Iron-based superconductors: nematicity and pairing symmetry

5 problems

• Does tetragonal FeSe1-xSx host Bogoliubov Fermi surfaces

  Contested

  Beyond the nematic critical point, $\mathrm{FeSe}1-x\mathrm{S}x$ keeps many zero-energy excitations inside its superconducting state, which may signal surfaces of zero-energy quasiparticles (Bogoliubov Fermi surfaces) or simply disorder.

• Pairing symmetry of heavily electron-doped FeSe without hole pockets

  Contested

  In some FeSe compounds only electron pockets remain at the Fermi level, so the standard sign-changing scenario between hole and electron pockets cannot apply.

• What drives electronic nematic order in iron-based superconductors

  Contested

  Electrons in iron-based materials spontaneously choose one crystal direction; it is debated whether magnetism, orbital occupation, or chemical bonds drive this choice, especially in FeSe where no magnetism follows.

• Do nematic quantum critical fluctuations enhance $T_c$

  Contested

  Near the point where nematic order disappears at zero temperature its fluctuations are strongest, and they might help electrons pair; data in different compounds disagree.

• Does the gap change sign between hole and electron pockets

  Partially resolved

  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.