I was brought up to believe that spin-singlet superconductors must be s-wave (elemental) or d-wave (cuprates) and spin-triplet must be p-wave (superfluid 3He) or f-wave, and so on... More generally, singlets (triplets) are associated with even (odd) parity.
However, like a lot of things we learn some of us tend to forget what are the necessary assumptions needed for the result/claim to be valid.
Thus, I was intrigued when my colleague Ben Powell showed me this preprint.
Unconventional superconductivity near a flat band in organic and organometallic materialsJaime Merino, Manuel Fernandez Lopez, Ben J. Powell
For a t-t'-J model on a decorated honeycomb lattice, they find an f-wave spin-singlet superconductor!
The explanation for this surprise is as follows.
The Cooper wavefunction must always be anti-symmetric under fermion exchange. However, additional internal degrees of freedom can change things.
Two other examples come to mind.
One is non-centrosymmetric superconductors, where the absence of inversion symmetry in the crystal, means that parity is no longer a good quantum number. Then spin-orbit coupling can lead to a pairing state which is a mixture of spin-singlets and spin-triplets.
The paper below argues that in multiorbital systems that new types of singlet pairing are possible, such as intra- and inter-orbital pairing. This may be relevant to some iron-based superconductors and heavy fermion superconductors. In particular, it can explain certain perceived inconsistencies between experimental results for different physical quantities. Some suggested an energy gap for quasi-particle excitations while others did not.
Emilian M. Nica & Qimiao Si
Ross, you forgot to mention the "time" label. Most, if not all SCs, have an order parameter that is even under time reversal. As you know, so-called odd-w SCs, introduce an additional minus sign, which can once again "reverse" the usual reasoning. People talk about odd under SPOT: S for spin, P for parity, O for orbital and T for time. Whether any bulk SC is odd-w is a matter of debate. However, there is good evidence that in junctions with ferromagnets and some other exotic situations, they are the most stable state.
ReplyDeleteEduardo, Thanks for the helpful comment.
Delete