Tuesday, October 25, 2011

Seeking a unified theory for unconventional superconductors

I am currently in Sydney at the Gordon Godfrey workshop on Spins and Strong Correlations.
Yesterday Rajiv Singh began his talk on orbital effects in the iron pnictide superconductors with some general remarks about the relationship between magnetism and superconductivity. The two were once considered inimical (one of Bernd Matthias' rules). But we now see classes of superconductors (heavy fermions, organics, cuprates, and pnictides) where they appear to be intimately connected.

Rajiv then took a philosophical position: there should be a common physics for all of these non-electron-phonon superconductors.

This certainly reflects a physicists desire for universality and simplicity. This is in distinct contrast to a chemists focus on particularity.

I am not convinced that it is or should be the case that there is some common underlying physics. Here are a few disordered thoughts.
  • For pnictides it seems that orbital (multiple band) effects matter. In contrast, in cuprates and organic it seems a single band is sufficient.
  • If there is a unified theory I think the strongest candidate is a weak-coupling spin fluctuation RPA picture (with renormalised interactions and Fermi liquid quasi-particles). To hold to this one will have to show that "exotica" (e.g., non-Fermi liquid effects) are just some higher order perturbative effects.
  • Similar sentiments of a unified picture of cuprates and pnictides is presented by Basov and Chubukov.
  • To me the two biggest problems for a "common physics" scenario are the pseudogap state in the cuprates and the spin liquid states in organics. They represent a "discontinuity" from the other materials and from any weak-coupling picture.
I welcome comments. Is there a common physics for non-electron-phonon coupled superconductors?

1 comment:

  1. My answer would be yes, at least in the sense that all where the mechanism is known or conjectured do not fit the dynamical screening scheme, where the hard core repulsion is evaded in time and the gap is isotropic in real space; rather they follow the Pitaevski-anderson-Morel-Thouless scheme of 1960 (later coopted by kohn-Luttinger) of evading it in space. So the gap is always anisotropic. I suspect that in general the interaction is not dynamically complex, i e no mediating boson, but rather a simple 4-Fermion vertex; or it can be modelled that way.

    One of the best arguments against your suggestion that weak-coupling spin-fluctuations are central is the work of Capone, Tosatti et al where they show that the region of the Mott transition gives a rising Tc which is wholly separate from the weak coupling domain, at least for their models which are based on buckyball physics.

    Finally, NFL phenomena of the strange metal are now a solved problem (Anderson & casey,HFL) and it is clear that they are a strong coupling phenomenon.