Editing Unconventional superconductor (section)

==Junction experiment supporting the ''d-wave'' symmetry==

There was a clever experimental design to overcome the muddy situation. An experiment based on pair tunneling and flux quantization in a three-grain ring of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7</sub> (YBCO) was designed to test the symmetry of the order parameter in YBCO.
<ref name="Tsuei1994">{{cite journal|author=C. C. Tsuei|journal=Phys. Rev. Lett.|volume=73|year=1994|title=Pairing Symmetry and Flux Quantization in a Tricrystal Ring of Superconductin YBa2Cu3O7- delta|doi=10.1103/PHYSREVLETT.73.593|bibcode=1994PhRvL..73..593T|last2=Kirtley|first2=J. R.|last3=Chi|first3=C. C.|last4=Yu-Jahnes|first4=Lock See|last5=Gupta|first5=A.|last6=Shaw|first6=T.|last7=Sun|first7=J. Z.
|last8=Ketchen|first8=M. B.|issue=4|pages=593–596|pmid=10057486|display-authors=etal}}</ref> Such a ring consists of three YBCO crystals with specific orientations consistent with the d-wave pairing symmetry to give rise to a spontaneously generated half-integer quantum vortex at the tricrystal meeting point. Furthermore, the possibility that junction interfaces can be in the clean limit (no defects) or with maximum zig-zag disorder was taken into account in this tricrystal experiment.<ref name="Tsuei1994" />
A proposal of studying vortices with half magnetic flux quanta in heavy-fermion superconductors in three polycrystalline configurations was reported in 1987 by V. B. Geshkenbein, A. Larkin and A. Barone in 1987.<ref>{{cite journal|author=V. B. Geshkenbein|journal=Phys. Rev. B|volume=36|issue=1|pages=235–238|year=1987|title=Vortices with half magnetic flux quanta in ''heavy-fermion'' superconductors|doi=10.1103/PhysRevB.36.235|bibcode=1987PhRvB..36..235G|last2=Larkin|first2=A.|last3=Barone|first3=A.|display-authors=etal|pmid=9942041}}</ref>

In the first tricrystal pairing symmetry experiment,<ref name="Tsuei1994" /> the spontaneous magnetization of half flux quantum was clearly observed in YBCO, which convincingly supported the ''d-wave'' symmetry of the order parameter in YBCO. Because YBCO is [[orthorhombic]], it might inherently have an admixture of s-wave symmetry. So, by tuning their technique further, it was found that there was an admixture of s-wave symmetry in YBCO within about 3%.<ref>{{cite journal|author=J. R. Kirtley|journal=Nat. Phys.|volume=2|page=190|year=2006|title=Angle-resolved phase-sensitive determination of the in-plane gap symmetry in YBa2Cu3O7-delta|doi=10.1038/nphys215|issue=3|bibcode=2006NatPh...2..190K|last2=Tsuei|first2=C. C.|last3=Ariando|first3=A.|last4=Verwijs|first4=C. J. M.|last5=Harkema|first5=S.|last6=Hilgenkamp|first6=H.|s2cid=118447968|display-authors=etal}}</ref> Also, it was demonstrated by Tsuei, Kirtley et al. that there was a pure ''d''<sub>''x''<sup>2</sup>-''y''<sup>2</sup></sub> order parameter symmetry in the [[tetragonal]] Tl<sub>2</sub>Ba<sub>2</sub>CuO<sub>6</sub>.<ref>{{cite journal|author=C. C. Tsuei|journal=Nature|volume=387|page=481|year=1997|title=Pure dx2 – y2 order-parameter symmetry in the tetragonal superconductor TI2Ba2CuO6+delta|doi=10.1038/387481a0|issue=6632|bibcode=1997Natur.387..481T|last2=Kirtley|first2=J. R.|last3=Ren|first3=Z. F.|last4=Wang|first4=J. H.|last5=Raffy|first5=H.|last6=Li|first6=Z. Z.|s2cid=4314494|display-authors=etal}}</ref>