto, California, used a scanning tunneling microscope to study the crystal surface of bismuth strontium calcium copper oxide. They observed stripes of charge along every fourth row of copper atoms, they report in a paper posted on the Los Alamos preprint server.
Finally, Yoichi Ando and colleagues at the Central Research Institute of the Electric Power Industry in Tokyo, Japan, report that in nonsuperconducting LSCO and YBCO, the electrical resistance is smaller for current flowing in the direction in which the stripes are thought to run. That indicates the stripes are conductive, they argue in a paper to be published in Physical Review Letters.
The new data suggest that stripes are a feature of all cuprate superconductors, says Steven Kivelson, a physicist at the University of California, Los Angeles, and a co-inventor of the stripes theory. But the larger question, says Douglas Scalapino, a theorist at the University of California, Santa Barbara, is whether stripes help superconductivity or--as most researchers believe--hinder it.