Do superconducting currents choose stripes?
Since the discovery of high-temperature superconductors more than a decade ago, scientists have puzzled over how these remarkable ceramics work (SN: 6/7/97, p. 351). One controversial theory holds that recently discovered stripe patterns in the magnetic and electronic features of the materials provide fast lanes for electrical charges. Doubters maintain that stripes either play no role in superconductivity superconductivity, abnormally high electrical conductivity of certain substances. The phenomenon was discovered in 1911 by Kamerlingh Onnes, who found that the resistance of mercury dropped suddenly to zero at a temperature of about 4.2°K;. or even stifle the phenomenon.
Now, experiments from California and Japan offer the first direct evidence that electrical charges move along the lanes. The findings, revealed in three separate reports this week, encourage stripe enthusiasts to believe they're on the right track but leave skeptics unmoved.
The new results are "very important and very striking," comments Steven A. Kivelson of the University of California, Los Angeles UCLA comprises the College of Letters and Science (the primary undergraduate college), seven professional schools, and five professional Health Science schools. Since 2001, UCLA has enrolled over 33,000 total students, and that number is steadily rising. . "These experiments in various ways strongly corroborate To support or enhance the believability of a fact or assertion by the presentation of additional information that confirms the truthfulness of the item.
The testimony of a witness is corroborated if subsequent evidence, such as a coroner's report or the testimony of other that stripes play a central role in the physics of the high-temperature superconductors," he says.
Although the experiments are "good physics," concedes Philip W. Anderson of Princeton University, the research teams involved "are not learning what's going on What's Going On is a record by American soul singer Marvin Gaye. Released on May 21, 1971 (see 1971 in music), What's Going On reflected the beginning of a new trend in soul music. that causes or is characteristic of high-temperature superconductivity." The narrowness of stripes would hinder electron pairing vital to superconductivity, he says.
A superconductor A material that has little resistance to the flow of electricity. Traditional superconductors operate at absolute zero (-459.67 degrees Fahrenheit or -273.15 degrees Celsius). Experiments in the 1980s raised the temperature to -321 degrees Fahrenheit. permits electric current to flow with zero resistance when the material is cooled below a critical temperature. The critical temperatures of high-temperature superconductors range up to roughly 150 kelvins. If researchers can understand how superconductivity arises in these materials, they may find ways to increase the critical temperature.
The compounds consist of repeating horizontal layers of copper and oxygen atoms separated by layers of transition metals, such as lanthanum lanthanum (lăn`thənəm) [Gr.,=to lie hidden], metallic chemical element; symbol La; at. no. 57; at. wt. 138.9055; m.p. about 920°C;; b.p. about 3,460°C;; sp. gr. 6.19 at 25°C;; valence +3. and yttrium yttrium (ĭt`rēəm) [for Ytterby, a town in Sweden], metallic chemical element; symbol Y; at. no. 39; at. wt. 88.9059; m.p. about 1,522°C;; b.p. 3,338°C;; sp. gr. about 4.45; valence +3. Yttrium is a highly crystalline iron-gray metal. , which contribute mobile, positive electric charges known as holes. Researchers led by Zhi-xun Shen Shen, in the Bible, place, perhaps close to Bethel, near which Samuel set up the stone Ebenezer. of Stanford University have now taken a close-up look at the electronic structure of a material--lanthanum-strontium-copper oxide with a smattering of neodymium--closely related to a superconductor.
The researchers report in the Oct. 8 SCIENCE that they used photons of ultraviolet light Ultraviolet light
A portion of the light spectrum not visible to the eye. Two bands of the UV spectrum, UVA and UVB, are used to treat psoriasis and other skin diseases. to eject electrons from the material. Then, they measured the particles' energy and momentum to infer how the holes left behind were behaving.
Their data show that holes move mainly in two perpendicular directions in the copper-oxygen layers. Shen says that the ejected electrons come simultaneously from multiple, tiny regions in the surface. In each region, the stripes run along one of the two perpendicular crystal axes. The direction of particle movement detected would be a composite of both types of regions. Therefore, the cross-shaped pattern observed indicates that the holes travel along the stripes. However, Shen says, "our data give no direct information about whether superconductivity is caused by stripes."
A second report in the issue of SCIENCE also looks at the movement of charges within the same type of copper oxide. However, scientists at the University of Tokyo “Todai” redirects here. For the restaurant called Todai, see Todai (restaurant).
The University of Tokyo (東京大学 , led by Shin-ichi Uchida, study the charges' bulk motion when a current induced by an electric field gets a sideways shove from a magnetic field--a phenomenon known as the Hall effect. This effect was reduced in an experiment in which the current ran parallel to the stripes. Apparently, stripes make it difficult for charges to move sideways.
In a different compound, yttrium-barium-copper oxide, Yoichi Ando and his colleagues at the Central Research Institute of Electric Power Industry in Tokyo used a magnetic field to turn the stripe pattern. Rotating the stripes from parallel to perpendicular with respect to a current flowing along the copper-oxygen lattice, they measured an increase in resistance. Reporting their findings in the Oct. 4 PHYSICAL REVIEW LETTERS Physical Review Letters is one of the most prestigious journals in physics. Since 1958, it has been published by the American Physical Society as an outgrowth of The Physical Review. , the researchers say that they have presented "strong evidence" that such stripes "have a considerable impact on electron transport."