Monday, April 4, 2011

It is not over until the fat lady sings

There is a nice informative article Iron superconductivity weathers another storm by Igor Mazin, which just appeared as a Trends piece in the overview journal Physics. It provided me with a nice succinct update on the very fast moving developments in the recently discovered iron-pnictide based superconductors. It seems everything is not as simple as originally thought. As is often the case it is the discovery of new materials which are leading the way and jolting us theorists.
Mazin describes the discovery of three classes of materials. The second two classes present problems for the picture that was developed for the first class.

I. The original Fe pnictide materials [e.g. La(O1-xFx)FeAs]  with the key features
  •  the electronic band structure is semimetallic, consisting of hole and electron Fermi surface pockets, separated by a (π,π) wave vector in momentum space. 
  • a spin excitation with the same wave vector.
  •  this spin excitation is the pairing agent for superconductivity 
  •  the superconducting order parameters for the holes and for the electrons have opposite signs, with the overall angular momentum being L=0 (s-type); hence the name s±. 
This means there are no nodes in the superconducting energy gap. This is implicitly a weakly correlated electron picture.

II.   Compounds, KFe2As2 , LaFePO, and BaFe2As2
These exhibited clear signs of a superconducting energy gap with nodes, inconsistent with class I and the picture above. Furthermore, calculations which claim to produce nodes for these materials also produce them in class I.

III. K0.8Fe2Se2 
Discovered just last November, this material has only electron Fermi surfaces and so one would expect d-wave pairing [and thus nodes] arising from nesting between the electron Fermi surface pockets.  But  experiments show that there are no nodes in the superconducting energy gap.

With regard to III. Mazin also recounts some experiments which show co-existence of antiferromagnetism and superconductivity [but do they co-exist on the microscopic scale] and uncertainty about stoichiometry and vacancy ordering in different samples.

The figure above is taken from a 2009 Physics Viewpoint, Are iron pnictides the new cuprates?  by Zlatko Tesanovic.

Aside: With regard to the title of this post, Wikipedia gives an interesting history of this colloquialism. I got idea of using the title from Phil Anderson, who used it for a talk in a session on the History of Superconductivity at an APS March Meeting, sometime in the nineties. I think he emphasized that BCS was not the end of the story because the question of gauge invariance, spontaneously broken symmetry, the Josephson effect, anomalous isotope effects, Rowell's tunneling spectra showing the electron-phonon glue, and strong coupling deviations from BCS all needed to be resolved/discovered.

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