page_title: High-temperature superconductivity
section_title: D symmetry in YBCO
hierarchical_section_title: High-temperature superconductivity / Theoretical models / D symmetry in YBCO
caption_attribution_description: Français&#160;: Effet Meissner: lévitation d'un aimant au dessus d'un supra-conducteur English: Meissner effect: levitation of a magnet above a superconductor
context_page_description: High-temperature superconductors are operatively defined as materials that behave as superconductors at temperatures above nearly 73.15 K.
  This is in fact the lowest temperature reachable by liquid nitrogen, one of the simplest coolants in cryogenics. All superconducting materials known at ordinary pressures currently work far below ambient temperatures and therefore require cooling. The majority of high-temperature superconductors are ceramics materials. On the other hand, Metallic superconductors usually work below -200 °C: they are then called low-temperature superconductors. Metallic superconductors are also ordinary superconductors, since they were discovered and used before the high-temperature ones.
  Ceramic superconductors are now becoming suitable for some practical use, but they still have many manufacturing issues and there are very few successful practical examples of employment.
  Most ceramics are brittle which makes the fabrication of wires from them very problematic.
  The major advantage of high-temperature ceramic superconductors is that they can be cooled by using liquid nitrogen.
context_section_description: An experiment based on flux quantization of a three-grain ring of YBa₂Cu₃O₇ (YBCO) was proposed to test the symmetry of the order parameter in the HTS. The symmetry of the order parameter could best be probed at the junction interface as the Cooper pairs tunnel across a Josephson junction or weak link. It was expected that a half-integer flux, that is, a spontaneous magnetization could only occur for a junction of d symmetry superconductors. But, even if the junction experiment is the strongest method to determine the symmetry of the HTS order parameter, the results have been ambiguous. J. R. Kirtley and C. C. Tsuei thought that the ambiguous results came from the defects inside the HTS, so that they designed an experiment where both clean limit (no defects) and dirty limit (maximal defects) were considered simultaneously. In the experiment, the spontaneous magnetization was clearly observed in YBCO, which supported the d symmetry of the order parameter in YBCO. But, since YBCO is orthorhombic, it might inherently have an admixture of s symmetry. So, by tuning their technique further, they found that there was an admixture of s symmetry in YBCO within about 3%. Also, they found that there was a pure dₓ²-y² order parameter symmetry in the tetragonal Tl₂Ba₂CuO₆.
caption: S magnet levitating above a high-temperature cooled by liquid nitrogen: this is a case of Meissner effect.
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