Super-unconventional superconductivity

 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 materials 
Jaime 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.

Multiorbital singlet pairing and d + d superconductivity 
Emilian M. Nica & Qimiao Si

Comments

  1. Eduardo MirandaFebruary 6, 2021 at 12:18 AM

    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.

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