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Title: Does the gap change sign between hole and electron pockets
Plain: The leading theory says the pairing amplitude has opposite sign on different pockets of the Fermi surface ($s\pm$ pairing); several experiments support it but a direct sign measurement is lacking.
Precise: In bulk iron pnictides with both hole pockets at $\Gamma$ and electron pockets at $M$ (for example Ba1-xKxFe2As2, LiFeAs), establish by a phase-sensitive probe whether the gap changes sign between them ($s\pm$) or not ($s++$). An answer is a measurement in which the sign enters directly, such as a Josephson interferometer or phase-referenced quasiparticle interference with known impurity potentials.
Settled by: A phase-sensitive interference or Josephson experiment with an unambiguous sign signature reproduced in two materials.
Kind: well-posed
Answer type: yes-no
References: (empty)
Literature status: partially-resolved
Status note: Neutron spin resonance and phase-referenced quasiparticle interference support $s\pm$; some impurity-scattering results were argued to favor $s++$.
Posed since: 2008