| MECHANISM OF ACTION |
How Ca doping boosts the supercurrent In its polycrystalline form, the high-temperature superconductor YBa2Cu3O7-* can carry more current across grain boundaries if some of the Y3+ ions are replaced with Ca2+ ions. This effect has generally been attributed to the fact that Ca2+ doping introduces excess holes (electron vacancies), which are the basis of the current. Now, an international team has shown that the ionic radius of calcium is more important than its valence in terms of boosting the supercurrent (Nature 2005, 435, 475). Robert F. Klie of Brookhaven National Laboratory and his collaborators find that Ca2+ not only substitutes for Y3+ but also replaces the larger Ba2+ and the smaller Cu2+ in strained grain-boundary regions. The substitution relieves strain and suppresses oxygen vacancies, which when unsuppressed reduce the local hole concentration. The researchers suggest that silver, which has roughly the same ionic radius as yttrium and calcium, might serve as an alternative dopant. They say it might increase the supercurrent across the grain boundary just as calcium does but without calcium's detrimental effects on superconducting properties within the grain. A University of Wisconsin, Madison, team reports related results (Nat. Mater. 2005, 4, 470). |
|
Want more information ? Interested in the hidden information ? Click here and do your request. |