Running in Circles

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Science  09 Jul 2010:
Vol. 329, Issue 5988, pp. 121
DOI: 10.1126/science.329.5988.121-d
CREDIT: GRASER ET AL., NAT. PHYS. 6, 10.1038/NPHYS1687 (2010)

While the debate about the origin of high-temperature superconductivity in the cuprates has been raging since their discovery more than 20 years ago, many of these materials have found practical use, most commonly as superconducting magnets and wires. However, a serious limitation exists, especially pronounced in the yttrium barium copper oxide (YBCO) family: Grain boundaries (interfaces between mismatched neighboring crystal orientations), present in the technologically interesting polycrystals, limit the observed supercurrent to values considerably below that attainable in a perfect single crystal. Graser et al. use molecular dynamics simulations and an effective tight binding model to simulate the flow of a supercurrent through a grain boundary in YBCO. They find that, even away from the boundary, the flow is disrupted and can run in closed loops (see arrows in the figure) and even backwards (blue lines). The experimentally established exponential dependence of the supercurrent on the angle of misorientation is recovered; the authors identify the accumulation of charge at the boundary as the primary cause, with the d-wave symmetry of the superconducting gap having, surprisingly, little effect. There are indications that similar behavior might occur in other complex superconductors, such as the ferropnictides.

Nat. Phys. 6, 10.1038/nphys1687 (2010).

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