Editing High-temperature superconductivity (section)

=== D symmetry in YBCO ===
[[File:Meissner effect p1390048.jpg|thumb|Small magnet levitating above a high-temperature superconductor cooled by [[liquid nitrogen]]: this is a case of [[Meissner effect]].]]

An experiment based on flux quantization of a three-grain ring of [[Yttrium barium copper oxide|YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7</sub> (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.<ref name=Gesh1987>{{cite journal |last1=Geshkenbein |first1=V. |last2=Larkin |first2=A. |last3=Barone |first3=A. |year=1987 |title=Vortices with half magnetic flux quanta in ''heavy-fermion'' superconductors |journal=[[Physical Review B]] |volume=36  |issue=1  |pages=235–238 |doi=10.1103/PhysRevB.36.235 |bibcode=1987PhRvB..36..235G |pmid=9942041}}</ref> 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. [[John R. Kirtley]] and C. C. Tsuei thought that the ambiguous results came from the defects inside the HTS, leading them to an experiment where both clean limit (no defects) and dirty limit (maximal defects) were considered simultaneously.<ref name=Kirt1995>
{{cite journal
 |last1=Kirtley |first1=J. R.
 |last2=Tsuei |first2=C. C.
 |last3=Sun |first3=J. Z.
 |last4=Chi |first4=C. C.
 |last5=Yu-Jahnes |first5=Lock See
 |last6=Gupta |first6=A.
 |last7=Rupp |first7=M.
 |last8=Ketchen |first8=M. B.
 |year=1995
 |title=Symmetry of the order parameter in the high-{{mvar|T}}<sub>c</sub> superconductor YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7−δ</sub>
 |journal=[[Nature (journal)|Nature]]
 |volume=373  |issue=6511  |pages=225–228
 |doi=10.1038/373225a0 |bibcode=1995Natur.373..225K |s2cid=4237450
}}
</ref> 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. By tuning their technique, they found an admixture of ''s'' symmetry in YBCO within about 3%.<ref name=Kirt2006>{{cite journal |last1=Kirtley |first1=J. R. |last2=Tsuei |first2=C. C. |last3=Ariando |first3=A. |last4=Verwijs |first4=C. J. M. |last5=Harkema |first5=S. |last6=Hilgenkamp |first6=H.
 |year=2006 |title=Angle-resolved phase-sensitive determination of the in-plane gap symmetry in YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7−δ</sub>
 |journal=[[Nature Physics]]
 |volume=2  |issue=3   |pages=190–194
 |doi=10.1038/nphys215 |bibcode=2006NatPh...2..190K |s2cid=118447968
|url=https://ris.utwente.nl/ws/files/6613933/angle.pdf }}
</ref> Also, they found a pure ''d''<sub>x<sup>2</sup>−y<sup>2</sup></sub> order parameter symmetry in tetragonal Tl<sub>2</sub>Ba<sub>2</sub>CuO<sub>6</sub>.<ref name=Tsue1997>
{{cite journal |last1=Tsuei   |first1=C. C. |last2=Kirtley |first2=J. R.
 |last3=Ren     |first3=Z. F.
 |last4=Wang    |first4=J. H.
 |last5=Raffy   |first5=H.
 |last6=Li      |first6=Z. Z.
 |year=1997
 |title=Pure ''d''<sub>x<sup>2</sup>−y<sup>2</sup></sub> order-parameter symmetry in the tetragonal superconductor Tl<sub>2</sub>Ba<sub>2</sub>CuO<sub>6+δ</sub>
 |journal=[[Nature (journal)|Nature]]
 |volume=387  |issue=6632  |pages=481–483
 |doi=10.1038/387481a0 |bibcode=1997Natur.387..481T |s2cid=4314494
}}
</ref>